A mining research contract report 

JANUARY 1985 

A STUDY OF RF HAZARDS 

AT LOW AND MEDIUM 

FREQUENCIES TO BLASTING IN 

UNDERGROUND COAL MINES 

Contract J0318023 

Franklin Research Center 

BUREAU OF MINES 

UNITED STATES DEPARTMENT OF THE INTERIOR 
The views and conclusions contained in this document are those of the 

author and should not be interpreted as necessarily representing the 

official policies or recommendations of the Interior Department's Bureau 

of Mines or of the U. S. Government. 
m-m 

1 REPORT -- - WCUMWTAnON I L mRT NO- I % 

PAGE 1 1 

LT)CI..rdaumma 

A STUDY OF RF HAZARDS AT LOW AND MEDIUM FREQUENCIES 

TO BLASTING IN UNDERGROUND COAL MINES 

Franklin Research Center 

A division of ~rvin/~alspan 

20th and Race Streets 

Philadelphia, PA 19103 

-Qr, 

January 1985 

& 

7. 

R.H. Thompson, J.G. Stuart and A. W. Cipkins L 

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l2knrwrnsOrprn~nl~ tt.t~d~~rcrw~ 

U.S. Department of Interior Final 

Bureau of Mines, Branch of Procurement 2/1/81 - 11/30/84 r I 

Cochrans Mill Road n 

Pittsburnh. PA 15236 

lswoa---wF-C5490 

aa--UllltIlr 

I FIRL Report F-C3102, p. 1-2 

Preliminary safe distances for blasting wiring from typical electromagnetic 

sources are presented. The distances are for underground coal mines and 

electromagnetic sources in the frequency range below 10 MHz. The distances 

are based on measurements performed at 3 locations in each of 3 coal mines. 

The measurement apparatus, measurement procedure, measurement results and the 

rationale used in the calculation of the safe distances are all described. 

The safe distances presented are dependent on the measurement results and 

their interpretation. More measurements are clearly necessary to establish 

the validity of the suggested distances. 

7. 

Blasting Hazards 

r - 

EM Hazards, Safety 

Elec troexplosive Hazards 
FOREWORD 

This report was prepared by Franklin Research Center, a division of 

Arvin/Calspan, Applied Physics Laboratory, 20th and Race Streets, 

Philadelphia, PA 19103 under USBM Contract No. 50318023. The contract was 

initiated under the mining research program. It was administered under the 

technical direction of U.S. Bureau of Mines, Safety Research Facility at 

Bruceton, PA with Mr. James R. Means, Jr. acting as Technical Project 

Officer. Mr. Joseph Gilchrist was the Contract Administrator for the Bureau 

of Mines. This report is a summary of the work recently completed as a part 

of this contract during the period February 1, 1981 to November 30, 1984. 

This report was submitted by the author on January 15, 1985. 

ACKNOWLEDGEMENTS 

This project was carried out by the personnel of the Applied Physics 

Laboratory of the Franklin Research Center. The mine measurements were 

carried out principally by the authors, Mr. Ramie H. Thompson, FRC Fellow, Dr. 

James Stuart, Research Scientist, and Mr. A. W. Cipkins, Technical Associate. 

Thanks are due to Messrs. Harry Dobroski and James R. Means Jr. of the 

U.S. Bureau of Mines in providing specialized equipment and actual help in the 

preliminary measurements. 
1 EXECUTIVE SUMMARY 

In many underground coal mining operations the use of electromagnetic 

field equipment, particularly communication systems, not only increases the 

overall efficiency of the mining operation but can be directly linked with 

mine safety considerations. The use of electromagnetic field producing 

equipment in underground coal mining operations can only be expected to grow. 

Several new communication systems have already been proposed. In addition, 

high-frequency producing equipment for many diverse uses in mining operations 

are under study. 

The use of these equipments in underground mining operations is hampered 

by the possibility of their electromagnetic fields interacting with the 

electric blasting cap operations commonly carried out in the mines. Such 

interactions can have at least two results bearing directly on mine safety: 

o premature initiation of the cap, either in its normal shot location or 

during hookup or transportation; 

o dudding of the cap so that normal firing operations do not cause 

initiation, thereby leaving unexploded high explosives after normal 

firing. 

Since the consequences of a premature initiation could, particularly in 

coal mines operations, result in catastrophe involving human life, the overall 

approach to prediction of possible hazard must be, as far as reasonable, of a 

"worst case" nature. 

A considerable body of work exists that deals with the overall problem 

for aboveground blasting operations and sources. Although some of the results 

of this work are directly applicable to underground mining operations, the 

factors of minimum distance between transmitting sources and caps, tunnel 

geometry, unknown electrical properties of the surrounding minerals, etc., 

that are inherent in underground operations limit complete application of 

these methods and results. Some limited theoretical investigations, tailored 

to a specific underground communication system, have been performed. In the 

end, electromagnetic hazard evaluation of the system was forced to rely on 

experimental data taken in a tunnel of the mine in which the communication 

system was to be installed. 

Under the U.S. Bureau of Mines Contract H0210068 The Franklin Institute 

undertook a program aimed at producing a worst case model for electromagnetic 

energy/blasting cap interactions in underground workings. The results were 

presented in FIRL Report F-C3102, "Evaluation and Determination of Sensitivity 

of Electromagnetic Interactions of Commercial Blasting Caps," R.H. Thompson, 

August 1973. 
The experimental program showed that, for blasting caps of American 

manufacture, the application of 40 mW of average RF power, either continuous 

wave or with a typical radar pulse signature, to the input impendance of the 

cap, either pin-to-pin or pin-to-case, could be expected to result in an 

initiation less than once in one thousand applications with a 95% confidence 

level. Thus the use of a 40 mW "no-fire" level for underground caps, a level 

that had been tentatively established for all American-made blasting caps, was 

confirmed. 

Under U.S. Bureau of Mines Contract H0252015, the Applied Physics 

Laboratory of The Franklin Research Center undertook measurements in coal 

mines that, coupled with the 40 mW "no-firen level, previously developed worst 

case wiring pick up antenna models, published electromagnetic mineral 

parameter data and a "lossy layer" model for propagation in the mine, could be 

used to predict safe distances for blasting operations underground. This work 

is reported in "RF Hazards to Blasting Caps in Coal ~ines."~ The predicted 

safe distances in that report for frequencies less than 10 MHz were quite 

large. This was the direct result of the demonstration by our measurement 

results that, at these frequencies, underground sources coupled well to 

longitudinal conductors along the drifts. Thus, the energy from the source 

could propogate long distances with relatively small attenuation. The 

analyses assumed a large coupling factor between the source and any nearby 

conductors, thus large values of the fields were predicted at very large 

distances. We recognized at the time that the assumption of very good 

coupling between the source and the conductors in the mine was in error but 

the measurements taken provided no way to produce a valid worst case estimate 

of smaller magnitude. 

The present project was aimed directly at this difficulty. Our overall 

effort was directed at source frequencies of 10 MHz, or less. In this 

frequency regime coupling between the source and the blasting cap can be 

considered to be completely by magnetic induction. If the magnetic field, B, 

is known in the vicinity of the blasting wiring, then an upper limit on the 

power absorbed by a blasting cap in the wiring can be made. Since the 

magnetic field can be thought of as having its source in currents driven by 

electromagnetic equipment, and this equipment can be related to the maximum 

currents it can produce, we see that a knowledge of the B field in the 

vicinity of known currents would allow us to predict safe distances for these 

currents. The experimental work on this project was directed to determining 

the B field distribution around typical underground current carrying conductor 

configurations. 
This overall approach to the problem is based on the observation that we 

are concerned only with the driven currents in the immediate vicinity of the 

blasting cap wiring. Currents far away will have a very much smaller effect 

on the total power pickup of the blasting wiring than those in the immediate 

vicinity. Thus, our approach takes no notice of the propagating modes of the 

electromagnetic field - which in any practical wire laden drift will, we 

believe, never be known without an extensive measurement program in that 

drift. We are concerned, in the 10 MHz and below frequency range, only with 

near field effects. We relate power pickup to the maximum driven currents in 

nearby loops and linear elements. 

In particular, we chose loop current sources to approximate the "back 

packn antennas and bandolier antennas being used for communication in coal 

mines. We also made measurements on a configuration approximating the 

conventional trolley line communcation systems. 

Measurements of magnetic fields from loop and line sources were made at 

each of 3 locations in 3 coal mines. 1744 data points were obtained for the 

loop sources and 128 data points were obtained for the trolley line simulator. 

Since safe distances can be readily computed from a knowledge of the 

magnetic field and since the "free spacen magnetic field can be readily 

computed from a knowledge of the transmitting equipment and antenna, the 

parameter of concern is how much the underground environment can increase the 

magnetic field over its free space value. Our best estimate for this value is 

5. 

Our overall recommendations for blasting wiring safe distances from loop 

or long straight wire current sources are given in detail in Section 6 of this 

report. They are assumed valid for frequencies up to 10 MHz. 

It should be carefully noted that the recommended distances are based on 

measurements performed in three locations in each of three coal mines. 

Although the locations and mines were selected to give a wide diversity of 

electrical characteristics, it is quite obvious that they can not be expected 

to give a statistically valid sample of all coal mines. 

Further, if the safe blasting distances derived herein are to be applied 

to "hard rockn or metal mines we are even further from a valid statistical 

sample. We remark in passing, that we would expect the derived safe distances 

to increase somewhat in metal rich surroundings and decrease a bit in 

conductor poor mines. 
Another important point in the validity of the safe distances is our use 

of a multiplying factor of 5 for the mine to "free spacen magnetic field. As 

discussed in Section 5.0, this is not a worst case assumption. We had 29 of 

our 1744 (approximately 1.7%) measured values exceed 5. We attribute most of 

these to measurements near our noise level, measurements very close to 

conductors [and very limited in extent] or errors in measurement. There are, 

however, some measurements we feel were valid that did exceed a factor of 5 

over the calculated free space value. We think that these "hot spots" are of 

very limited spacial extent and they would thus not make significant 

differences in our safe distance calculations. We do not know. 

We recommend that further measurements of the type made here be performed 

in several more coal mines and, if non-coal mines are to be considered, 

several metal mines. The measurement apparatus should incorporate more gain 

to alleviate our small signal problem and the measurement procedure should be 

changed somewhat. Tables of values of the calculated free space pickup 

multiplied by 5 for each frequency/source can easily be prepared using the 

procedures given in Sections 6.2 and 6.3. We recommend that tables of this 

type be used during the measurements and any measured pickup in excess of 5 be 

closely investigated for its area of extent. 

The measurement equipment is already in existence, the measurement 

procedures are documented, the software for reduction of the data has already 

been developed and checked out. Thus, the additional measurements and data 

reduction can be carried out quickly with relatively small expense. 

The results of such additional measurements will result in a higher 

multiplying factor or confirm our recommended value of 5. The safe distance 

calculating procedure is structured so that a change of the multiplying factor 

is easy to incorporate. 
CONTENTS 

Sect ion Title Page 

ACKNOWLEDGEMENTS. 4 

EXECUTIVESUMMARY ........................ 5 

1.1 Background ......................... 13 

1.2 Early Efforts ........................ 17 

1.3 Final Measurements ..................... 18 

2.0 MINE MEASUREMENT DESCRIPTIONS .................. 19 

2.1 The Mines and the Measurement Locations ........... 19 

2.2 Apparatus .......................... 22 

2.3 Apparatus Check out ..................... 23 

2.4 Measurement Procedure at Each Mine Location ......... 25 

3.1 Loop Measurement Results .................. 26 

3.2 Transmission Line Results .................. 26 

4.1 Loop Data Analysis ..................... 27 

4.2 Transmission Line Data Analysis ............... 31 

6.0 SAFE DISTANCES FROM SOURCES ................... 49 

6.1 Critical Values of the Field ................ 49 

6.2 Free Space Values of the Field from Various Sources ..... 54 

6.3 Safe Distances from Carrier Phone Line Conductors ...... 63 

6.4 Safe Distances from Loop Sources .............. 76 

6.5 Summary of Safe Distances and Comment ............ 80 
Contents (~ont.) 

Sect ion Title 

A Monograph on Computation of RF Hazards 

B Raw Data for the Loop Measurements 

C Raw Data for the Transmission Line Measurements 

D Plots of the Nonnalized for the Loop Measurements 

E Normalized Data for the Loop Measurements 

F Breakdown of High Values of the Measured/Calculated 

Ratio by Occurrence 

G Plots of Current & Frequency Normalized Transmission 

Line Data 

H Output from Computer Program HECOMP Straight Line 

Line Currents of 10 Amps (Rectangular - 10 Meters 

by 1 ~eter) for 1 MHz 

I Listing for All Transmission Line Data Normalized 

to 10 Amps and 1 MHz 

J Trolley Carrier Phone Data 
Fig . No . 

LIST OF FIGURES 

Title Page 

SafeDistance~ .......................... 15 

Low-Frequency Transmitting Antenna ................ 24 

Measurements of 10-MHz RF Current ................. 24 

Calculated Z Pickup ........................ 28 

Calculated X Pickup ........................ 29 

Bar Graph for Variable: PKUP ................... 39 

Bar Graph for Variable: PKUP ............... .. .. 40 

Bar Graph of Measured/Calculated Ratio Distribution - 

Coarse Division .......................... 46 

Bar Graph of Measured/Calculated Ratio Distribution - 

FineDivision ........................... 47 

Solid Wire Skin Effect Quantities Compared with d-c Values .... 51 

(R/R~~ - 0.25) vs . Frequency for a Circular Wire PR = pR = 1 52 

Trolley Wire Coordinate System for H Field Sampling ........ 64 

Trolley Wire. 2 M Sep.. 1 A. 1 MHz ................ 65 

Trolleywire. 2MSep.. 1 A. 1MHz ................ 66 

Trolley Wire. 1 A. 1 MHz. 2 Meters Separation ........... 67 

Trolleywire. 2M. Sep.. 1 A. 1MHz ................ 68 

Trolley Wire. 2 M. Sep.. 1 A. 1 MHz ................ 69 

Loop. Radius = 0.5 Meters. 1 A. 1 MHz ............... 70 

Loop. Radius = 0.5 Meters. 1 A. 1 MHz ............... 71 

Loop.Radius~0.5Meters.1A.1MHz. .............. 72 

Loop Coordinate System for H Field Sampling ............ 73 

H Values of Various Points Around the Transmission Line ...... 74 

H Values at Various Points from the 0.5 M Radius Loop ....... 75 

Loop. Radius = 0.5 Meters. 1 A. 1 MHz ............... 76 
LIST OF TABLES 

Table NO. Title Page 

1 Computed Pickup for a 0.5 CM Radius Loop Source Current 

is a 10 Ampere Loop of 0.25 M Diameter. . . . . . . . . . . . . 30 

2 Computed Pickup for a 0.5 CM Radius Loop Source Current 

is a 10 Ampere Loop of 0.25 M Diameter. . . . . . . . . . . . . 30 

3 Ratio of Normalized Data to Computed Value for the Data Normalized in Frequency and Drive Current to 1 MHz and 10.0 Amps. . . 32 

4 Ratio of Normalized Data to Computed Value for the Data Normalized in Frequency and Drive Current to 1 MHz and 10.0 Amps. . . 33 

5 Ratio of Normalized Data to Computed Value for the Data Normalized in Frequency and Drive Current to 1 MHz and 10.0 Amps. . . 38 

6 Normalized Pickup for All Mines . . . . . . . . . . . . . . . . 41 

7 Ratioof NormalizedLoopDatatoComputedValue fortheData 

Normalized in Frequency and Drive Current to 1 MHz and 10.0 Amps 41 

8 Free Space H Field and Pickup from Transmission Line Wire 

Configuration - 10 Amps at 1 MHz. . . . . . . . . . . . . . . . 43 

9 For Transmission Line Data All Data Normalized to 10 Amps and 

~MHz............................. 44 

10 Transmission Line - Measured/Calculated . . . . . . . . . . . . 45 

11 Frequencies from 0.1 to 10 MHz. . . . . . . . . . . . . . . . . 55 

12 Frequencies from 0.1 to 1 MHz . . . . . . . . . . . . . . . . . 58 

13 Values for Safe Distance (z) for APF Transmitter Antennas . . . 79 
1.0 INTRODUCTION 

1.1 BACKGROUND 

In many underground coal mining operations the use of electromagnetic 

field equipment, particularly communication systems, not only increases the 

overall efficiency of the mining operation but can be directly linked with 

mine safety considerations. The use of electromagnetic field producing 

equipment in underground coal mining operations can only be expected to grow. 

Several new communication systems have already been proposed. In addition, 

high-frequency/high field producing equipment for many diverse uses in mining 

operations are under study. 

The use of this equipment in underground mining operations is hampered by 

the possibility of their electromagnetic fields interacting with the electric 

blasting cap operations commonly carried out in the mines. Such interactions 

can have at least two results bearing directly on mine safety: 

o premature initiation of the cap, either in its normal shot location or 

during hookup or transportation; 

o dudding of the cap so that normal firing operations do not cause 

initiation, thereby leaving unexploded high explosives after normal 

firing. 

Since the consequences of a premature initiation could, particularly in 

coal mines operations, result in catastrophe involving human life, the overall 

approach to prediction of possible hazard must be, as far as possible, of a 

"worst casen nature. 

The general problem of predicting possible RF hazards for any 

electroexplosive device (EED) is best treated by reference to Appendix A of 

this report. Appendix A considers the general problem from a military/space 

agency viewpoint but blasting caps are the primitive models for most EEDs so 

the material is directly applicable to blasting cap problems. 

In brief, the specific problem of analytically predicting possible RF 

hazards to blasting caps can be reduced to finding answers to the following 

three questions: 

(1) How much power, at the frequency of concern, is necessary to 

function the cap? 

(2) How much power is coupled to the cap by a given field existing in 

the vicinity of the cap and its associated wiring? 

3) What fields will exist in the vicinity of the blasting cap and its 

wiring for a given source, physical environment, frequency and 

separation? 
A considerable body of work1, *f3* exists that deals with questions (2) 

and (3) for aboveground blasting operations and sources. Although some of the 

results of this work are directly applicable to underground mining operations, 

the factors of minimum distance between transmitting sources and caps, tunnel 

geometry, unknown electrical properties of the surrounding minerals, etc., 

that are inherent in underground operations limit complete application of 

these methods and results. Some limited theoretical investigations4f5, 

tailored to a specific underground communication system, have been performed. 

In the end, electromagnetic hazard evaluation of the system was forced to rely 

on experimental data taken in a tunnel of the mine in which the communication 

system was to be installed. 

Under the U.S. Bureau of Mines Contract H0210068 The Franklin Institute 

undertook a program aimed at producing a worst case model for electromagnetic 

energy/blasting cap interactions in underground workings. The results were 

presented in FIRL Report F-C3102, "Evaluation and Determination of Sensitivity 

of Electromagnetic Interactions of Commercial Blasting Caps. n6 R.H. 

Thompson, August 1973. 

The experimental program showed that, for blasting caps of American 

manufacture, the application of 40 mW of average RF power, either continuous 

wave or with a typical radar pulse signature, to the input impendance of the 

cap, either pin-to-pin or pin-to-case, could be expected to result in an 

initiation less than once in one thousand applications with a 95% confidence 

level. Thus the use of a 40 mW "no-fire" level for underground caps, a level 

that had been tentatively established for all American-made blasting caps, was 

confirmed. 

Under U.S. Bureau of Mines Contract H0252015, the Applied Physics 

Laboratory of The Franklin Research Center undertook measurements in coal 

mines that, coupled with the 40 mW "no-firen level, previously developed worst 

case wiring pick up antenna models, published electromagnetic mineral 

parameter data and a "lossy layern model for propagation in the mine, could be 

used to predict safe distances for blasting operations underground. This work 

is reported in "RF Hazards to Blasting Caps in Coal ~ines."~ The predicted 

safe distances in that report and the previously mentioned report (F-C3102), 

for frequencies less than 10 MHz, were quite large. They are given in Figure 

1. This was the direct result of the demonstration by our measurement results 

that, at these frequencies, underground sources coupled well to longitudinal 

conductors along the drifts. Thus, the energy from the source could propogate 

long distances with relatively small attenuation. The analyses assumed a 

large coupling factor between the source and any nearby conductors, thus large 

values of the fields were predicted at very large distances. We recognized at 

the time that the assumption of very good coupling between the source and the 

conductors in the mine was in error but the measurements taken provided no way 

to produce a valid worst case estimate of smaller magnitude. 

"superscripts refer to numbered references of Section 8. 

The present project is aimed directly at this difficulty. Our overall 

effort was directed at source frequencies of 10 MHz, or less. In this 

frequency regime coupling between the source and the blasting cap can be 

considered to be completely by magnetic induction. If the magnetic field, B, 

is known in the vicinity of the blasting wiring, then an upper limit on the 

power absorbed by a blasting cap in the wiring can be made. Since the 

magnetic field can be thought of as having its source in currents driven by 

electromagnetic equipment, and this equipment can be related to the maximum 

currents it can produce, we see that a knowledge of the B field in the 

vicinity of known currents would allow us to predict safe distances for these 

currents. The experimental work on this project was directed to determining 

the B field distribution around typical underground current carrying conductor 

configurations. 

This overall approach to the problem is based on the observation that we 

are concerned only with the driven currents in the immediate vicinity of the 

blasting cap wiring. Currents far away will have a very much smaller effect 

on the total power pickup of the blasting wiring than those in the immediate 

vicinity. Thus, our approach takes no notice of the propagating modes of the 

electromagnetic field - which in any practical wire laden drift will, we 

believe, never be known without an extensive measurement program in that 

drift. We are concerned, in the 10 MHz and below frequency range, only with 

near field effects. We relate worst case power pickup to the maximum driven 

currents in nearby loops and linear elements. 

In particular, we chose loop current sources to approximate the "back 

pack" antennas and bandolier antennas being used for communication in coal 

mines. We also made measurements on a configuration approximating the 

conventional trolley line communcation systems. 

Our overall measurement scheme was guided by the following approach to 

computing safe distances in the frequency band below 10MHz. 

The maximum voltage induced in a loop of blasting wiring containing a cap 

can be written as 

where V is the RMS open circuit voltage (volts), R 

B is the RMS magnetic field (maxwells), 

linking the loop, and 

2 

A is the area of the loop (meters ) 
Total worst case power to the cap (wR) is 

where WR is the worst case power (watts), Zb is the total 

impedance of the loop and, 

Rdc is the resistance of the cap -- 

about 1 ohm. 

If the received power is less than 40 milliwatts, we assume the cap is 

safe in its environment. Now, we could easily compute B at the blasting 

wiring location if we knew the direction, magnitude and location of all driven 

currents in the vicinity and the overall conductor configuration were located 

in free space. The actual fact is that the driven currents exist underground 

and there are other conductors present in the mine. The direction, magnitude 

and location of the driven currents can be estimated -- in a worst case sense 

-- from the equipment used in the mine and the conductors they drive, thus we 

need a correction factor that can be applied to the free space calculation of 

the B field that compensates -- in a worst case sense -- for the presence of 

the additional conductors in the mine and the presence of the minerals that 

make up the overall mine environment. 

The determination of such a correction factor was the object of our 

measurements. 

Various loop antennas and transmission line simulating antennas were 

employed in late 1981 in a gallery of the U.S. Bureau of Mines Bruceton 

experimental mine. Measurements were taken of the variation of magnetic field 

with distance from the known currents in the transmitting antennas. The 

measurements, as expected, showed values larger than those predicted from free 

space calculations. The various conductor locations -- metal roof mesh, water 

pipes, electrical conduit, pipes, etc. -- were closely determined in the 

mine. A complete mockup of the mine -- constructed of wooden 2x4's was 

erected in an open field in Millville, N.J. 

All conductors were simulated in this mockup. The overall idea was to 

determine if the conductors or the minerals of the mine contributed most to 

the elevated B field measurements obtained in the actual mine. Hindsight is 

much better than foresight. We found our aboveground measurements to be 

completely useless. All our measurements were contaminated by large pickups 

-- large in relation to our previous measurements -- from AM & FM radio 

stations, even though these stations were many miles away. In the main, this 

entire result was due to the fact that the 300 foot long conductors were 

excellent antennas for the commercial radio stations. 
This entire phase of our effort provided no insight into our problem but 

did produce improvements in our source equipment and measurement techniques. 

1.3 FINAL MEASUREMENTS 

The measurements presented herein were aimed at the determination of B 

field variation -- in coal mine settings -- from known straight line and loop 

driven current sources. The measurements were performed at three locations in 

each of three mines. The individual locations were selected to provide a wide 

variation in local conduction density and degree of wetness. 
2.0 MINE MEASUREMENT DESCRIPTIONS 

2.1 THE MINES AND THE MEASUREMENT LOCATIONS 

The measurements were performed in two mines close to Raton, New Mexico 

and a mine in Ashland, PA. In the data presented the measurement locations 

are given by a number 1 to 9. 

The York Canyon Mines 

Two underground coal mines called respectively "Main" and "Central" are 

in York Canyon near Raton, New Mexico. Franklin Research Center personnel 

made electromagnetic measurements in three locations in each of these two 

mines. The mines were temporarily shut down at the time of measurements with 

only maintenance and supervisory personnel present. 

The mines are owned by the Kaiser Steel Co. The cooperation of the 

following people, among others, was greatly appreciated: Mr. Harry Elkin, 

Manager; also Fred Revera, M.A. Bertola, Paul Starkovich and Joe Romero. Help 

and hospitality from Mike Sepich and Larry Stolarczyk of A.R.F. Products 

Incorporated, manufacturers of mine-communications equipment, is also 

acknowledged . 

(1) Location 1 was in York Canyon Main Mine, number nine left intake, in 

front of the entrance to cross-cut thirty-six. This was a dry 

location. Density of metallic conductors present was high. A 

three-inch steel water-pipe ran along one wall along with many heavy 

electric cables. Thin steel plates called "mats", approximately 

fifteen inches wide by fifteen feet long, with their long dimension 

oriented across the tunnel, were bolted to the roof at four-foot 

intervals. The circular antenna loops were hung with the top of the 

loop approximately eighteen inches beiow the roof. This location 

was about three miles in from the mine portal and close to a 

long-wall continuous-mining apparatus. 

(2) Location 2 was not far from the previous location: York Canyon Main 

~ine, number nine left intake, in crosscut twenty-seven. This was a 

dry location. Density of metallic conductors present was medium. 

There was just one electric cable hung along the side of the 

crosscut, and no pipes. There were steel mats on the roof like 

those described previously. Transverse steel I-beams were used as 

roof supports. The antenna-loop was hung with its top two feet 

below the roof. 

(3) Location 3 was York Canyon Main Mine, number nine left intake, in 

crosscut 26. This was a dry location. Density of metallic I conductors present was low. There were no cables or pipes running 

along the side of the crosscut. Steel mats were on the roof in an 
irregular pattern. The roof was held up by steel I-beams going from 

one side of the crosscut to the other, supported at the ends by 

sturdy wooden posts. These I-beams were about eight feet apart. 

The loop antenna was hung so its top was about two feet below the 

roof, in a spot fourteen feet into the crosscut from the place where 

it opened into the haulageway. Thirty feet from the loop in the 

other direction was a concrete-block wall with a small door leading 

to a beltway. 

(4) Location 4 was York Canyon Central Mine, main intake, first south, 

in the haulageway between crosscuts eleven and twelve. This was a 

dry location. Density of conductors was medium. There were two 

three-inch-diameter high-voltage electric cables running along the 

side of the tunnel; these were the only longitudinal conductors. 

Steel I-beams ran across the roof every eight feet, supported by 

vertical logs at the tunnel sides. Between these I-beams were one 

or two of the "matsn described previously, running across the tunnel 

and bolted to the roof. The antenna-loop was hung with its top 

three feet below a roof mat, in the center of the haulageway. 

(5) Location 5 was York Canyon Central Mine, first south intake, near 

crosscut thirteen. This was a wet location. Density of conductors 

was high. Because this was close to the face there were a great 

many longitudinal conductors, pipes and electric cables running 

along the sides of the tunnel. The roof, about five feet five 

inches high, was steel mats, oriented transversely, bolted to the 

roof, spaced about four feet apart. There were steel I-beams 

running across the roof, supported by vertical logs at the ends. 

These I-beams were about twenty feet apart in this tunnel. The 

floor was covered with puddles. Our radiofrequency apparatus was 

put onto duckboards. The roof dripped water everywhere. Icicles up 

to three feet long hung from the roof; we had to clear a small space 

through the icicles so we could set up the antennas to make 

measurements. The loop antenna was hung with its top approximately 

eighteen inches below the roof, one foot away from a steel I-beam 

which was at roof-level. 

(6) Location 6 was York Canyon Cental Mine, first south intake, in the 

haulageway near crosscut twelve. This was a wet location. Density 

of conductors was medium, The only longitudinal conductors were a 

few electric cables and a telephone wire. There were puddles all 

over the floor. Water dripped from many places on the roof, and 

icicles hung all around, The steel roof mats were like those in 

location 5. 
Pioneer Tunnel 

This is a 70-year-old coal mine in Pennsylvania's anthracite 

region. The mine has three levels, three hundred vertical feet 

apart. All the tunnels have steel rails on the floor, and many 

places have electricity and compressed-air pipe. The mine was not 

operating, i.e. not producing coal at the time we were there. The 

author would like to express gratitude to George Staudenmeier, 

manager, and to William Whyne, foreman. 

(7) Location 7 was the so-called Little Buck Gangway. There were steel 

rails underfoot. Two wires ran overhead, along the center of the 

tunnel, attached to insulators nailed to logs or to the roof. Those 

were the only conductors present except for water, which dripped 

slowly from many places on the roof, but did not form puddles on the 

floor. The "Little Buck" vein of anthracite coal was two or three 

feet wide and slanted up at about a forty-five-degree angle. Above 

and below this vein were layers of hard rock. The bottom layer of 

rock had been blasted and excavated to make the tunnel. It was ten 

feet wide, with a horizontal floor and a smooth, sloping rock roof 

which was nine feet from the floor at its highest point. The vein 

of coal could be seen clearly. It continued up on one side, and 

down on the other side of the tunnel, parallel with and just below 

the tunnel's slanted rock roof. In most places no roof-supports of 

any kind were necessary. Infrequently (about fifty-foot intervals) 

there were a few vertical logs at the walls, and horizontal logs 

across the top, to give support at places where a chute led into 

this gangway from levels further up. The transmitting-loops for our 

measurements were hung with their centers about five feet from the 

floor, in the middle of the tunnel. 

(8) Location 7 was called "West number two Buck Mountain Gangway." 

There were no conductors at all here. The rails and electric wires 

had stopped farther back, and the roof did not drip water. The roof 

was twelve feet off the floor at its highest point, sharply slanted 

over a two-foot-wide coal seam. At the end of the gangway, where we 

were, the coal seam pinched down to one foot wide. In other 

respects this location was like the one described above. The 

transmitting coils were hung with center approximately 

four-and-a-half feet above the floor, in the center of the tunnel. 
(9) Location 9 was called "West Orchard Gangway." The roof dripped much 

water, and there were puddles on the floor. There were rails 

underfoot, and spare rails, pipes, tools and metal cans were 

stacked against the wall. This gangway had logs against the walls 

about every two feet, supporting horizontal logs overhead. 

Evidently the roof here needed more support than the roof in the 

other two gangways where we took data. This gangway was about ten 

feet wide, seven feet high, and seventy feet long. The transmitting 

coils were hung about twelve feet from the face at the end of this 

gangway. The coils were hung in the middle of the gangway with 

centers about four feet from the floor. The gangway ended abruptly 

at a face or vertical wall where the coal seam, three feet wide here 

and tilted up at forty-five degrees, could be seen clearly in crosssection. There was a single electric power line overhead for 

lighting. 

2.2 APPARATUS 

Measurement of the B field was performed using a circular loop antenna 

source at frequencies of approximately 0.175, 0.5, 1.0, 5.0 and 10.0 MHz in 

each mine location. Measurements were also made using a rectangular loop 1 by 

10 meters at approximate frequencies of 0:175, 0.5 and 1.0 MHz. 

All circular loops were one-half meter in diameter. At the lower 

frequencies (175 KHz, 500 KHz, 1 MHz) a loop with fifteen turns was used, with 

appropriate matching-circuits which were different for each frequency. At 5 

MHz a two- turn loop was used, and at 10 MHz a one-turn loop. Typical antenna 

currents varied from 10 to 12 amperes (peak to peak) in each turn. 

Current flowing in the antennas was measured by a clip-on current probe, 

Hewlett-Packard type lllOA. Peak-to-peak current, displayed on the 

oscilloscope described below, was entered by hand in a notebook. The transfer 

impedance of this current probe was 1 ohm, so that voltages displayed on the 

oscilloscope were of equal magnitude to currents enclosed by the probe jaws. 

The mobile transmitter which drove the antennas consisted of a low-power 

oscillator (General Radio 1211-B serial number 1215, or Hewlett-Packard 204B 

serial number 416-08150) and a 35-watt RF power amplifier (E.N.I. 440LA serial 

number 141). The change in transmitter current due to the reflected impedance 

of the clip on current probe used to measure the current was investigated by 

observing the change in pickup coil output when the clip on probe was removed 

and reattached to the transmitting antenna lead. The pickup coil was 

positioned on axis at 1 meter from the 10 MHz transmitting loop antenna and 

aligned for maximum output as indicated on the oscilloscope. In this 

configuration some slight change in the observed pickup could be observed when 

the clip on probe was attached to the transmitting antenna; however, the 

effect was transient and, to the best of our observation, no change in 

amplitude could be detected. We estimate that we would be able to resolve a 2 

percent change. 
The electromagnetic-field sensor consisted of a one-turn pickup loop of 

10 cm diameter, with a 160-ohm load paralleled by the 50 ohm measurement cable 

leading to the oscilloscope. The loop can thus be considered to be loaded 

with 38.1 ohms. The small size and low "Q" eliminated resonance effects over 

the entire frequency band of interest. The coaxial cable, terminated in its 

characteristic impedance, carried the pickup signal without reflections to a 

small battery-powered oscilloscope (Leader LBO-308S, serial number 1080193). 

Peak-to-Peak voltage, displayed on this oscilloscope, was entered by hand in a 

notebook. It should be noted that all data recorded were the maximum voltage 

observed on the scope when the pickup loop was oriented in various 

directions. Thus, the recorded datum is associated with the maximum B field 

magnitude at the measurement point irrespective of B field direction. 

The low-frequency transmitting-antenna, a 15-turn loop, is shown in 

Figure 2. Figure 3 shows the apparatus for measurement of RF current on the 

single-turn 10-MHz loop. 

The rectangular loop or long wire antenna was one meter high and ten 1 

meters long, with the lower conductor right on the ground or floor of the 

tunnel. This long-wire running down the center of the tunnel was intended to 

serve as a model or mockup of a trolley wire carrying radiofrequency signals. 

The RF magnetic field was then measured at points near the center of this 

wire, far enough from the ends that this mockup might be approximated 

theoretically by a wire of infinite length carrying uniform current. 

Naturally this long-wire could only be used at the lower frequencies (0.175, 

0.5, 1 MHz) where the current was approximately uniform over its length. The 

B field measurements were made using the same 10 cm probing loop described 

above. Preliminary measurements were made in every case to insure that the 

current in the rectangular loop was approximately uniform. 

1 2.3 APPARATUS CHECK OUT 

In order to determine if the source antennas and the measurement 

apparatus were functioning properly we performed all anticipated measurements 

aboveground in conductor free environments. These check out measurements were 

performed on an open field here in Philadelphia outside our laboratory and 

again on the grounds of the Br'uceton experimental mine. 

The pickup results were very close to those calculated by free space 

assumptions. 

23 

2.4 MEASUREMENT PROCEDURE AT EACH MINE LOCATION 1 

A circular loop antenna was hung in the center of the tunnel, at eye 

level, with its axis pointing along the tunnel. Current in the loop was 

driven by the transmitter set to that antenna's tuned frequency. Loop current 

was measured by the current probe. The strength of the magnetic field was 

first measured at points along the axis of the loop, and then at points in the 

plane of the loop. Approximately twenty-four measurements were made, at 

distances up to three meters from the center of the loop. This was a 

practical limit because at greater distances the signal level was generally 

too low to measure. All recorded data are the maximum values observed when 

orienting the pickup loop for maximum signal. 

Then the loop was rotated so its axis pointed across the tunnel. This 

was important because we expected the tunnel-loop coupling to be dependent 

upon the relative orientation of the loop and the tunnel. Another twenty-four 

measurements were then made like those described above. 

Having thus finished with the first frequency, the loop antenna was 

exchanged for another one, tuned to another frequency, and the whole procedure 

described above was followed again. In all, data was taken at five 

frequencies (approximately 0.175, 0.5, 1, 5, 10 MHz) in each mine location. 

After the loop measurement the long-wire antenna was set up. The 

long-wire antenna has the form of a long narrow rectangle, 1 by 10 meters. 

The wire is suspended on wooden forms so that one of the long sides was 1 

meter above the floor for its 10 meters of length. The opposite long side of 

the rectangle was on the floor. The rectangular loop was driven by the same 

transmitters used for the circular loops and current in the source loop 

measured with the current probe. The magnetic field was measured at a point 5 

meters (half way down) from the driven end at heights of 0.5 and 1.0 meters at 

various distances from the plane of the rectangular loop. Measurements were 

carried out at approximately 0.175, 0.5 and 1.0 MHz. 

Note that all data were recorded for B field measurements as millivolts 

-- (peak-to-peak) displayed on the oscilloscope. All source antenna currents 

were also recorded as total amps (number of turns times current, peak-to-peak) 

so that no conversion from RMS to peak-to-peak was needed for our 

calculations. Thus, all references to transmitting loop currents can be 

assumed to be in ampere turns. 
3.0 MINE MEASUReMENT RESULTS 

3.1 LOOP MEASUREMENT RESULTS 

The raw loop data is given in Appendix B. The loop is always assumed to 

be in the X,Y plane with the axis of the loop perpendicular to this plane. 

Further, the loop is assumed to be suspended along the Y axis. Thus if the 

axis of the loop is pointed along the drift, Z is measured along the drift 

from the center of the loop. X is then measured to the drift wall from the 

center of the loop. This orientation, with the axis of the loop in the 

direction of the drift axis is referred to in Appendix B as direction 1. 

Direction 2 is associated with the axis of the loop pointing to the drift 

wall. In this case also X and Z are defined as above. Thus, the X,Y,Z 

coordinate system is associated with the loop, not its orientation in respect 

to the tunnel. 

In particular, if X=O for a data point, then the measurement is at a 

point along the axis of loop and if 2=0, the measurement point is in the plane 

of the loop. Note that both X and 2 values can be greater than or less than 

zero. Measurements were made in both directions along the axes. 

In Appendix B the column headed "Current" gives the total (number of 

turns times single turn peak to peak current) current in the source loop, the 

column headed "Pickup" is the peak-to-peak voltage recorded from the 

oscilloscope connected to the 10 cm search coil. 

The order of measurements in Appendix B is the Actual order in which the 

measurments were taken. 

3.2 TRANSMI SSION LINE RESULTS 

Appendix C gives all the raw data for the transmission line (long line 

simulator) measurements. Notation is the same as in Section 3.1; note that 

here, however, we have no direction notation since the long line always runs 

down the drift. With the transmission line, measurement data were always 

taken at the midpoint of the line. The Z dimension was taken as vertical 

height from the floor. The Y direction was measured toward the mine wall from 

the middle of the drift (the location of the transmission line). A sketch of 

the coordinate system is given as the first page of Appendix H. 
4.0 ANALYSIS OF MEASUREMENT RESULTS 

4.1 LOOP DATA ANALYSIS 

The raw data listing of Appendix B is difficult to interpret even if 

exactly the same frequencies were used at each location and exactly the same 

loop source currents were obtained. In order to compare the results at 

differing locations we have lumped all frequencies that are close together, 

e.g. all 0.173, 0.174 and 0.175 frequencies are lumped together for clarity. 

Specifically we have lumped all data for frequencies 1.71 to 1.75 MHz into 

1.73 MHz data; all 0.52 MHz data into 0.53 MHz data; all 5.7 to 5.8 MHz data 

into 5.0 MHz data, and all 9.0 to 9.8 MHz data into 10 MHz data. In every 

case we have scaled the pickup for the frequency change assuming that the 

pickup is proportional to frequency. Also, we expect all results of these 

experiments to be linear in relation to source current. Therefore, all source 

currents can be normalized to a single value if the "pickups" are changed by 

the same factor. Applying these rules and normalizing all the data to a 10 

ampere source current, we have plotted the results for all frequencies (1.73, 

0.5, 1.0, 5.0 and 10.0 MHZ) for both possible directions of the loop axis and 

for both X=O and Z=0 data. These plots are given in Appendix D. Note that 

some individual curves show a sawtooth pattern. This is the result of our 

plotting both the result at a negative coordinate and a positive coodinate at 

the same positive value of the coordinate. Each individual curve on a given 

graph thus presents all data for a single location and direction along the 

indicated coordinate. 

The curves of Appendix D, then, present a good visual picture of the 

variation from location to location for the same conditions. : .r overall 

interest is in determining how far our measurement data deviate from a 

computed free space value. The free space values of induced voltage in a 

10 cm diameter loop at various distances from a 0.5 meter diameter receiving 

loop along both the X and 2 axes can easily be computed for a frequency of 1 

MHz. These data are shown in Figures 4 and 5. Note that Figure 4-B expands 

Figure 4-A at the 1 meter point where our first data point in the Z direction 

was taken. Figure 5 provides the same data for the X direction. Tables 1 and 

2 present the same data in tabular form. Thus, we know the calculated data 

for all the data points at 1 MHz. 4 

aB The pickup values at all frequencies are assumed to be proportional to - , thus all pickups can be scaled to any other frequency. For the 10 MHz 

%ta we can simply divide the measured pickup by 10 to scale it to 1 MHz. 

Appendix E presents all the loop data scaled to 1 MHz and 10 amperes source 

loop current. It also presents the free space calculated value for data and 

the ratio of measured over calculated values. This ratio of measured to 

calculated values is precisely the "correction factor" mentioned in the 

Introduction. 

(text continued on page 31) 
DRIVEN LOOP: R=0.250 1=10.000 CENT 0.0 0.0 0.0 AXlS 0.0 0.0 1.0 

PKUP LOOP R=0.050: PICKUP VRS DISTANCE FROM LOOP CENTER: FMZ=1 .OO 

D l STANCE FROM LOOP CENTER-METERS 

THE POINTS ARE ALONG THE VECTOR 0.000 0.000 1.000 

DRIVEN LOOP: R=0.250 1=10.000 CENT 0.0 0.0 0.0 AXlS 0.0 0.0 1.0 

PKUP LOOP R=0.050: PICKUP VRS DISTANCE FROM LOOP CENTER: FMZ=1 .OO 

0 1 2 3 4 5 

DISTANCE FROM LOOP CENTER-METERS 

THE POINTS ARE ALONG THE VECTOR 0.000 0.000 1.000 

Figure 4. Calculated Z Pickup 
DRIVEN LOOP: R=0.250 1=10.000 CENT 0.0 0.0 0.0 AXIS 0.0 0.0 1.0 

PKUP LOOP ~=0.050: PICKUP VRS DISTANCE FROM LOOP CENTER: FMZ=I.OO 

0 1 2 3 4 5 

D l STANCE FROM LOOP CENTER-METERS 

THE POINTS ARE ALONG THE VECTOR 1.000 0.000 0.000 

(A) 

DRIVEN LOOP: R=0.250 1=10.000 CENT 0.0 0.0 0.0 AXIS 0.0 0.0 1.0 

PKUP LOOP R=0.050: PICKUP VRS DISTANCE FROM LOOP CENTER: FMZ=l .OO 

DISTANCE FROM LOOP CENTER-METERS 

THE POINTS ARE ALONG THE VECTOR 0.000 0.000 1.000 

Figure 5. Calculated X Pickup 
TABLE 1. - Computed Pickup for a 0.5 CM Radius Loop 

Source Current is a 10 Ampere Loop of 0.25 M Radius 

Distance Z 

(Meters) 

Pickup 

(Volts) 

TABLE 2. - Computed Pickup for a 0.5 CM Radius Loop 

Source Current is a 10 Ampere Loop of 0.25 M Radius 

Distance X 

(Meters) 

Pickup 

(Volts) 
(text continued from page 27) 

Comparison of data from location to location or direction to direction is 

difficult in Appendix E. Table 3 gives a breakdown of the ways the ratio of 

measured to calculated pickup varies with direction. Here, all measurements 

along a given axis, at a given frequency, are lumped together. Note the table 

suggests that changing the direction of the loop axis has only a small effect 

on the data obtained. As explained earlier, direction refers to the loop axis 

direction. 

Table 4 gives much the same data as Table 3 but here direction of the 

axis is ignored. 

Table 5 breaks down the variation in the ratio for each measurement 

distance and direction. Note that high values of the ratio seems to be 

associated most frequently with high values of X or 2. 

Appendix F investigates this association in detail. Here, each ratio 

larger than 2, 3, 4, 5 ... is listed so that distance, location, direction 

can all be evaluated. There were approximately 193 data points taken at each 

location. Study of Appendix F shows that these were 294 points greater than 

2. Of these, 103 occurred at Location 5 -- our most conductor rich location. 

Study of the locations and conditions for values of the ratio greater than 5 

shows that there were 29 such values and, of these, only 3 occur at distances 

less than 2.5 or 3 meters. At these distances (2.5 and 3 meters) we are close 

to our noise level and small changes in field can make large changes in our 

ratio. Also, note that 16 of the 29 values are from location 5 and location 1 

-- our high conductor density locations. 

For ratios equal or greater than 7 all 10 occur close to the wall of the 

drift -- except one occurring in location 5. The values of the ratio greater 

than 8 all show the same pattern. We checked these against the data notebook 

and some show marginal notations showing that they were taken close to 

conductors. 

Figures 6 and 7 are bar graphs of the occurrence of various values of the 

ratio on differing scales. We see that we had only 29 of 1744 occurrences of 

the ratio larger than 5 -- approximately 1.7% -- and it seems that most of 

these are associated with measurements close to conductors and relatively far 

(2 to 3 meters) from the source loops. 

Table 6 is the output of a commonly available statistics program (STP) 

that evaluated the basic statistics for the 1744 values of the measured to 

collected ratios. Table 7 is an evaluation of the same data by the program 

that prepared the previously presented breakdowns. We present it here to show 

the agreement. 

(text continued to page 42) 
TABLE 3, - Ratio of Normalized Data to Computed Value 

for the Data Normalized in Frequency and Drive Current 

to 1 MHz and 10,O Amps 

Direction is Loop Axis Direction 

WITM X=OmO AND DIRECTION= ALONG THE DRIFT 

FREQ (HHZ 1 PICKUP RATIO 8 OF ITEMS SIGMA 

MAX HIN AVG, 

,173 2m4 l 7 1m4 99 e3 

,530 5e4 el 1e6 107 m9 

1eOOO 2 l 7 05 1 l 3 108 3 

5,000 2.7 e6 1.3 8 1 a4 

10,000 3m7 m4 1m4 104 6 

NOTE THAT SIGMA (S) IS COMPUTED FROM: 

S**2=(1/(~-l)l*(SUNi(X-MEA~l**2) 

WHERE: K=# OFITEMS; X IS PICKUP; AND REAN IS THE HEAN OF THE PICKUPS 

WITH XtO.0 AKD DIRECTION= ACROSS THE DRIFT 

FREQCHHZ) PICKUP RATIO I OF iTCKS SIGMA 

MAX MIN AVG. 

.I73 2.0 e5 la2 85 • 3 

,530 4.8 09 1 m7 88 l 8 

1,000 4.1 04 1.6 87 07 

5,000 4 l 2 6 1 e4 67 a5 

10,000 305 6 1.4 66 05 

WITH' Z=OmO AND DIRECTION= ALONG THE DRIFT 

FREQ(MHZ) PICKUP RATIO t OF ITEMS SIGHA 

WAX HIN AVG. 

,173 302 8 1.7 7 1 04 

,530 7.1 07 2 • 0 87 1.3 

1,000 2.8 1.0 1.6 85 .C 

5,000 8.0 04 1.9 63 1m2 

10,000 13.0 04 2m9 52 2 l 4 

WITH Z=0,0 AND DIRECTION= ACROSS THE DRIFT 

FREQ(MHZ1 PICKUP RATIO I OF ITEMS SIGMA 

MAX MIN AVC. 

.I73 3m3 a9 1.6 89 m4 

,530 Sm8 1 e8 100 09 

1.000 5 l 3 09 1m8 104 8 

5,000 6.1 .5 2mO 79 1.1 

10,000 7.5 l 5 2.4 82 1.5 
TABLE 4. - Ratio of Normalized Data to Computed Value 

For the Data Normalized in Frequency and Drive Current 

to 1 MHz and 10.0 Amps 

Direction is Loop Axis Direction 

WITH X=O,O 

CREQ (HHZ 1 PICKUP RATIO # OF ITEMS SIGMA 

MAX MIN AVCe 

m173 2e4 l 5 le3 184 l 3 

a530 5e4 l 1 1 e6 195 l 9 

1*000 4*1 l 4 1 e4 195 l 5 

5,000 4e2 l 6 1-4 148 • 4 

10,000 3e7 l 4 1e4 170 l 5 

WITH Z=OeO 

.I73 3 3 l 0 1e7 160 4 

.530 7e1 l 7 1.9 187 1-1 

1*000 5.3 l 9 1-7 189 .7 

5,000 8e0 l 4 2 l 0 142 lel 

10.000 13,O ,4 2e7 174 2eO 

FOR ALL DATA 

a173 3 l 3 l 5 1-5 344 .4 

.530 7,1 1 1.8 382 t ,O 

1,000 5e3 l 4 1 e6 384 e6 

5.000 8.0 l 4 1.6 290 l 9 

10,000 13,O a 4 2.0 344 le6 

NOTE THAT SIGMA (S) IS COMPUTED FROM: 

S**2=(l/(N-l))*(SuM((X-MEAN)**2) 

WHERE: N=# OF ITEMS; X is PICKUP; AND MEAN IS THE MEAN OF THE PICKUPS 
Table 4 (~ont.) 

WITH Xt0.Q ; Z=1.5 AND DIRECTION= ALONG THE DRIFT 

FREQ (HHZ 1 PICKUP RATIO t OF ITEMS SfGKA 

%AX MI N AVG 

el73 1.6 07 102 19 02 

.530 3.8 01 1.5 18 8 

1.000 2.7 09 1.4 18 04 

5,000 1.8 0 1 e2 13 • 3 

lO.OU0 2.0 . 7 1.2 20 . 4 

WITH X=O.O ; Z=l.S AND DIRECTION= ACROSS THE DRIFT 

FREQ(HHZ1 PICKUP RATIO 8 OF ITEMS SIGMA 

MAX HIN AVG. 

0173 1 e5 l 7 102 18 02 

,530 4.7 1.1 1.7 18 1.0 

i.000 3.1 1.1 1.6 18 6 

So00O 2.3 07 1.3 16 • 4 

10.000 1e7 6 102 15 3 

WITH X=O;O ; 2=2.0 AND D~RECTION= ALONG THE DRIFT 

FREQ(MHZ1 PICKUP RATIO 8 OF ITEHS SIGMA 

MAX AVG. 

.I73 1.7 'IN 1.0 1.4 19 02 

.530 3.9 • 9 1.6 18 0 

1.000 1.5 09 1.3 I0 2 

Se0OO 1 .b 1.0 1.2 12 • 2 

10.000 2.3 6 1e4 17 06 

WITH X=O.O ; Z=2,0 AND DIRECTION= ACROSS THE DRSFT 

FREQ (MHz 1 PICKUP RATIO I GF ITEMS SIGH& 

MAX HIN AVGo 

el73 1.9 05 1.3 14 04 

.S3O 3.6 1.1 1.6 1 S 8 

1.000 3.6 8 1.6 15 07 

50000 1e6 l 6 1.3 8 • 4 

1O.OOO 2.4 e9 1.4 10 e5 

WITH X=O.O ; 2=2.5 AND DIRECTION= ALOiiG THE DRIFT 

FREQ (MHz 1 PICKUP RkTIO # OF ITEMS SIGMA 

MAX HI AVG. 

0173 1.9 1.1 1.7. 45 e2 

.530 4e8 07 1.7 1 Fj 1.1 

1eOOO 1.6 100 1 3 10 02 

5.000 1 • 7 1.3 12 04 

lO.000 2.5 e4 1.5 15 6 
Table 4 (~ont.) 

WITH X=O.O ; 2~2.5 AND DIRECTION= ACROSS THE DRIFT 

FREQ (MHz 1 PICKUP RATIO 8 OF ITEMS SIGMA 

MAX MI& AVG 

0173 1 e9 • 7 lo3 11 e4 

,530 3e4 • 9 lef 13 8 

1,000 4e1 100 1.8 11 l 9 

5~000 2e2 • 9 1.5 7 l S 

lOe000 2-3 06 1.5 6 07 

WITH X=O.O ; Zs3.0 AND DIRECTIOt4= ALONG THE DRIFT 

FREQ (MHz 1 PICKUP RATIO t OF ITEMS SIGMAMAX MIN AVG, 

0173 2.4 1.4 1.8 8 • 4 

0530 5e4 lei 1.8 17 102 

1.000 107 1.0 103 18 • 2 

5.000 2,7 -6 1.6 12 • '1 

1OoOOO 3.7 • 7 202 12 .9 

WITH X=O.O ; Zt3.O AND DIRECTIOK= ACROSS THE DRIFT 

FREQ(MHZ1 PICKUP RATIO # OF ITEMS SIGMA 

MAX HlN AVG. 

.I73 200 1 • 1 1.3 6 • 4 ' ,530 3eO 1 l 1 1.7 6 a 7 

1.000 2 l 5 w 4 1.6 7 a 8 

5.000 4 l 2 • 7 2e1 4 1.5 

100000 ' 3e5 1 a 1 2 l 1 3 1 e2 

WITH X= .5 ; Z=OeO AND DIRECTION= ALONG THE DRIFT 

FREQ(MHZ1 PICKUP RATIO # OF ITEMS SIGMA 

MAX MIN AVC 

0173 3e2 -8 lo8 17 w 6 

,530 7.1 1eO 2-1 18 1.5 

1.000 2e5 1.0 1.6 18 • 4 

5,000 2e7 • 9 let 16 -4 

100000 3e4 l 8 3eO 20 a 8 

WITH X= 05 t Z=0.0 AND OXRECTIOh= ACROSS THE DRIFT 

FREQ(MHZ1 PICKUP RATIO 8 OF ITEMS SIGMA 

MAX MIN AVCe 

0173 2.4 • 9 1.4 10 e4 

,530 ' 3.6 l 8 1.6 18 8 

1 .OOO 5.3 102 202 16 1e1 

5,000 2 5 101 1.7 16 • 4 

10.000 4.0 l 8 1.7 16 • 9 
Table 4 (~ont.) 

WITH X=l,O ; Z=0.0 AND DIRECTION= ALONG THE DRIFT 

rREQ (MHz) PICKUP RATIO # OF ITEMS SIGMA 

MAX WIN AVG 

8/73 2 0 1.0 1.5 17 3 

.S30 4.6 - lo1 1.8 10 1.0 

1,000 2.2 1.0 1.4 18 3 

5,000 108 09 1.4 16 3 

10aOOO 202 04 1.4 20 • S 

WITH X=1.0 ; Z=0.0 AND DIRECTION= ACROSS THE DRIFT 

PICKUP RATIO # OF ITEMS SIGMA 

MIN AVG, 

,173 2.4 lo1 1.5 18 3 

,530 4.1 1.1 1.6 18 8 

1,000 403 102 1.8 18 8 

5,000 1.9 05 103 15 04 

10.000 201 oS 1.5 16 04 

WITH X=1.5 ; Z=0.0 AND DIRECTION= ALONG THE DRIFT 

FREQ MAX 

PICKUP RATIO # OF ITEMS SIGMA 

MIN P.VGo 

,173 1.9 101 lo6 17 02 

,530 5 0 7 108 18 1 • 1 

1,000 2 8 1.1 1 5 18 04 

5,000 4.0 04 1.7 13 09 

10,000 3.6 8 200 19 07 

WITH X=1.5 ; Z=0,0 AND DIRECTION= ACROSS THE DRIFT 

FREQ (MHz 1 PICKUP RATIO t OF ITEMS SIGMA 

WAX MIN hVGo 

,173 2.7 09 1.5 18 04 

,530 4.1 101 1.6 18 8 

1,000 3.8 09 1.6 18 07 

5,000 305 07 1.8 14 07 

10,000 2.9 1.1 1.9 15 6 

WITH Xz2.O ; Z=0.0 AND DIRECTION= ALONG TtiE DRIFT 

FREQ (MHz 1 PICKUP RATIO # OF ITEHS SIGH4 

WAX. HI (r ~VC l 

,173 2.1 1.0 1.7 - 12 3 

,530 4.8 lo1 1.9 15 1.1 

1,000 2.3 1 1 1.5 15 3 

5,000 305 1.1 2 • 2 8 09 

10,000 6.7 102 3.3 15 1.6 
Table 4 (~ont.) 

WITH 

FREQ CHHZ 1 

WITH 

FREQ (UHZ 

WITH 

FREQ CMiiZ 1 

WITH 

X=2.0 ; Z=OmO AND DIRECTION= ACROSS THE DRIFT 

PICKUP RATIO # OF ITEMS SIGMA 

MAX MIti AVG l 

2 8 1.4 1m8 16 m5 

3,8 lm1 1.6 18 l 8 

4m1 09 1 m6 18 m7 

2m8 m9 1.9 12 7 

4m3 1m3 2m6 14 9 

X=2mS ; 2=0.0 AfiD DIRECTION= ALONG THE DRIFT 

PICKUP RATIO t 'OF ITEMS SIGMA 

MAX MIN AVGm 

2 l 7 1.8 2 0 6 a4 

6m1 1 l 1 2m3 13 1m6 

2 l 4 tm3 1.8 13 l 3 

6m1 1m4 2m9 6 1m7 

8m3 2m4 5.2 ti 1m9 

X=2.5 ; Z=0.0 AND DIRECTIOK= ACROSS THE DRIFT 

PICKUP RATIO # OF ITEKS SIGMA 

WAX MIN AVCm 

2.6 1m3 1.9 13 l 4 

4 l 0 1m3 1m9 18 1.1 

4 l 3 1.0 1m8 17 .7 

3m5 1mo 2m2 11 l 9 

6.2 1.6 3m5 11 1 m6 

Xz3.O ; Z=OmO AND DIRECTIOti= ALONG THE DRIFT 

PICKUP RATIO # OF ITEMS SIGMA 

MAX MIN AVGm 

1.9 1m9 1.9 2 

6 l 9 1m4 2 l 7 5 2m4 

2m5 1.4 lm8 3 6 

8mO 1.4 4mO 4 3m1 

13,O 3m3 8.1 7 3.7 

Xz3.0 ; Z=OmO AND DIRECTION= AC3OSS THE DRIFT 

PICKUP RATIO t OF ITEMS SIGMA 

MAX MIN AVGm 

3.3 1.9 2r2 6 6 

5 l 8 lm4 '2m4 10 1.5 

4m4 1.1 2mO 15 8 

6m1 8 3m3 11 1 m9 

7.5 2 l 8 4m6 lo 1 m9 
TABLE 5. - Ratio of Normalized Data to Computed Value 

For the Data Normalized in Frequency and Drive Current 

to 1 MHz and 10.0 Amps 

Direction is Loop Axis Direction 

WITH X=OaO ; Z= .5 AND DIRECTION= ALONG THE DRIFT 

FREQ (MHz 1 PICKUP RATIO t OF ITEMS SIGMA 

MAX MIN AVG, 

a173 1 .f .9 102 19 -a 2 

.530 4.2 1.1 1.6 18 .8 

1,000 1.7 8 la3 18 3 

5,000 1 e9 1.2 la4 16 a 2 

10aOOO 1 a7 .6 la3 20 a 3 

NOTE THAT SIGVA CSI IS CDYPUTED FROM: 

S**2=(l/(Nm111*(SUM(~X-~EANW) 

WHERE: N=l OFITEMS; X IS PICKUP; AND MEAN IS THE MEAN OF THE PICKUPS 

WITH X=OeO ; Z= 05 -AND DIRECTION= ACROSS TliE DRIFT 

FREQ (MHz 1 PICKUP RATIO # OF ITEMS SIGMA 

WAX HIN AVG, 

.173. 1.8 a 7 1.4 18 a 3 

,530 4e4 la0 1.7 18 09 

1,000 4.1 1.0 la6 18 a7 

5,000 1 e7 1.3 1 es 16 .I 

10eOOO 2.1 1 1 1.5 16 3 

WITH X=OeO ; 2=1.0 AND DIRECTION= ALONG THE DRIFT 

WITH 

FREQ c MHz 1 

PICKUP RATIO $ OF ITEMS SIGMA 

MAX ClIN AVGe 

1 e6 09 la2 19 2 

3.6 la0 1e6 18 a 7 

2 a 3 l 5 la3 18 4 

1.5 a 6 102 16 .2 

i a 8 a6 1.2 20 e3 

X=O.O ; Z=1.0 AND DIRECTION= ACROSS THE DRIFT 

PICKUP RATIO $ OF ITEMS SIGMA 

MAX MItJ AVGa 

1.6 .7 1.2 -1 8 a 2 

4.8 la1 1.7 18 a 9 

3.2 .7 la5 18 a 7 

2e1 • 9 1.3 16 3 

la6 .9 1.2 16 .2 
PKUP = Measured Value/Expected Free Space Measured Value 

***** BAR GRAPH FOR VARIABLE: PKUP ***** 

RANGE OF VALUES 

.oooo - 1,000 

1.000 - 2,000 

2.000 - 3.000 

3,000 - 4.000 

4.~00 - 5,000 

5,000 - 6,OOO 

6.000 - 7.000 

7.000 - 8,000 

8,000 0 16.00 

FREQ 

184 

1282 

157 

52 

40 

10 

10 

4 

5 -0.0 

1744 

PCENT 

10,6 

73,s 

9mo 

3.0 

2m3 

oe6 

0.6 

0 . 2 

0.3 

+oom~+oooo+oo~o+~m~~+ 

IXXX 

IXXXXXXXXXXXXXXXXXX 

IXX 

IX 

IX 

I 

I 

I 

I 

+~00D+~000+0m00+0~~I+ a a n m . 20,O 40.0 60,O 80-0 

PERCENTAGE 

Figure 6. Bar Graph for Variable: PKUP 
PKU? = Yeasured ~alue/~'x~ected Free Space Measured Value 

*I*** BAR GRAPH FJR VARIABLE: iKUP **ltr 

RANGE OF VALUES FREQ PCENT t----+----+----+----+ 

.lOOOE+OO- ,3580 1 001 I 

,3580 - ,6160 20 1.1 IX 

,6160 - .a740 42 2.4IX 

0740 1,132 272 15.6 IXXXXXXXX 

1,132 - 1.390 347 19.9 IXXXXXXXXXX 

1,390 1.648 518 29.7 IXXXXXXXXXXXXXXX 

1.648 - 1,906 233 13.4 IXXXXXXX 

1,906 ' 2.164 61 3.5 IXX 

2,164 2.422 52 3,OIX 

2,422 2.680 35 200 IX 

2,680 2,938 32 1.8 IX 

2,938 - 3.196 17 1.0 I 

3.196 - 3.454 18 1.0 IX 

3.454 - 3,712 16 0.9 I 

32 - 3.970 8 005 1 

3,970 4,228 14 0.8 1 

4,228 4.486 8 005 I 

4,486 - 40744 8 0.5 1 

4.744 - 5,002 13 0.7 I 

5.002 5.0 260 0 0.0 I 

5.260 - 5.518 6 Om3 I 

5.518 5.776 1 0.1 I 

5.776 - 6,034 3 0.2 I 

6,034 6.292 5 0.3 I 

6,292 - 6,550 0 0.0 I 

6,550 6.808 1 0.1 I 

6,808 - 7.066 4 0.2 I 

7.066 - 7.324 1 Oei I 

7.324 7.582 2 0.1 I 

7,582 - 7.840 0 0,O I 

7,840 - 8.098 1 001 I 

8,098 - 8,356 1 0.1 I 

8,356 8,614 0 0.0 I 

8,614 8,872 0. 0.0 I 

8,872 9,130 1 001 I 

9.230 - 9.3aa o 0.0 I 

9,388 9,646 0 OeO I 

9,646 9.904 0 OeO I 

9.904 - 10,16 0 0.0 I 

10.16 - 10.42 0 0.0 I 

10.42 - 10.68 2 Om1 I 

10.68 - 10,94 0 0,O I 

10.94 11.19 0 000 I 

11.19 - 11.45 0 000 I 

11.45 11,71 0 000 I 

11.71 - 11.97 0 000 I 

11.97 - 12.23 0 Om0 I 

12.23 12-48 0 000 1 

12.48 - 12m74 0 000 I 

12.74 13.00 1 0.1 I --m- +----+----+----+----+ 

1744 a a a a 

10.0 20,O 30,O 40-0 

PERCENTAGE 

Figure 7. Bar Graph for Variable: PKUP 

40 
TABLE 6. - Normalized Pickup for All Mines 

There are 1 variable and 1744 observations. 

Var. Means Std. Dev. 

Pkup 1.696101 1.010665 

Var. Median Mode 

Pkup 1.400000 1.400000 

Var. Std Err of Mean Skewness 

Pkup 0.24202043-01 3.968134 

Variance 

1.021444 

Maximum Minimum 

13.00000 0.1000000E+00 

Coef. of Var. 

59.58756 

TABLE 7. - ~atio of Normalized Loop Data to Computed Value 

for the Data Normalized in Frequency and Drive Current 

to 1 MHz and 10.0 Amps 

For All Loop Data 

Pickup Ratio 

Max Min Avg . 13.0 .1 1.7 

No. of Items Sigma 

1744 1.0107 
(text continued from page 31) 

4.2 TRANSMISSION LINE RESULTS 

We have processed the transmission line (long line simulator) data in 

much the same manner as the loop data. In order to compare data at different 

mines the nearby frequencies are first grouped, e.g. 0.175, 0.74, 0.72 MHz, 

all go to 0.170 MHz, then all pickup data is normalized to a line current of 

10 amperes. The data are all also normalized to a pickup for 1.0 MHz. The 

resulting data is plotted in Appendix G. Note that plots are presented for 

both 2 = 0.5 and 2 = 1.0 meters. This allows a comparison of the spread in 

the data for both mine location and 2 distance. 

We next computed the free space pickup from a 10 amp., 1 MHz current. 

This computation was performed by a specialized computer program. The results 

are given in Table 8. The computer program only computes the magnitude and 

direction of the H field. Pick up for the 10 cm diameter search loop was 

computed from: 

Pickup (volts) = A w IH ( 

where A = area of the pickup coil (radius 0.05 m) 

w = 2n frequency = 2n x lo6 for f = 106 

p = 4n x h/m IH I = magnitude of the H field linking the search coil 

Appendix H gives the computer program output. 

Using the calculated pickup and the current normalized data we can 

compare the measured results to the calculated free space values. The 

comparison is given for each measurement in Appendix I. Table 9 breaks down 

the data by location and measurement point. Table 10 gives the basic 

statistics for all the transmission line measured/calculated ratios and 

Figures 8 and 9 are bar graphs of the distribution. 

Note that there are only 4 of the 128 ratios more than 3 (about 3.2%) and 

of these, 2 occur at location 7 and 2 at location 8. Study of the notebook 

entries -- and Appendix C -- shows that all three values greater than 4 are 

associated with a measurement at 170 or 500 KHz at a point 0.5 meters from the 

plane of the rectangular loop and 1 meter from the floor. This measurement 

point is the one nearest to the driven wires of the loop. The field varies 

more rapidly at this point than at any other mesurement point. Errors in 

pickup measurement due to pickup loop location uncertainty would be expected 

to be maximum at this point. 

(text continued on page 48) 
TABLE 8. - Free Space H Field and Pickup From Transmission Line 

Wire Configuration -- 10 Amps at 1 MHz 

z 

Meters 

Y H 

Meters Amps/Meter s 

Pickup 

- (mV) 
TABLE 9. - For Transmission Line Data 

All Data Normalized to 10 Amps and 1 MHz 

RAT10 OF MEAS[JFED/CALCULATED FOR DATA 

LOCATION MIN MAX AV(; 8 ITEMS 

RATIO OF MEASUWED/CAI,CULATED FOR ALL DATA 

UIN MAX AVG I, ITEMS 

,591 5,104 lo361 128 

RATIO OF HEASURED/CALCULATED FOR DATA 

FOR ALL Y=.5 DATA 

MIN MAX AVG UITEhS 

,591 5,104 1,725 52 

RATIO OF 'fEASURED/CALCULATED FOR DATA 

FOR ALL Y=l.O DATA 

MIN MAX AVC I ITEKS 

,620 2,231 1 167 46 

RATIO OF YEASURED/CALCULATED FOR DATA 

FOR ALL Y=1.5 DATA 

MIN MAX AVG I ITEMS 

,697 1,581 1,026 30 

RATIO OF MEASURED/CALCULATED FOR DATA 

FOR ALL 2=0.5 DATA 

MIN MAX AVG I ITEMS 

.591 1,914 ,980 64 

RATIO OF ~EASURED/CALCULATED FOR DATA 

FOR ALL 2=1.0 DATA 

MIN H AX AVG I ITEMS 

.731 5,104 1,741 64 
I TABLE 10. - Transmission Line Data Basic Statistics 1 

l~ransmission Line - Measured/Calculated 

 h here are 1 variable and 128 observations. 

Var. Means Std. Dev. Variance 

p*UP 1.360641 0.7601704 0.5778590 

Var. Median Mode Maximum Minimum 

1.116000 0.6200000 5.104000 0.591000 

More than 1 mode exists - only the first is shown. 

Var. Std Err of Mean Skewness Coef. of Var. 

P~UP 0.6719020E-01 2.552539 55.86856 
PKUP = Measured Value/Expected Free Space Measured Value 

+momm+----+-m--+----+ 

J 

IX~XXXXXXX~XXXXXXXXX 

lXXXXXXXXXXXXXXXAXX 

lXXXXXXX 

IXXX 

IXX 

I 

1u.t' 2u.O 30,G 40,') 

PERCENTAGE 

Figure 8. Bar Graph of Measured/Calculated 

Ratio Distribution-Coarse Division 
PKUP = Measured Value/~xpected Free Space Measured Value 

XMISSION LINE - MEASURED/CALCIILATED 

***** BAR GRAPH FOR VARIAULE: PKUP *re** 

RANGE OF VALUES FREQ PCEtiT +----+----+---a+----+ 

.S~IO - .6a13 s 3.9 JXXXX 

e6813 ,7715 6 4.7 IXXXXX 

07715 ,8618 19 14.8 IXXXXXXXXXXXXXXX 

08618 - ,9520 11 8.6 IXXXXXXXXX 

m9520 1,042 . 14 10.9 IXXXXXXXKXXX 

1.042 - 1.133 12 9.4 IXXXXXXXXX 

1.133 1,223 8 6.3 IXXXXXX 

1,223 - 1,313 7 5.5 IXXXXX 

1,313 1,403 7, 5.5 IXXXXX 

1,403 - 1,494 5 3.9 IXXXX 

1,494 - 1,583 3 2.3 IXX 

1,584 - 1,574 0 0.0 I 

1,674 a 1,764 5 3.9 IXXXX 

1,764 - 1,955 4 3.1 IXXX 

1,855 - 1,945 2 1.6 IXX 

1,945 - 2,035 5 3.9 IXXXX 

2,035 - 2,125 2 1.6 IXX 

2,125 - 2.216 0 0.0 I 

2,216 - 2,306 3 '2.3 IXX 

2,306 - 2,396 1 0.8 Ix 

2.336 - 2.486 1 0.8 IX 

2.486 - 2,577 0 0.0 I 

2,577 - 2,667 1 0.8 IX 

2,667 - 2,757 1 0.8 IX 

2,757 - 2,847 0 0.0 I 

2,847 - 2,938 1 0,8 IX 

2,938 - 3,OZd 1 0.8 IX 

3,028 - 3,llr3 0 0.0 I 

3,118 - 3,2GY 0 0.0 I 

3,209 - 3,299 0 0.0 I 

3,299 - 3,389 1 0.8 IX 

3,3s9 - 3,479 0 0.0 I 

3,479 - 3,570 0 0.0 I 

3,570 - 3,660 0 0.0 I 

3.660 - 3,750 0 0.0 I 

3,750 - 3,840 0 0.0 I 

3,840 - 3.931 0 0.0 I 

3,931 - 4,021 0 0.0 I 

4,021 a 4.111 0 0.0 I 

4,111 - 4.201 0 0.0 I 

4.201 4,292 0 0.0 I 

4,292 - 4,382 0 0.0 I 

4,382 - 4,472 0 0.0 I 

4,472 - 4,562 1 0.8 IX 

4,562 - 5,653 0 0.0 I 

4,653 - 4,743 1 0.8 IX 

4,743 - 4,833 0 0.0 I 

4,833 - 4,923 0 0.0 I 

4,923 5,014 0 0.0 I 

5.014 5,104 1 0.8 IX ---- +~~~-+----+----+----+ 

128 n n L L 

5.0 1000 15.0 20.0 

PEXCENTAGE 

Figure 9. Bar Graph of Measured/~alculated 

Ratio Distribution - Fine Division 
(text continued from page 42) 

5. CONCLUSIONS FROM MEASUREMENTS 

Study of our data indicates that, for frequencies of 10 MHz and below, an 

adequate approximation for the magnitude of the magnetic field, for use in 

blasting safe distances, can be computed from the free space predictions of 

the magnetic field if we use a multiplying factor of 5. 

Only one of our transmission line measurements showed ratios as high as 5 

and of the 1.7% of our loop measurements that were greater than 5, most seem 

to be associated with measurements taken very close to conductors or in -- or 

close to -- our noise level for the measurement. Further, those ratios 

greater than 5 that were not associated with measurement near conductors or 

close to the noise level are isolated, i.e. the neighboring values are not 

greater than five. This indicates a "hot spot" of small extent or errors in 

the measurements. For use in the calculation of blasting safe distances, we 

speculate that "hot spotsn of very localized extent will not contribute unduly 

to the overall pickup and they can be safely ignored. 

This is, quite clearly, not a worst case assumption. We believe that 

existence and extent of "hot spotsn should be investigated experimentally with 

equipment having greater sensitivity and lower noise levels than that used in 

the work described in this report. A battery powered, well shielded, wide 

band (up to 15 MHz) amplifier used before the scope would probably solve the 

problem. We feel that such a program would show that any "hot spotsn are of 

very localized nature and thus a factor of 5 can be safely used in prediction 

of hazard. If not, a higher multiplier can be used to modify the safe 

distances derived in this report. 
6.0 SAFE DISTANCES FROM SOURCES 

6.1 CRITICAL VALUES OF THE FIELD 

In order to evaluate safe distances for blasting wire from transmission 

line and loop sources we assume -- as a worst case estimate -- that the 

shorted loop of blasting wiring is arranged normal to the B field produced by 

the source and that it is formed as a circle of area A square meters. This 

assures maximum area for a given lead length. Further, we assume that the B 

linking the loop is the maximum B anywhere in the loop, Thus, if the edge 

of the loop is at a given maximum of the source field and the field is less at 

other portions of the loop, we will use the maximum value of B everywhere in 

the loop. This is a worst case assumption, We further assume that the 

blasting wiring loop is loaded with only one cap and that its resistance is 

Rdc and Rdc equals 1 ohm. This, too, is a worst case assumption in that 

more resistance lessens power pickup and the nominal resistance of American 

made caps is 1.2 ohms or greater (Reference 7, p. 4-3). This is, of course, 

true only for low frequencies where the radiation resistance of the pickup 

antenna is much less than one ohm. That is the case here considered. 

The maximum open circuit voltage included in the blasting wiring is then 

d 

V,, = A - 1~~ 1 cos wt volts at 

where w = ZnefMHz x 10 6 

f~~z = frequency in megahertz 

IB, 1 = the maximum B field magnitude linking the loop (maxwells). 

Note that we will use this notation throughout this report, 

i.e. IAI 

is read as "magnitude of the vector (or phasor) A*" 

2 A = area of the blasting cap (m ) 

Vb = open circuit voltage (volts) 

In terms of the H field (B = pH) 

where p = 4n x lo-' henryslmeter, and 

IH I = maximum IH I linking the loop - amps/meter. 
The total current IB that will flow in the blasting wiring loop is 

where Zb is the total impedance of the blasting wiring loop. This impedance 

includes Rdc -- the blasting cap resistance -- any reactance of the cap, any 

internal drop of the wire forming the loop and the external inductance for the 

loop. The external inductance term is of considerable importance for our 

application. It represents the reduction in current flow due to the reduction 

of the linking B field by the field produced by the current in the loop. 

Reference 8, Chapter 5, gives an excellent outline of these points. We 

recommend that the interested reader review this chapter if difficulty is 

encountered in the following exposition. 

Thu sf 

where Zi is the internal impedance of the blasting wire forming the loop, 

Lo is the external inductance of the loop (henrys) and, 

Xcap is the reactance of the cap (ohms) . 

We will assume that the blasting wire that forms the pickup loop is 20 

gauge (B & W) copper wire. Caps are usually supplied in 22 gauge for short 

leads (30 feet or less) and 20 gauge for longer wires. The assumption of 20 

gauge wire results in a tiny bit more pickup than the 22 gauge wire 

assumption. Thus, using the 20 gauge wire in the calculations is 

conservative. 20 gauge wire has a diameter of 31.96 mils (0.8118 millimeters) 

and a dc resistance of 10.15 ohms/1000 feet. Figure 10 is from reference 8, 

p. 297. It shows the variation of the internal impedance terms for round 

wires in terms of Ro (the dc resistance) , ro (the radius of the wire) 

and 6 (the skin depth). Figure 11 gives the same data in a more accessible 

way for the internal resistance term. In this figure is resistivity in 

relation to copper and p~ is permeability in relation to free space. 

Using these figures, we compute that 

where fMHz is frequency in megahertz. 

For a circle of this wire of area A square meters, 
2, Rotio of rodius to depth of penetrotion 8 

Figure 10. Solid Wire Skin Effect Quantities 

Compared with d-c Values 
I V 

10- 10-I loo 10' lo2 lo3 lo4 lo5 

FREQUENCY (Mc) 

Figure 11. (R/RDC - 0.25) vs. Frequency for A 

Circular wire PR = UR = 1 
since the circumference equals 2T times the radius and the radius is equal to 

the square root of the area divided by T. 

The Xca term is of the form wLC, where LC is the external 

inductance of the small rectangular loop (approximately lmm x 1 cm) formed 

where the cap leads enter the base of the cap through the cap plug. The 

external inductance for a circular loop of the same area can be evaluated 

using (see reference 8, page 311). 

henrys, 

where ro is the radius of the loop (m), V= 4T x (h/m) and a, is the 

radius of the wire (m). Since the area of our loop is x lo-') = 

10-5 m2, a, = 8.1 x 10-4m and r = JAT for a circle, we compute 

LC = 0.00196 microhenrys 

The Lo term of (6-4) is evaluated in the same way from equation 8; we obtain 

k micr ohenrys 

0.8118 10'~~ 

We can now write equation (5) as 

=ii = Rdc 0.016 + 0.204 (l+j) 

MHz 
At frequencies as high as 10 MHz and areas as large as 10 square meters, 

all of the terms can have magnitudes comparable to Rdc = 1 so we will retain 

them all in our calculations. 

The no-fire power level (0.1% probability with 95% confidence) for 

American made caps is 0.04 watts (see Ref. 6). For a 1 ohm cap this gives a 

no-fire current level of 0.2 amperes. If we substitute this value for current 

in equation (4) we form an expression for IH I that will just produce the 

no-fire current in the cap. Thus 

where here we have written lHc 1 to call attention to the face that this is 

the critical value of IHI and we have indicated that the magnitude of the 

impedance Zb is to be used. Note that Zb is given completely in terms of 

frequency and area (A) of the blasting cap wiring. We have used a cpmputer 

program to evaluate equation (12). Table 11 lists the values of IHC I for 

frequencies up to 10 MHz and areas up to 10 square meters. Table 12 shows 

iHC Ifor a finer gradation of frequencies for the 0.1 to 1 MHz range. 

Note that Tables 11 and 12 also give values for the Lo term of equation 

(9) (the column labeled microhenrys) and the ratio of the external reactance 

(ao) to the dc resistance term (the column labeled X/RDC). RDC was assumed 

equal to 1 ohm. 

If we can now calculate the free space values of (HI due to various 

sources, we can determine safe distances for the blasting wiring 

configurations. 

6.2 FREE SPACE VALUES OF THE FIELD FROM VARIOUS SOURCES 

Our main concern is with small loop antennas such as those used in "back 

pack" applications and bandolier configurations. Also we are interested in 

the typical "trolley wire" communication systems. 

(text continued on page 63) 

54 
TABLE 11. - Frequencies from 0.01 to 10 MHz 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 

k C!F 1)ICf.llt' AHEk= 0,00031b SO, b3ETEi23 
TABLE 11. - Frequencies from 0.01 to 10 MHz (Cont.) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 
TABLE 11. - Frequencies from 0.01 to 10 MHz (Cont.) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=l OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 

ECH FlCkllF AREA= 10,000000 Sy, fdE+l'El!b 
TABLE 12. - Frequencies from 0.1 to 1 MHz 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 
TABLE 12. - Frequencies from 0.1 to 1 MHz (Cant.) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=l OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 

FUR lJICKIIF' ALEA= 0.0031b2 SO. YETENS 
TABLE 12. - Frequencies from 0.1 to 1 MHz (Cant.) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 
TABLE 12. - Frequencies from 0.1 to 1 MHz (Cont.) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 
TABLE 12. - Frequencies from 0.1 to 1 MHz (Cont. ) 

VALUES OF HAZARD CRITICAL H FIELD FOR VARIOUS CONDITIONS 

RDC=1 OHM, CRITICAL HAZARD CURRENT = 0.2 AMPS 
(text continued from page 54) 

For the trolley wire configuration, we consider the geometry of Figure 

12. Figures 13 through 17 give, for a current of 1 ampere, the magnitude of 

the H field along various lines indicated in Figure 12. Figures 18 through 20 

give the same sort of data for the lines indicated in Figure 21. Study of 

these plots shows that the magnitude of the H field ( IH I in aim) around the 2 

meter separation transmission line varies from about 0.318,half way between 

the driven lines, to about 0.15 at 1 meter away along the y axis. Figure 22 

shows some of the values. For the 0.5 meter radius loop IH I varies as shown 

by the points in Figure 23. For the moment we can note that IH I = 0.32 a/m 

for a 1 meter separation from the transmission line current carriers and IH I = 

0.1 a/m for a 1 meter separation from the loop source. We will use these 

values later. 

6.3 SAFE DISTANCES FROM CARRIER PHONE LINE CONDUCTORS 

A. D. Little's Mr. Robert LaGace has supplied us with data on mine 

trolley carrier phones -- see Appendix J -- that indicates that the carrier 

frequencies are no more than 0.190 MHz and the maximum short circuit current 

for the transmitters -- and therefore the maximum current on the trolley line 

-- is approximately 2.6 amperes MS. 

For a carrier phone operating at the worst-highest-frequency, say 200 

KHz, and running into a shorted line with the short located precisely right -- 

about a half wavelength away (a distance somewhat smaller than 3000 meters or 

about 2 miles) with no loss in the lines, then the maximum current close to 

the end of the line near the short will be 2.6 amperes. The values of H 

given in Figure 22 multiplied by 2.6 and again multiplied by our correction 

factor of 5 (to account for the difference between free space and the mine 

environment) indicate that the maximum H field one meter from either 

transmission conductor line would be 4.16 or less. Indeed, since the return 

would be split through the tracks, the 4.16 a/m figure would be high for a 1 

meter separation from either rail. Note, however, that even if one of the 

rails carried none of the return current, the 4.16 a/m value would still apply. 

If we consult Table 12, we see that a blasting wiring loop of area 0.0316 

square about 17.8 cms (about 8 inches) on a side or a circular loop formed of 

63 cm (about 2 feet) of wire. 

With the "worst case" presently existing carrier phone and a loop of this 

size at a point one meter from the carrier phone conductor wire we would still 

need to orient the loop correctly to receive maximum power. At that 

orientation we assume 

(text continued on page 76) 
figure 12. h-olley Wire Coordinate System for H Field Sampling 
ALONG X= 0.00 Y= 0.00 Z= 1.00 TO X= 0.m Y= 3,- Z= f.00 

TROLLEY WIRE, 2 M SEP,, 1 A, 1 MHz 

DISTANCE ALONG Tti€ LINE CMEYERS) 

Figure 13 

ALONG Xu 0.00 Y= 0.25 Z= 0.00 TO X= 0.00 Y= 0.25 Z== 3.00 

TROLLEY WIRE, 1 A, 1 MHz, 2 METERS SEPARATION 

Figure 15 
ALONG X- 0.00 Y= 0.00 Z- 0.0'5 TO X= 0.00 Y= 0.m Z= 1.95 

TROLLEY WIRE, 2 M, SEP., 1 A, 1 MHz 

Figure 16 

ALONG X= 0.00 y= 0.00 Z= 0.00 TO X= 3.00 Y= 0.00 Z= 3.00 

LOOP, RADIUS= 0.5 METERS, 1 A, I MHz 

Figure 18 


The loop is in the x,y plane, centered at the origin. 

Figure 21. Loop Coordinate System for H Field Sampling 
2 

meters 

b' TRANSF 

1 X 1 meter 

JllSSlON LINE WIRES 

Figure 22, H Values at Various Points Around the Transmission Line 
H Values at various points 

from the 0.5 M Radius Loop 

LOOP CONDUCTORS 

Figure 23. H Values at Various Points from the 0.5 M 

Radius Loop 
(text continued from page 63) 

that the "no fire" current -- the current that will function the cap with 0.1% 

probability at 95% confidence, i.e. 1 in 1000, 95% of the time, -- will flow 

in the cap. Our analysis has "worst cased" several other factors however; we 

assume that the maximum B field can be normal to our loop over its total 

area of the blasting wiring; we assume that the cap lead wires are only as 

long as the loop periphery -- they are in general longer and will tend to 

reduce the current due to inductance and resistance -- in addition and most 

important, we assume that our correction factor of 5 applies at all times and 

all places in the mines. 

Each of these factors, however, cannot be shown to be improbable. At 

some time they may all occur naturally together. So our overall result is 

that blasting cap wirinq with areas of qreater than 0.0316 sq. meters -- a 

square about 8 inches on a side -- should not be brought closer than 1 meter 

to any trolley phone wire or track return. Also note that this figure is 

dependent on a study of presently existing equipment. If differing equipments 

that produce more current are developed in the future, corrections in this 

predicted figure must be made. 

Lest we convey the idea that our assumptions are so "worst case" as to be 

restrictive of cap use, consider a cap with six foot leg wires and the short 

still on the wires. If the leg wires were pulled apart in the middle forming 

an approximate 1 sq. meter loop and this loop were placed coplaner (plane of 

the loop in the plane of the carrier phone conductors) with the conductors of 

a n.ormally operating carrier phone at 200 KHz with about 1.32 amps in the 

conductors, we can see from Figure 15 that the H field would be more than 0.3 

x 1.32 = 0.4 amp/meter over the loop assuming no correction factor of 5. 

Table 12 shows the Hc for this area to be about 0.85 a/m. So we would be 

about a factor or two from the "no fire" level even if we were in free space. 

It is also wise to consider that the spread in currents for blasting caps from 

the "no fire" level to the "all fire" level -- 99.9% probability with 95% 

confidence -- is about 1.2 to 1.4, e.g. 1.25 times the "no firen current level 

for at least one American-made cap gives the "all fire." So the total safety 

factor separating us from a "sure fire" would be about 2 x 1.2 = 2.4. Study 

of the data taken in our mine measurements shows that, under some conditions, 

"correction factors" of much greater than 2.4 have been measured. This would 

convince me to leave the area quickly. 

6.4 SAFE DISTANCES FROM LOOP SOURCES 

Study of Figures 18 and 20 shows that the magnetic field of the loop 

along the loop's axis is larger -- at a given distance from the center of the 

loop -- than the magnetic field in any other direction unless we get very 

close to the current carrying conductors. The field variation along the axis 

has a particularly simple form. 
where Ha is the H field on the axis (amps/meter), 

Z is distance (meters) from the center of the loop, 

R, is the loop radius in meters, and 

I is the loop current in ampere-turns. 

For Z >> R 

2 IR~ IW IA, 

where A is the area of the loop in square meters. 

If we define the magnetic moment of the loop as 

Mm = IA, 

where M, is the magnetic moment in amps-meters2, 

then for Z >> R 

m H 'La= 3 a /m 

2~z 

This definition allows us to treat small loop antenna sources without 

regard to the geometrical differences and actual number of turns. Note that 

equation (15) is valid only for Z >>R . How much bigger is "much, much" 

bigger. Figure 24 plots H field variation for a R, = 0.5 meter loop along the 

Z axis. On this "log log" plot the slope of -3 at distances such that 

Z > 2R is readily apparent. Now, for equations of the form 

log y = log c + q log x and 

d (log y) =q. 

d (log x) 

Thus the slope on "log log" paper gives the value of the exponent in the 

equation. Since the slope of Figure 24 is -3 for Z >2R we know that the form 

of the equation in that range conforms to the approximation equation (15). We 

therefore write, with only a very small error 

M 

m H 2- 

a- 3 a/m for Z > 2R 

2rZ 

To determine safe distances from loops we need only compare 5.0 times 

Hz in (6-15) to the l~~ 1 values of Figures I1 and 12 for various values of 

blasting wiring loops. 

Mr. Larry Stolarczyk of the ARF Company of Raton, New Mexico, has 

informed us, by private communication, that the "back pack" and vehicular 

communication systems, that they manufacture for underground use, operate at 

frequencies of less than 400 KHz and that the magnetic moments of the antennas 

are 1.5 ampere turns meter2 for the "back packn system and 11.09 (6.3 x 11 x 

0.16) for the vehicular system. Table 13 shows the results of substituting 

these values for Mm in 

M (5.0) 

Hc (A) 1 - m 

3 a/m 

f=400 KHz 2n Z 

and solving for 2. Note that use of this equation overestimates Ha for 

values of Z < 2R - unless we approach a current carrying conductors very 

closely. Thus, the equation gives a conservative estimate of safe distance 

for distances less than 2R for all cases of concern. The values of 

Hc (A) 1 are taken from Table 12. Note that we have included the 

f=400 KHz 

"correction factor" of 5 in Eq. (17). 

~ransmitting systems of this type are usually tuned for peak current 

and any malfunction of the system results in less current flowing in the 

antenna than the design value. We therefore feel that the above distances are 

valid even for presently existing equipment malfunction conditions. 

TABLE 13. - Values for Safe Distance (2) for ARF Transmitter Antennas 

Safe Distance (2) 

(meters) 

Vehicular "Back Pack" 

Blasting Wire 

Area (Sq. m) 

System 

Mm=ll. 088 

System 

Mm=l. 5 

IHC I 

amp/me te r 
Note that a blasting wiring configuration having a 10 square meter area 

is large indeed. This is the area of a typical blasting face layout -- the 

maximum we would expect to see underground. Table 13 shows that we would be 

safe -- even for 10 sq. meters of blasting wiring loop area -- if such 

portable transmitter equipment is always kept about 3 meters from any blasting 

wiring. This is a convenient number in that any electronic or electrical 

equpipment should always be kept five (about 1.55 meters) or ten feet (about 

3.06 meters) from any portion of a blasting wiring layout to eliminate 

potential premature firings due to dropping the equipment on the wiring or due 

to a man carrying (body mounted) equipment falling on or into the wiring and 

having the equipment come in contact with the blasting wiring. 

Safe distances from other underground loop sources such as "bandolier" 

antennas can be calculated by the same methods as used above if the frequency 

exceeds 400 KC and/or the magnetic mount exceeds 11.09 ampturns-meters 

squared. 

6.5 SUMMARY OF SAFE DISTANCES AND COMMENT 

Our derivations of safe distance have been of a worst case nature with 

the following exceptions: 

We have used data for resently existing trolley phone equipments. 

We have used a correction factor of five for free space to underground 

magnetic fields. This factor needs investigation by further 

measurement. 

Section 6.3 recommends that blasting wiring with areas of greater than 

0.0316 square meters -- a square about 8 inches on a side or a circle formed 

from 2 feet of wire -- not be brought closer than 1 meter to any carrier phone 

wire or track return. Certainly no blasting wiring should approach even this 

close to a "hot" dc trolley wire. 

This recommendation can be implemented most easily by prohibiting any 

blasting operation in a haulage way containing an operating carrier phone. 

The recommendation could allow, however, a shot line from a remote blasting 

wiring configuration to be run into (or along) a haulage way that contains an 

operating carrier phone. Thus firing could be done from the haulage way if 

care is taken to keep the shot line conductors close together so that any 

shorted loop is less than 64 sq. inches in area. 

Section 6.4 recommends that the "back pack" and vehicular antennas 

(operating at less than 400 KHz) made by ARF' be kept respectively 5 feet and 

10 feet from blasting wiring of 10 square meters area to preclude RF hazard. 

In essence this recommendation says that as long as these antennas are 

restrained from being within 10 feet of any blasting wiring configuration we 

eliminate the possibility of RF hazard. Since the hazard separation distance 

would be smaller for smaller areas, one might conclude that smaller 

separations are desirable for smaller blasting wiring areas. We do not 

believe so. Some minimum distance should be provided to eliminate accidental 

contact between the blasting wires and the transmitting equipment. 

80 
Note that we have considered here only the shorted blasting wire 

configuration. For frequencies and sources below 10 MHz at the distances 

considered, the entire possibility of RF hazards to blasting caps is 

eliminated by keeping the blasting circuit open. We do not recommend this 

procedure however. Caps are shipped shorted with the leg wires configured so 

as to form a minimum coupling area. Good blasting practice -- dictated by 

consideration of electrostatic hazards and stray currents -- requires that 

blasting circuits be kept shorted. We agree. 

Also note that we have considered only the pin-to-pin firing mode of the 

caps. American-made caps require very high voltages to function in the 

pin-to-case case mode for the frequency range up to 10 MHz. The open circuit 

voltages produced by the sources considered in the blasting wiring pin-to-case 

loops is extremely small for the separations recommended and thus safety for 

the pin-to-pin mode insures -- for the conditions considered here -- safety 

for the pin-to-case mode. To eliminate confusion we note that other 

references, even ours, use differing notations for pin-to-case mode firing. 

It is sometimes written pins to case, pins-to-case or pin to case; they all 

refer to the same phenomena although a real difference can be involved in 

devices that "arc over" from one pin (or lead) to case when the other pine (or 

lead) is raised to a high potential in relation to the case. In this rare 

instance a device can be "safe" for a given potential in the pins-to-case mode 

and "unsafe" in the pin-to-case made for the same potential. The actual mode 

of firing in this rare, although possible, case is pin-to-pin. 
7.0 CONCLUSIONS 

Our overall recommendations for blasting wiring safe distances from loop 

or long straight wire current sources are given in detail in Section 6 of this 

report. They are assumed valid for frequencies up to 10 MHz. 

It should be carefully noted that the recommended distances are based on 

measurements performed in three locations in each of three coal mines. 

Although the locations and mines were selected to give a wide diversity of 

electrical characteristics, it is quite obvious that they can not be expected 

to give a statistically valid sample of - all coal mines. 

Further, if the safe blasting distances derived herein are to be applied 

to "hard rock" or metal mines we are even further from a valid statistical 

sample. We remark in passing, that we would expect the derived safe distances 

to increase somewhat in metal rich surroundings and decrease a bit in 

conductor poor mines. 

A further limitation on the safe distances presented is that they are 

based on presently used mine equipment. If newly designed equipment is 

introduced it may change required safe distances. 

Another important point in the validity of the safe distances is our use 

of a multiplying factor of 5 for the mine to "free spacen magnetic field. As 

discussed in Section 5.0, this is not a worst case assumption. We had 29 of 

our 1744 (approximately 1.7%) measured values exceed 5. We attribute most of 

these to measurements near our noise level, measurements very close to 

conductors [and very limited in extent] or errors in measurement. There are, 

however, some measurements we feel were valid that did exceed a factor of 5 

over the calculated free space value. We think that these "hot spots" are of 

very limited spacial extent and they would thus not make significant 

differences in our safe distance calculations. We do not know. 

We recommend that further measurements of the type made here be performed 

in several more coal mines and, if non-coal mines are to be considered, 

several metal/non-metal mines. The measurement apparatus should incorporate 

more gain to alleviate our small signal problem and the measurement procedure 

should be changed somewhat. Tables of values of the calculated free space 

pickup multiplied by 5 for each f requency/source can easily be prepared using 

the procedures given in Sections 6.2 and 6.3. We recommend that tables of 

this type be used during the measurements and any measured pickup in excess of 

5 be closely investigated for its area of extent. 

The measurement equipment is already in existence, the measurement 

procedures are documented, the software for reduction of the data has already 

been developed and checked out. Thus, the additional measurements and data 

reduction can be carried out quickly with relatively small expense. 

The results of such additional measurements will result in a higher 

multiplying factor or confirm our recommended value of 5. The safe distance 

calculating procedure is structured so that a change of the multiplying factor 

is easy to incorporate. 

82 
8.0 REFERENCES I 

"Investigations of the RF Hazards to Electric Blasting Caps," FIRL 

Technical Report F-B2256, R. H. Thompson, P. F. Mohrbach, et al., 

prepared for 'the Institute of Makers of Explosives, 1965. 

"RF Pickup of Antenna Simulating Blasting Wiring Configurations, 

Measurement Results," FIRL Technical Report F-B2256-1, R. H. Thompson, 

prepared for the Institute of Makers of Explosives, 1966. 

"Measurements of the RF Coupling Between an Antenna Simulating a Blasting 

Wiring Configuration and Nearly Mobile Transmitting Antenna," FIRL 

Technical Report F-C1951, R. H. Thompson and K. Fertner, prepared for the 

Institute of Makers of Explosives, 1967. 

"The Possible RF Hazard to Blasting Caps Caused by a Lossy Communication 

System," FIRG Technical Report F-C2399, R. H. Thompson and P. F. 

Mohrbach, prepared for Motorola, Inc., 1969. 

"RF Pickup of Blasting Face Wiring," FIRL Technical Report F-C2942, R. H. 

Thompson, prepared for Motorola, Inc., 1970. 

"Evaluation and Determination of Sensitivity and Electromagnetic 

Interaction of Commercial Blasting Caps," FIRL Technical Report F-C3102, 

H0210068, August, 1973. 

"FU? Hazards to Blasting Caps in Coal Mines," FIRL Technical Report 

F-C4075, R. H. Thompson, prepared for U.S. Bureau of Mines under Contract 

H0252015, October, 1976. 

"Fields and Waves in Communication Electronics," Ramo, Whinnery & Van 

Duzer, John Wiley & Sons, 1965. 

APPENDIX A 

MONOGRAPH ON 

COMPUTATION OF RF HAZARDS 
Monograph M-C2210-1 W. B. Ligett, Vice President 

"Computation of RF Hazards" Director of Laboratories 

Paul F. Mohrbach, Ramie H. Thompson, 

Robert F. Wood, Daniel J. llullen 

The Franklin Institute Research Laboratories 

July, 1968 

for National Aeronautics and Space 

Administration/~oddard Space Flight Center 

ABSTRACT 

This monograph presents a method for analyzing the potential 

RF susceptibility to the electrical components and systems used in typical 

space vehciles. It presents the philosophy, applicability and limitattions of this approach. While not exhaustive, enough mathematics is 

presented to permit analysis of a very large percentage of the types of 

problems which normally occur. Where the actual development of equations 

is not given in detail, suitable references are provided. Familiarization 

wj-th the test and the cited references should provide the reader with the 

necessary information to analyze most systems and the general procedures 

to handle those situations which are beyond the scope of this monograph. 

ACKNOWLEDGEMENTS 

This monograph is based on work performed and techniques and 

ideas developed by the Applied Physics Laboratory, P. F. Mohrbach, Principal Scientist. Inquiries concerning this work should be referred to 

him at Area Code 215-448-1236 or to the sponsoring agency. 
TABLE OF CONTENTS 

ABSTRACT ........................... i 

LIST OF SYMBOLS ....................... v 

1 . INTRODUCTION ......................... 1-1 

1.1 Approach ........................ 1-5 

2 . DETAILED ANALYSIS PROCEDURES ................. 2.1 EED Disconnected .................... 2.1.1 Multipin Connector Type ............. 2.1.2 Wire Lead Type .................. 2.1 -2.1 Unshorted Wi re Lead Type 

(Pi n-to-Pin) .............. 2.1.2.2 Shorted Wire Lead Type 

(Pin-to-Pin) ............. 2.1 .2.3 Wi re Lead Type (Pi ns-to-Case) ..... 2.1.2.4 Wire Lead Type(High Frequency 

Calculations .............. 2.2 EED Connected ...................... 2.2.1 Circuits with Shielding Gaps ........... 2.2.2 Completely Shielded Ci rcui ts ........... 2.2.2.1 Calculation of the Maximum Power 

Density at the Outer Surface of the 

Braided Shield ............. 2.2.2.2 Calculation of the Power Density 

at the Inner Surface of the 

Braided Shield ............. 

3 . EXAMPLEOFAN EVALUATION ................... 3-2 

4 . SUMMARY ........................... 4-1 

5 . REFERENCES .......................... 5-1 

$kE FRANKLIN INSTITUTE RESEARCH LABOMTORm 
TABLE OF CONTENTS (cont) 

APPENDICES 

A - Notes on RF Sensitivity of EEDs and the Need 

forTesting.. .................... A 

B - Notes on Multiple Sources ............... B 

C - Notes on Fie1 d Tests. ................. C 

D - Notes on Protection of EED Systems Against RF . D 
LIST OF FIGURES 

Fi qure Page 

2-1 Transmission Coefficient of Circular Apertures 

of Radius a (meters), Straight Line Approximation.. . . . . . . . . . . . . . . . . . . . . . . . 2-6 

Unshorted EED and Its Antenna Model . . . . . . . . . 2-7 

Coordinate System Employed in Calculating Electric 

Field of the Antenna Configuration. . . . . . . . . . 2-11 

2- 4 Antenna Equivalent Circuit for Wire Lead EED 

in Pins-To-Case Mode. . . . . . . . . . . . . . . . . 2-15 

2-5 Maximum Di recti vi ti of Three Known 

Antenna Configurations. . . . . . . . . . . . . . . . 2-18 

2-6 A Typical Shielding Gap Configuration . . . . . . . . 2-21 

2- 7 Basic Antenna Model for a Shielding Gap . . . . . . . 2-21 

2-8 Aperture of a Typical Shielding Gap . . . . . . . . . 2-23 

3-1 Schematic Drawing of System to be Analyzed. . . . . . 3-1 

3-2 Loop Approxlma ti on of System Shown in Figure 3-1. . . 3-1 

3-3 Antenna Configuration for Eva1 uation 

Derived From Figure 3-2 . . . . . . . . . . . . . . 3-2 

3-4 Aperture of Loop Configuration at Paddle 

(Pin-to-Pin Mode) . . . . . . . . . . . . . . . . . . 3-3 

3-5 Comparison of 0.1% Firing Levels of Initiator 

Installed in Pad le with Maximum Power Developed 9 in a 100 Watts/M Field (Pin-to-Pin Mode) . . . . . . 3-5 

LIST OF TABLES 

- Table Paqe 

4- 1 Summary of Analytical Methods for RF 

Hazard Evaluation . . . . . . . . . . . . . . . . . . 4-3 

89 
LIST OF SYMBOLS 

a = Radius of Connector Shell Opening - Meters 

A = Area - Square Meters 

*c = Circular Aperture - Square Meters 

= Effective Aperture - Square Meters 

Aec = Effective Composite Aperture - Square Meters 

A = Maximum Effective Aperture - Square Meters em 

A = Maximum Effective Aperture of qth gap - Square Meters 

emq 

Am = Area of mth loop - Square Meters 

b = Length of Lead Wire or Largest Length of Opening in 

Shield Braid - Meters 

- 

B = Magnetic Flux Density -Webers per Square Meter (Vector 

Not at ion) 

C 

8 = Velocity of Light, 3 x 10 Metemper Second 

d = Lead Wire Spacing or Pin Spacing - Meters 

dB = Decibels 

D = Directivity of Antenna 

- 

E = Electric Field Intensity - Volts per Meter (Vector 

Notation) 

IT 1 = Magnitude of Electric Field Intensity - Volts per Meter 

IFr 1 = Square of Magnitude of Electric Field Intensity at radius rl 

EED' = Electroexplosive Device 

HZ = Frequency - Hertz 

fMHz = Frequency - Megahertz 

G = Gain of Antenna 

- 

H = Magnetic Field Intensity - Amperes per Meter (Vector 

Notation) 

Ix 1 = Magnitude of Magnetic Field Intensity - Amperesper Meter 

I0 = Current on Antenna - Amperes 

Im{zc) = Imaginary Part of Surface Impedance - Ohms 

K 2rr =-- -1 

h Meter 
LIST OF SYMBOLS (cont) 

= Length - Meters 

max 

= Length of Dipole - Meters 

= Maximum 

= Total Number of ~oo~s or Gaps 

= Poynting Vector 

= Power Density - Watts per Square Meter (vector ~otation) 

= Magnitude of Power Density - Watts per Square Meter 

= Incident TEM Field Power Density - Watts per Square Meter 

= Average Power Density at Inner Surface of Shield - Watts 

per Square Meter 

= Power Density atiOuter Surface of Shield - Watts per Square 

Meter 

= Ratio of Solid Area to Hole Area in the Shield 

= Radius of Poynting Vector - Meters 

= Real Part of Surface Impedance- Ohms 

= Radio Frequency 

= Loss Resistance of Antenna - Ohms 

= Radiation Resistance of Antenna - Ohms 

= Termination Resistance - Ohms 

= Thickness of Braid - Inches 

= Transverse Electromagnetic Wave 

= Transmission Coefficient 

= Maximum Radiation Intensity - Watts per Square Radian 

= Average Radiation Intensity - Watts per Square Radian 

= Total Voltage Reduced in Antenna - Volts 

= Power Dissipated in Load - Watts 

= Termination Reactance - Ohms 

= Antenna Reactance - Ohms 

= Surface Impedance - Ohms 

= Impedance of Free Space, 377 Ohms 

= Pins-to-Case Impedance of EED - Ohms 

91 
LIST OF SYMBOLS (cont) 

= Pins-to-Case Firing Mode Impedance of EED (Z ) Transformed 

Along Connecting Lines - Ohms PC 

= Pin-to-Pin Impedance of EED - Ohms 

= Pin-to-Pin Firing Mode Impedance of EED (Z ) Transformed 

Along Connecting Lines - Ohms PP 

= Unknown Pin-to-Pin Impedance Looking Toward Arming Circuits - Ohms 

= Unknown Pins-to-Case Impedance Looking Toward Arming Circuits - Ohms 

= Attenuation - Nepers per Unit Length 

2 11 = - - -1 

, Meter 

= Partial Derivative of with Respect to Time 

-12 = Permittivity of Free Space, 8.85 x 10 Farad per Meter 

= Permeability of Free Space, 12.5 x 10~' Henry per Plater 

= Angles - Radians 

= 2?lfHz 

= Skin Depth - Inches 

= Reflection Coefficient 

= Wave Length - Meters 

= Cutoff Wavelength - Meters 
1. INTRODUCTION 

The determination of the potential radio frequency (RF) 

hazard to any system exposed to an incident RF field is a very complex 

problem. Consider, for example, a typical electroexplosive device 

(EED) and its associated firing circuit mounted in a missile. To begin 

with, the missile may be transported to the launch site with some or all 

of its circuits installed and could conceivably be exposed to a wide 

variety of RF signals along the way. At the launch site it may be 

necessary to install some of the EEDs or electronic components while in 

an RF environment. This would permit the possibility of the individual 

components being irradiated during handling and,subsequently, after installation in its circuit. In addition check out procedures often 

result in altering the circuits, connecting temporary new circuits to 

the potentially vulnerable component and such actions as the opening 

and closing of vents and ports in the missile skin. Furthermore, there 

would probably be constant movement of vehicles and personnel in the 

area and this movement would cause continual fluctuation in local RF 

field intensities. All of these factors would contribute to a constantly changing and very difficult to define set of conditions with 

respect to RF hazards. It should be noted that localized field intensity conditions can exceed the overall field intensity that would 

be determined by measuring the field produced at a given point by a 

radiating transmitter. Unless one can measure the field at the exact 

point of interest, under the actual conditions and with all equipment 

that will be in the area and without serious perturbation of the field 

by the measuring equipment one can be certain only of an approximation 

of the actual field conditions. 

Even if one could accomplish a testing program which would 

cover all of the conditions, the inherent variation from missile to 
missile would introduce another large variable. Slight changes in the 

arrangement of the wiring or in the orientation of the missile with 

respect to the RF field might well produce large variations in the amount 

of RF energy delivered to the device under investigation, identical 

electrical impedance conditions cannot be maintained from missile to 

missile and on board transmitters may directly interact with the 

vulnerable circuits. 

Of course, if the circuit designer were free to design his 

circuits with nothing else in mind but to make them insensitive to RF, 

the RF problem could be essentially eliminated. Complete continuous 

shielding of the entire systems would in general reduce RF levels at 

the components to safe values. However, this is often almost impossible, 

for in our modern complex electric circuits it is usually necessary to 

break branch circuits out of the shield, to terminate on circuit boards 

open to RF signals or to follow other procedures which compromise RF 

safety. In addition, other design groups may argue for and obtain 

different concepts for wiring to accomplish their ends, and in so doing 

may also seriously compromise the RF protection. 

On the other hand it is often suggested that even with circuits poorly designed from the RF viewpoint, there have been relatively 

few accidents directly attributed to RF and therefore the problem must 

be negligible. This could be a very dangerous viewpoint. First of all, 

information on accidents of any nature is usually very poorly disseminated so that it is difficult to know what accidents have occurred and 

what situations surrounded such accidents. This is particularly true 

of accidents which do not result in severe injury to personnel or very 

large property damage. Second, the determination of the cause of an 

accident after it has happened is a very difficult business. This is 

particularly true when trying to evaluate the after-the-fact influence 

of anything as variable as the potential RF hazard. Furthermore if the 

investigators do not fully understand how RF energy can be transferred 

they will easily miss many possibilities. Third, at the present time 

94 
most RF ffelds in proximity of vulnerable systems are of 

reasonably low intensity or are turned off during possibly critical 

periods. Every year, however, the RF environmental levels are increasing, and RF silence may not always be possible. Systems which are now 

marginal may eventually become quite vulnerable. 

With all of these complicating and generally uncontrollable 

factors, how can one even evaluate the potential RF hazard to any 

critical system? Unfortunately, the answer at the present state-of-theart is that it cannot be done with great precision for anything but a 

very specifically defined case; however, the hazard can sometimes be 

evaluated in such a manner that it can be conclusively stated that no 

hazard exists if this should be the case. 

Two methods are now in general use. Both of these require 

that the RF sensitivity of the device in question be known. There are 

laboratory techniques for determining this with reasonable precision; 

unfortunately, the RF sensi'tivity is of the device is not always so 

determined and this in general will negate the effectiveness of either 

method unless suitable precautions are taken. 

The first method, stated briefly, is to directly radiate the 

system in question with a variety of high powersd transmitters and to 

observe the RF levels that arrive at the device under rest. The method 

is appealing, if expensive, since it is a direct approach which superficially appears to simulate the actual conditions that will occur. 

But, while such tests are much used, and have a definite place in the 

scheme of things, there are many pitfalls that generally make them unsatisfactory for a really valid hazard determination. The chief weaknesses of the method include inadequacy of present RF detectors, inability to determine field strengths accurately, the very large expense 

of suitably powerful transmitters, the risk of assuming that tests on 

one or two systems can be extended to all such systems and the lack of 

complete understanding by most field testers of the mechanisms of RF 

damage on the vulnerable devices. 

95 
the results are always on the safe side, and should it be demonstrated 

by this approach that a system is safe in a given field and at a 

specific frequency, its safety can practically be guaranteed; second, 

the actual analysis is reasonably inexpensive. The main expense comes 

from the fact that to perform the analysis properly the RF sensitivity 

of the device in question must be determined, but as was pointed out 

earlier, this should also be done in the case of the direct radiation 

method. The one exception to this occurs when the circuits are so well 

designed from an RF standpoint that it can be demonstrated analytically 

that protection levels are so large that the sensitivity of the device 

is not a factor after installation in these circuits. The main objection to the analytic method in its present form is that it can put 

unusually stringent restrictions on the circuits so that only the very 

well designed systems can be shown to be safe; in other words, the 

safety factor afforded thereby can be unreasonably large. In contrast 

to the irradiation method, it should be noted that the only positive 

9 6 

FRANKLIN tN!3TRWE RESEARCH LABORATORIES 

---. . --- - -- 
result of the analytical approach is to show that a given system is 

safe. Specifically, the analysis can not show tllat a system is 

hazardous since the worst case assumptions implicit in the analysis 

can never be guaranteed to exist. 

1 .I General Approach 

The procedure for establishing the extent of the RF hazard 

to any system by means of the analytic method is as follows: 

a. The RF sensitivity of the particular device or devices in 

each of the circuits in the system is determined over the entire frequency range of interest, for both continuous wave (CW) and pulsed RF 

signals and for all possible modes of damage such as through the regular 

leads or between the leads and the case or any other potential damage 

mode which exists. 

b. Using circuit diagrams, wiring diagrams, observation of 

the actual systems, observations and discussions of the handling, installation and checkout procedures and discussions with the engineers 

directly concerned the'details of the actual physical systems are 

established. These details include such things as length of cables, 

locations of wiring breakouts, and separation of distance between firin6 

leads and between the firing leads and the ground plane. 

c. Mathematical models are constructed which closely resemble 

the actual wiring systems, and which can be handled with analytic 

techniques. These models are constructed for all phases.of the problem; 

i.e., handling, installation, check out and installed; and treat circuits, in the case of EED'S for example, for pin-to-pin, pins-to-case 

and bridgewire-to-bridgewire effects, as applicable. All known parameters of the circuits are used such as the length of unshielded portions, and the physical shape; but wherever a parameter cannot be 

properly defined a worst case assumption is made. For example it is 

normally assumed that a given circuit is oriented with respect to the 

97 
RF field for maximum pick-up of energy, that the entire circuit is in 

a single plane and that all impedances in the circuit are matched for 

optimum pick-up and transfer of energy. 

d. The mathematical model is analyzed to establish the 

amount of RF energy that can be extracted from any incident RF field 

and subsequently transferred to the device under consideration, for 

example, the EED terminating the circuit. The analysis gives, for a 

particular circuit, a quantity known as "aperture" a measure of 

ability to pick up energy. The aperture as a function of frequency 

plot can be applied LO any assumed field intensity. 

e. For any assumed field intensity and frequency the amount 

of RF energy that could be delivered to the test item is obtained by 

the product of the incident power density and the aperture and this 

value compared with its RF sensitivity. The degree of potential 

hazard is thereby established. Under the assumptions which are made, 

an indicated safe condition should be quite safe; an indicated 

hazardous condition may or may not be hazardous. 

These data are usually presented graphically and in such a 

manner that as long as the same circuits and test items are employed, 

the analysis can be immediately applied to any change, present or 

future, in the incident field desnities. Only those circuits which are 

completely different need be analyzed; for example, in the case of 

redundant circuits only one analysis need be conducted if the two circuits are very similar. In a few rare cases the evaluation of the RF' 

sensitivity of the device under test can be eliminated. The usual case 

occurs when preliminary investigations of the circuits indicates that 

they are so well designed from an RF' standpoint that only a small amount 

of energy can be extracted from even a very strong incident field; then 

the sensitivity of the test device may be of secondary importance. 

However, KF sensitive EEDs should always be avoided if possible. 

fi FRANKLIN WST'KUTE RESEARCH LABORATORIES 
This approach is often designated a "worst case" analysis, 

however, it should be noted that this is a mild misnomer. In actual 

fact, all of the known or reasonably obtained data bearing upon any 

circuit is used. For example, such details as actual sizes of loops, 

length of unshielded wire runs, separation distance of cable from frame, 

pin configuration of test device, RF sensitivity of test device, impedance of test device, quality of shielding material used and attenuation provided by switches and arming devices used in the circuit are 

carefully determined and actual values are used in the calculations 

wherever possible. On the other hand, those characteristics which 

could be variable from test vehicle to test vehicle or very expensive 

to determine are assumed to be at their worst. For example; orientation of all circuits is assumed to be optimized in the incident field, 

impedances throughtout the circuit are generally assumed to be matched 

in such a manner as to give maximum transfer of RF energy to the test 

device, RF pickup from all loops is assumed to be in phase and missile 

skins, except under unusual circumstances, are assumed to offer no 

attenuation. Experience has shown this last assumption to be quite 

valid. 

As a result, the analysis produces values of RF power delivered 

to the test device which are always on the conservative side, occasionally by rather large amounts. This leads to the statement made earlier 

, that if under the worst case approach a system is found to be safe, 

it is most likely quite safe; if on the other hand a hazard is indicated, the system may still be safe. 

Three additional points should be noted, however. First, experience has shown that if the missile system is considered across a 

wide frequency band there is a good probability that at some point in 

the frequency spectrum the worst case assumptions will come close to , 

being satisfied and the analysis and the real conditions will come 

close to coinciding. Second, attempts to assign probability values to 

99 

%??THE FRANKLIN INSTITUTE RESEARCH UI)ORATORIES 
-E FRAM RlfTmm RESEARCH LABORATORIES 

- - -- 
2. DETAILED ANALYSIS PROCEDURES 

It is the purpose of this section to describe in detail most 

of the mathematical procedures necessary to conduct an RF analysis on 

a component. From the start it should be carefully noted that when 

analyzing the potential hazard to a component such as an EED every 

pertinent aspect of its history must be carefully considered in its own 

specific situation. For example, the circuit attached to an EED when 

it is installed in a space vehicle may have very different RF pickup 

characteristics than the circuit which might be temporarily attached 

to check the resistance or some other parameter of the EED. If the EED 

is installed in a vehicle with the shorting cap attached and the shorting cap is removed to attach the functioning circuit while an RF field 

is present, possible RF hazard must be considered for the EED with 

shorting cap, without shorting cap and installed in circuit. Should a 

monitoring circuit be included in the EED, the RF pickup associated 

with this circuit must be considered along with its possible coupling 

to the EED functioning circuit. In short, the engineer performing the 

analysis must become intimately familar with all aspects of the device, 

its associated circuits usually back to the power source and its history 

insofar as handling, installation, checkout and final installed condition are concerned. 

In addition the engineer must consider all of the possible 

functioning modes of a device. For a wire bridge EED this would include 

the following: through the bridgewire, between the bridgewire and the 

case and between the bridgewires,if applicable. 

For each condition, the engineer must characterize the system 

as to its most likely manner of acting as a receiving antenna. In its 

simplest form one might consider a wire lead EED with its leads twisted 

together at the end. This system could probably be most directly 
of the EED or device under consideration. It is assumed that the incident RF field is essentially TPI; i.e., far field. Under these conditions the power density P can be expressed as 

where 

is the power density, 

is the electric field, 

is the magnetic field, 

Z, is the impedance of free space, 377 ohms. 

The lines above the letters indicate vector notation. 

fiE FRANKLIN MSmVn RESEARCH LUORATORfES 
With an incident TEM field the basic antenna formulas can be 

applied and the hazard expressed in terms of the effective aperture (A,) 

which is defined by 

= meter 2 (2-2) 

where 

Ae = effective aperture (square meters), 

P = power density, (watts/square meter), 

W = power dissipated in the antenna load, the EED, (watts). 

This concept of aperture is used in all of our analyses. 

A general equation(') for expressing the effective aperture is 

where 

V = the total voltage induced in the antenna, 

RR = radiation resistance, 

2 = loss resistance of the antenna, 

% = termination resistance, 

XT = termination reactance, 

XR = antenna reactance. 

This basic equation is used to formulate many of the analyses. 

In an actual computation the effective aperture must be calculated for each frequency of interest using the applicable equations, 

If the product of the effective aperture and incident power density at 

any given frequency is now formed, the result is the actual RF power 

delivered to the EED under the assumed conditions. This value can then 

be compared with the sensitivity of the EED at that frequency to establish the possibility of RF susceptibility. 

With respect to specific cases we are concerned with only two 

conditions for the EED: disconnected; i.e., not attached to any firing 

103 

*E FRANXLIN INS'ITIWTE WEARCH LABORATORIES 
or testing circuit; and connected. In the former, we are concerned with 

the physical and electrical structure of the EED alone including any 

shorting or shielding caps. This would be the normal condition for the 

various analyses designated as hand held, transportation (when the EEDs 

are not installed) and installation (before the circuits are attached). 

In the latter or connected condition we are concerned with the EED as a 

component in an electrical system. This would be the normal condition 

whenever the device is installed or during check out or other electrical 

testing procedures. It is these two conditions that we will now examine 

in more detail. 

2.1 EED Dl sconnected 

In order to determine the potential hazard to an EED resulting from exposure to an incident RF field during handling and installation (hand-held mode) it is necessary to analyze the physical body of 

the initiator in terms of its ability to pick up and deliver energy to 

its explosive components. The method of analysis depends heavily upon 

the connector type: twin-lead, coaxial or others. Various analytical 

methods are available which include similitude to a small loop, a coaxial aperture, or a circular aperture. In all of these methods, we 

assume that the field is essentially TEM or far field. With an incident TEM field, the basic antenna formulas can be applied considering 

the initiator or initiator assembly positioned for maximum power pick 

up. The various firing modes (pin-to-pin, pins-to-case and bridgewireto-bridgewire) must also be considered both for continuous wave (CW) 

and for pulsed power. 

2.1 .I Mu1 ti pin Connector Type 

In this section we are concerned with EEDs in which the input 

uses some form of the standard type metal shelled, multipin connector. 

The analysis applies even if there is only a single pin such as in the 

coaxial type. Over the years we have developed numerous analysis 

104 
procedures for models in which it was assumed that the model was a coaxial line; lee,, the pins in the connector are assumed to be the inner 

conductor of a coaxial line and the connector body the outer connector, 

a two wire end driven line (the connector body if assumed to be removed 

and the exposed pins end driven) and a small loop formed by the connector 

pins (connector body removed). However, it was determined that the 

worst case aperture exists when the axis of the connector on the EED 

lies along the direction of propagation of the incident radiation. In 

this case we assume that the power delivered to the initiator is not 

more than that which would be transmitted through a circular aperture 

(of the same diameter as the inside diameter of the pin shield) in an 

infinite conducting screen normal to the direction of propagation. 

This approach is now used for all connector type EEDs whether shorted 

or unshorted and for all excitation modes (pin-to-pin, pins-to-case or 

bridge-to-bridge),and while this approach produces a "worst-worst caset' 

value of aperture, the values are in general so low that no hazard 

exists in reasonable incident RF fields and the overall calculation is 

simplified. The circular aperture is given by 

where 

A = area of the opening of the pin shield in square meters, 

Twc = transmission coefficient as given in Figure 2-1. 

Figure 2-1 is a straight line approximation we developed from 

the relationship of transmission coefficient to the radius of the circular aperture which is given in reference (2), page 126. 

In practice, for any given frequency we can compute a value 

27r of Ka where K - - X and a - the radius of the aperture; a and A should 

be in the same units. Twc can then be obtained from Figure 2-1 for the 

calculated value of Ka and the circular aperture can be calculated from 

Equation 2-4. This calculation must be repeated for each frequency of 

interest . 105 

$kE FRANIUtN INSTRJTE RESEARCH LMORATORIP 

2.1.2 Wire Lead Type 

The other most common type of EED which is in usage is the 

wire lead type in which the pins to which the bridgewire is applied are 

wires which extend through the base plug and are used to make connections 

to the EED. These wires may be very short or as long as several feet 

although the most common length is 6 to 8 inches. For the EED disconnected we are interested in both the shorted and unshorted case. 

2.1.2.1 Vns horted Ni re Lead Type (Pin- to-Pi n) 

Figure 2-2 sketches this type configuration and its antenna 

model. This configuration is also often formed by firing system wiring. 

We can evaluate the maximum possible aperture of this configuration by 

1 

using (from reference (1)) 

Fig. 2-2 - Unshorted EED and Its Antenna Model 
Here we must compute the gain G of the antenna, where G is defined for 

a lossless antenna, (which is clearly necessary for prediction for maximum aperture) ae equal to the directivity D. D in turn Is defined by 

D = maximum radiation intensity Iaverage radiation intensity Uo 

The units of U are watts per square radian and since a sphere contains 

4n square radians, 

4n Um 

I total power radiated 

At large values of r we will haye TM propagation and therefore the 

Poynting vector will be real and perpencidular to the surface of a 

sphere centered at the antenna. Using P as the magnitude of the 

r. 

Poynting vector at a large radius r and the definition of Urn at radius 1 

then 

= rlL P 

r max 

r1 1 

Combining Equations (2-5) and (2-6) gives, for a lossless antenna, 

where r is a very large radius. The Poynting vector at a large radius 1 

may be computed from Equation (2-I), and we obtain 

108 
where I is the current on the antenna and \ is the radiation resistance. 

0 

I must be the current actually passing thru % in the equivalent cir0 

cuit. The denominator of Equation (2-10) is the total power radiated 

and \ is also equal to the total power. 

If in Equation (2-3), an impedance match is assumed; i.e., 

we obtain 

since this must be the maximum aperture. This may be equated to 

Equation (2-ll), yielding 

If we now can find IE I 2 

r max for our configuration we will have found V , 1 the induced voltage squared. Substitution of this in Equation (2-3) 

will then yield 
If we now maximize Equation (2-13) in relation to the unknowns we obtain, for \ = -XT, 5 = 0, \ = 0, 

and our only unknown is . Figure 2-3 shows the configuration lE 1: II max 

to be evaluated for the E field at a large r. A similar case with a 

different phase relationship between the currents has already been analyzed(3). Substitution of our value of phase difference (1.e.. 180') in 

this analysis yields 

2 2 2 6d (1 - sin $ sin 8) sin (T cos 8) 

X r 1 

where 

This expression has a maximum value, at 8 = 0, of 

2 2 ,2 

zo I0 2 6d l'rl12 max * A 22 r 

sin \j-1 

1 

Substituting this result in Equation (2-S), yields 

2 

4 Zo e 2 Bd 

Ae * R, . sin 

The above derivation is subject to the restriction that R << X since we 

have considered the currents as linear, whereas they are actually, to 

a first approximation at least, distributed sinusofdally along the 

wires. 
Fig. 2-3 - Coordinate System Employed in Calculating Electric Field 

of the Antenna Configuration 
For 1 cc A, d < k, 

Bd nd sin (2) + - X 

and 

A can be considered the area of the antenna given by the product of d 

and a. 

2.1 -2.2 Shorted Wi re Lead Type (Pi n-to-Pi n) 

The standard method of shorting a wire lead type EED is to 

twist the ends of the wires together. This fonns the leads into a loop 

antenna . 

On page 171 of Reference (1) it is shown that for a small 

2 loop whose area, A, is less than r /lo0 the radiation resistance is 

given by: 

and that the directivity, D, of the small loop is 3/2. Using the 

(1) formula for maximum effective aperture of a lossless antenna , 

which is evaluated when the terminating resistance equals the radiation 

resistance and the reactances cancel, we can obtain for the induced 

voltage 

$% FRANKLIN MSllTLR'E WCH LABORATORIES 
Substituting for RR, we get 

Substituting this expression and % = 0, XR = -XT in Equation (2-3) we 

obtain 

4 At large A, the X term in the expression for the radiation 

resistance (Equation 2-20) dominates and the radiation resistance be2 comes very small (for reasonable areas, <0.01 m ) in relation to the 

other resistance in thqcircuits; we therefore may assume \ = 0. 

Using this approximation, Equation (2-24) becomes 

This equation represents the aperture of a small loop assuming a reactive match between antenna and load, no dissipation of power in the 

radiation resistance (which we have seen is very law for small loopsj, 

and orientation of the loop for maximum pickup. 

An alternate method of deriving the maximum aperture of a 

small loop is to obtain an expression for the voltage induced in the 

loop. Consider that the magnetic flux density is uniform over the 

loop. The total voltage around the loop is then given by 

where 

A = area of the loop, 

8 

w = 2~f = 6~ x 10 /A, 

f = frequency, 

X = wavelength, 
-7 

0 = permeability of free space, 12.5 x 10 h/m, 0 

B = magnetic flux density. 

If we express li12 in terms of P and Zo from Equation (2-1) and frequency in terns of wavelength and substitute these into the square of 

Equation (2-26) we obtain 

where 8 

c = fA = 3 x 10 m/sec. 

If we now make use of 

1 6 

C- -- 300 x 10 and r I/>- 377 dE 0 

0 

where E is the permittivity of free space, we can write Equation 2-27 

0 

Substitution of ~quation(2-28) in (2-3) with RL = 0, icR = -XT gives 

Using the assumption that 5 0 as before, Equation (2-29) 

can be rewritten as 

which is identical to Equation (2-25). 
Furthermore, if we now compare this result with Equation 

(2-191, the expression for unshorted wire lead configuration, we find 

that the two expressions are also identical. Therefore, the effective 

aperture for a wire lead EED in the pin-to-pin mode is the same whether 

the leads are shorted together or not,if the physical dimensions are 

the same. 

2.1,2.3 Wire Lead Type (Pins-to-Case) 

The two preceeding sections discuss the case for pin-to-pin 

or "through-the-bridgewire" conditions; however, a pins-to-case 

functioning mode is also possible. Figure 2-4 shows a typical EED and 

the corresponding antenna equivalent. As shown, the approximation of 

this configuration as gn antenna is an end driven short dipole where 

the impedance that must be used is 

the real part of the pins-to-case 

impedance. This must be obtained 

by measurement at the frequencies 

of 

culating the maximum 

interest. The formula for 

power pickup 

cal- a3 

in this impedance from an end driven 

dipole is as follows: tsD1 

R.<Z ,,c) 

-2 2 

w = lEl 'D (2-31) Fig, 2-4 - Antenna Equivalent 

Re (Zpcl Circuit for Wire Lead EED 

i n Pi ns-to-Case Mode 

where 

W = maximum power in watts, 

1i1= magnitude of field density in volts/meter, 

kD = length of dipole in meters, 

Re i.Z = real part of pins-to-case impedance in ohms. 

PC 

This formula utilizes the fact that the effective height of a 

short dipole is its physical lenkth and therefore the total open circuit 

voltage in the antenna equivalent circuit will be equal t.o t!le magnitude 

115 
of the electric field times the dipole length. The worst case assumption for series impedances of this model is that the radiation and 

terminating impedances have equal and opposite reactances and that the 

radiation resistance is zero. From Equation (2-2) it can be seen that 

Equation (2-31) can be expressed as an aperture by dividing both sides 

by the incident power density (Pi). 

W tDC z 

=-I O Meter 2 

A pi ~ei~~~) 

where Zo = impedance of free space in ohms. 

2.1.2.4 Wire Lead Type (High Frequency ~alculations) 

In the preceding three sections we have discussed the methods 

of analyzing the RF pickup of a wire lead device in all of its various 

configurations and hazard modes. However, each of these approaches 

has the limitation that the wavelength must be long with respect to the 

physical dimensions of the receiving antennas. When the wave length 

becomes too short the assumptions which lead to the various calculations 

are no longer valid due to non uniform current distribution in the 

antennas. For the unshorted loop the shortest applicable wavelength 

occurs at X 20t where L is the length of one of the leads. For the 

shorted loop the shortest applicable wavelength is X 2P where k is 

the perimeter of the loop. For the end driven dipole, the shortest 

applicable wavelength is X - 10LD where kD is the length of the dipole. 

In each case the equations are valid for any wavelength longer than these 

conditions. 

FMNKLW MSTITVITl RESEARCH LABOMTONES 
At the shortc wave lengths; i.e., the higher frequencies, tile 

maximum effective aperture (A ) can be calculated from 

em 

which holds for a lossless antenna. In this formula Aem is the maximum 

possible aperture, assuming a complete impedance match, and D is the 

directivity of the antenna. Generally, at these higher frequencies the 

directivity of the actual configuration under consideration as a function 

of frequency is not known; but if we assume that it can be no mare than 

that of an antenna of known directivity we can calculate Aem, the maximum effective aperture. 

Another referin~e'~), shows curves of directivity for three 

types of antennas: the unterminated rhombic, the long wire and the 

circular loop. It is reasonable to assume that our configuration will 

be no more directive than these, since these are among the most directional linear antennas known. 

Figure 2-5 is a composite plot of the greatest directivity of 

these antennas types as a function of overall lead length. The plot 

was made directly from the above reference. Using Figure 2-5 and 

Equation (2-33) the maximum effective aperture of our antenna configurations can be calculated. The maximum effective aperture (A ) is calem 

culated under the assumption that the lead configuration will be no 

more directive than an unterminated rhombic, a long wire or a circular 

loop antenna of equal linear dimension. The calculation is straightforward. 

It is interesting to note at this point the previous determinations for effective aperture (A ) at the lower frequencies were 

e 

calculated with the following assumptions: the terminating bridgewire 

resistance is no less than the dc resistance, the antenna is reactively 

matched, loss resistance is zero, and the radiation resistance is zero. 

11 7 
OVERALL LEA0 LENGTH (WAVELENGTHS) 

Fig. 2-5 - Maximun Di rectivi ty of Three Known Antenna Configurations 
Note that these last assumptions effectively maximize the Ae expression 

2 (see Equation (2-3) where V is considered constant). These calculations 

contain a seeming anomaly since the effective aperture curve, if continued, would rise above the maximum effective aperture curve. This is 

a result of considering the radiation resistance to be equal to zero in 

our maximizing procedure of the effective aperture. If the radiation 

resistance were taken into consideration the curves would not intersect 

so abruptly but the effective aperture curve would roll over at the 

higher frequencies to meet the maximum effective aperture curve. 

2.2 EED Connected 

The preceding discussions provided the necessary formulas to 

determine the worst cake RF pick up of the majority of EED disconnected situations that one is likely to come upon and which would be 

applicable for hand-held, installation and transportation considerations. 

In turning our attention to the EED connected in its various circuits, 

for example installed and checkout, it is important to restate that the 

most important and necessary part of the analysis is to properly characterize the antennas represented and that this procedure is considerably more complicated when the EED is connected. However, experience 

has shown that the majority of present missile circuits fall into one 

of two categories. The first of these is the circuit which contains 

breakout of the shields to go to circuit boards, through bulkhead 

connectors, to other circuits or at the EED itself. A common occurrence 

is for the shielding to terminate just prior to the EED, for example. 

Most of these breakouts can be characterized as loops of varying dimensions. One must pay particular attention to possible pins-to-case lo~ps 

in these systems. The second type is the circuit which is completely 

shielded from end to end and through 360'. In this section we will 

treat these two possibilities. 
Circuits with Shielding Caps 

Many EED firing systems use shielded cables between the safe/ 

arm device and the EED, or if no safelam unit is used, between the 

timers or firing switches and the EED. For such circuits the first 

assumption used in arriving at the antenna models to be used is that the 

power coupled to the EED firing mode impedances through the braided 

shield of the cables is negligible in relation to that coupled to these 

impedances by the non-shielded portions of the wiring. In consequence 

the models chosen represent the physical characteristics of the gaps or 

breaks in the shielding. Figure 2-6 diagrams a typical break or gap 

in a shielded firing lead and Figure 2-7 diagrams the equivalent antenna 

model used for this gap. The dimensfons given are representative of 

commonly used separation switches. 

The impedances Z and Z are considered to be completely unu1 u2 

known while Z and Z represent the firing mode impedances (2 and 

PPt PCt PP 

2 ) of the EED transformed along the connecting lines to the separation PC 

switch. The models for pin-to-pin and pins-to-case pickup are thus seen 

to be, for the lower frequencies at least, small loops loaded with the 

indicated impedance. We further assume that the transmission lines 

formed by the shielded cables that connect the gaps and the EED are lossless. This is to be expected since these cables are constructed of good 

conductors and good insulators. In addition we have made measurements 

on many typical types of two wire twisted shielded cable in the low frequency ranges and although attenuation is not zero it is usually small for 

the lengthe of cable considered in these ranges. The only worst case assumption that can be made, without extensive and expensive measurements, is that 

the loss. is zero. 

Once the loop has been reduced to Its diagrammatic representation as shown in Figure 2-7, the aperture for this loop can be calculated 

from the same equations as developed before. For wavelengths up to twice the 

perimeter of the loop, Equation (2-25) applies. For shorter wavelengths 

Equation (2-33) applies. 120 
I ,,r--- CONNECTOR BACKSHELL 

ARMING 

CIRCUITS 

SHIELD 

L- PIGTAIL 

Fig. 2-6 - A Typical Shielding Gap Configuration 

Fig. 2-7 - Basic Antenna Model for a Shielding Gap 

*E FRANKLIN INSTITUTE RESEARC?i LABORATORIES 
The above methods allow us to predict the maximum possible 

aperture of a single loop across the frequency range of interest. If 

more than one loop exists in the same firing circuit the composite 

aperture of the combined loops is obtained, at all frequencies such that 

2a < A, from 

where Aec is the composite effective aperture of n loops and A is the 

m 

- area of the mth loop. This result reflects the fact that the methods 

employed in this frequency range are based on a maximum voltage and since 

the voltage contributions of the individual loops could add in phase, 

we must consider this worst case possibility. In fact, at the lower 

frequencies where the wavelengths could be considerably longer than the 

circuit considered, this is a distinct possibility. 

At the higher frequencies such that 2L - > A a similar procedure 

must be used, here the composite aperture is calculated from 

where Aem is the maximum aperture of the qth gap and Aec is the composite a$rture. 

Figure 2-8 shows the pin-to-pin aperture computed by the above 

methods for a small shielding gap in a 6.4 ohm (dc resistance) EED 

firing circuit. The geometry of the gap is shorn on the figure. 

122 

2.2.2 Completely Shielded Circuits 

The case where circuits are completely shielded with no gaps 

is comparatively rare and it should be noted immedi.ately that when this 

is done there is rarely any RF hazard problem involved with such circuits. However, it is sometimes necessary to demonstrate by analysis 

that such is the case. 

Since in such a system the shields completely enclose the EED, 

shorting switch, power supply and interconnecting wires, the analysis 

can be broken into the following parts: 

1. Determining the total power into the outer surface 

of the shield as a function of frequency and of 

the incident field assuming matched conditions 

inside the shield. 

2. Determining the total power loss of the shield 

assuming matched conditions inside the shield. 

3. Using the results of steps one and two to cslculate the maximum possible power that can be 

delivered to the EED as a function of frequency and incident field, assuming the EED to be 

installed in the longest firing circuit. 

4. Comparing the results of step three to a composite 0.1% firing level that is the minimum 

0.1% level for all the EEDs under consideration for any firing mode. 

Dividing the analysis into the parts given above implies the 

assumption that the field that is reradiated by any structure within 

the shield or by the boundary between the inner surface of the shield 

and the region internal to the shield will, at the outer surface of 

the shield, be very small in relation to the field induced on the outer 

surface by the incident radiation. A worst case approach which insures 

the above is to assume that all power that penetrates the shield is 

perfectly matched to the EED. If this were the case, there would be no 

reflection from either the shield/internal region boundary or any 

FRANKLIN MSmVTr RESEARCH LABORATORIES 
internal structure. 'ye may further assume that the power that penetrates 

the shield can be matLhed to any of the firing modes' impedances. 

2.2.2.1 Calculation of the Maximum Power Density at the Outer Surface 

of the Braided Shield 

As a start toward determining.the maximum power density at 

the surface of the braid, we assume that the entire surface of the 

cable is illuminated by a TEM field at normal incidence and the field 

has a power density Pi. We realize that generation of such a field 

(normal to an irregular convex surface) is well nigh impo'ssible, but it 

is surely the worst case TEM field assumption. The maximum power 

density (PT) at the surface of the shield is given by 

where 

P is the power density at the surface of the braid, T 

Pi is the incident TEM field power density, 

Zo = 377 ohms, 

zc = (1 + j) x 2.59 x log4 vz = the surface impedance of 

a copper sheet in ohms, 

f~z = frequency in megahertz. 

If we note that Re {Zc} = Im 12 1 << Z we can write Equation C 0 

(2-36) as 

4 Re {Zcj 

P~=Pi Zo = Pi x 2.75 x IO-~~F- MHz 

with negligible error. It can be shown that this equation provides a 

worst case estimate of surface power density. 

-E FRANKLIN INSTIIm RESEARCH UM)RATORDS 
2.2.2.2 Calculation of the Polder Density at the Inner Surface of the 

Braided Shield 

Power passes through the shield by two separate paths: propagation through the copper ribbon of the braid and propagation through 

the interstices. The loss in the metallic path is a dissipative attenuation, and that produced by the small holes is due to reflection. The 

copper loss can be evaluated by the planar attenuation which is 

where t is the thickness of the braid and 6 is the skin depth. The 

justification of the use of planar attenuation instead of the attenuation of the curved surface of the phield can be found on page 248 of 

reference6)where it is shown that as long as the radius of the cable 

divided by the skin depth is more than 7.55, the planar approximation 

leads to very small errors. If we use the minimum thickness of the 

ribbon that makes up the braid as t and calculate 6 for copper we obt ain 

dB 

copper 

where fmz is frequency in megahertz. 

The attenuation of the small holes can be computed from that 

of a waveguide operating below cutoff frequency. From page 346 of 

reference 6),we obtain, for a cutoff rectangular guide 

where a is the attenuation in nepers per unit length, 

Xc is the cutoff wavelength of the guide, 

X is the free space wavelength of the propagating 

energy. 

The applicable cutoff wavelength is given by 

126 
where b is the largest dimension of a rectangular guide. Converting 

to nepers gives 

dB = 8,68at 

where t is the thickness of the shield, 

Since the power density at the surface of the shield has been 

assumed constant and since the ratio of open to solid area of the shield 

can be determined, a symbolic equation can be written for the average 

power density out of the inner surface of the shield. If Po is the 

average power density at the inner surface of the cable, PT is the 

outer surface power density and Q is the ratio of solid area to hole 

area in the shield, 

t Po = (1-Q) PT [down 8.68at dB] + QPT [down 8.6% dB] 

It should be noted that shielded cable varies greatly in construction and quality and to apply the above system it is necessary to 

carefully investigate the shield in question to determine thickness, 

material and ratio of hole area to solid area. General construction 

should also be noted. 

It is now possible at any one frequency to use Equation (2-36) 

and Equation (2-43) to establish the maximum amount of RF power arriving 

at the inside of the shield in terns of the RF field incident upon the 

cable, If this is now adjusted for the total area of cable exposed, 

the total maximum RF power inside the shield will have been determined. 

By our original assumptions all of this power is assumed to be delivered 

to the EED. For consistency we could construct a symbolic aperture 

/- SHWTING CAP 

/ TAPED TOGETHER 

+ 

DETONATOR 

PADDLE 

fig. 3-1 - Schematic Drawing of System to be Analysed 

APPROXIMATE AREA OF LOOP = 9.5 CM* 

PERIMETER OF LOOP = 38 CM 

Fig. 3-2 - Loop Approximation of System Shown in Fig. 3-i 
3. EXAMPLE OF AN EVALUATION 

Figure 3-1 is a schematic drawing of a simple system configuration actually used on a space vehicle. Figure 3-2 is an approximation 

of this configuration shown as a simple loop and finally Figure 3-3 

shows the antenna configuration for analysis derived 

. i AREA : 9.5 CM* 

Roc: 1.4 A 

from the actual circuit. PERIMETER : 38CM 

Rdc is the dc resistance of 

the initiator. 

While this is an 

fig. 3-3 - Antenna Configuration installed mode, the final for Evaluation Derived 

antenna configuration is a ftm Fig. 3-2 

single loop. Therefore Equation (2-25) can be used to compute the 

aperture for all wave lengths up to X = 2 times the perimeter of the 

loop, i.e., for all wave lengthe up to 76 cm or a frequency of 395 MHz. 

Abovethis frequency Equation (2-33) is used. For each frequency of 

interest, and sufficient frequencies should be chosen to define the 

curve, one must calculate an aperture using the appropriate equation. 

Figure 3-4 is a plot of such calculations made for the circuit uader 

consideration here. 

The final step consists of using this aperture versus frequency data to produce a plot of RF power received at the EED as a function of the RF field incident on the eystem and to compare this RF 

pick-up with the RF sensitivity of the EED established by testing. 

Figure 3-5 showe such a plot where the incident RF power density was 

2 assumed to be 2 watts/meter2 up to 50 MHz and 100 watts/meter above 

50 MHz. The data for this plot was obtained by multiplying chosen 

points on the aperture curve of Figure 3-4 by the assumed incident 

power density at the same point. Superimposed on the power pick-up 

130 

curve of Figure 3-5 is the RF sensitivity curve of the EED used in the 

installation. The conclusion one would draw from this plot is that 

should this system be exposed to 100 watts/meter2 f ieldo across the 

5 frequency spectrum from 10 MHz to 10 MHz safety could be guaranteed 

on the basis of the analysis only from 10 MHz to 80 MHz and from 

approximately 1600 MHz to 8500 MH~. 

$&E FRANUN INSTITUTE RESEARCH LABORATORIES 

4. SUMMARY 

As indicated in the beginning of this monograph there has 

been no attempt to be exhaustive in coverage since it is almost impossible to predict all of the possible circuit characteristics that 

one may be faced with in any given analysis. However, our experience 

has shown that most problems fall into the general categories discussed 

here. We have not touched on the special case of near field, for example, nor have we considered device8 other than wire bridge type EEDs. 

It should be apparent, however, that the general philosophy and methods 

of approach can be used for any type of field, any type of circuit and 

component. In general,, to apply the techndque three conditions must be 

met: 

a) Knowledge of all of the failure modes for the component 

being considered and the RF levels that will cause failure or degradation of these components. 

b) Proper construction of a mathematical model which 

accurately simulates the actual circuits involved so that the system 

connected to the device in question can be characterized in terms of 

a workable RF receiving antenna. Once again it is essential to consider 

all possible failure modes. 

c) Proper application of electromagnetic theory principles 

to this model. 

No one part of this sequence can be taken lightly since 2 

failure to properly conduct any one part could cause a failure of the 

entire apprcach. To be successful the engineer must be painstaking and 

methodical in his approach and must accept no unsupported heresay regarding any elements of the device or circuits. 

It should be remembered that, properly applied, the analysis 

approach is only semi worst case and every attempt should be made in 
constructing the model to use actual conditions wherever possible. Note 

that in a very general way the analytical approach can prove that a 

circuit is safe, but cannot always prove that a system indicated to be 

in trouble is really unsafe. In contrast the field-test approach can 

only show that a system is in trouble and cannot prove that a system is 

absolutely safe. 

In conclusion Table 4-1 summarizes the conditions and equations we have presented in this monograph. 
Table 4-1 - Summary of Analytical Methods for RF Hazard Evaluation 

Cant iqur a t ion 

I 

UID KWQ 

(#anD) 

1 = 2b+2r 

QDlLTtm tm 

tun nonlrn CAP) 

L 

.u UAD EVIQ 

(-1 

COIK'IDI tm 

~ro mmtn w) 

L 

= Lorn 

I = 2b + 24 

U17 1R t 1- ' 

A- 2Uof 

Configuration 

Designation 
I Table 4-1 (Cant,) 

Configuration 

Ccri~nation 

TOTAL AWWUFE 

INSTALLEI) 

(360' Corr,?lete;y Shielded 1 

P A 

0 A t 

A -- - - { (1-u) P [don, 0.6Bat de] a F,. [dour. 8.66 - db]) 

mnJ Pi *I T 1 6 
Table 4 -1 (Concl . ) 

~onf iiura t ion 

Designation 

---- INSTALLED WIT W VALDEY I (360' Corriplr:e;y St.ielde?) c 
5. REFERENCES 

1. Kraus, J. D., Antennas, McGraw-Hill, New York 1950. 

2. King and Wu, The Scattering and Diffraction of Waves, Harvard 

University Press, Cambridge, Mass. 1959. 

3. Plonsey and Collin, Principles and Application of Electromagnetic 

Fields, New York, 1961 p. 404. 

4. Rawls, Stilwell, McDonald, RF Radiation Hazards, Air Force Missile 

Test Center Report AF'MTC-TM-59-4; July 1959, p.16. 

5. Ramo and Whinnery, Fields and Waves in Modern Radio, John Wiley & 

Sons, New York, 1953. 
Appendix A 

Notes on RF Sensitivity of EEDs and the Need for Testing 

Throughout this monograph reference is made to various 

functioning modes of EEDs. While it is not the purpose of this document to present a comprehensive picture of EED1s behavior under RF irradiation a brief discussion should help clarify the situation. 

A standard EED normally contains a transducer to which 

electrical energy can be applied. Some of this energy is usually converted to heat which in turn initiates the explosive mix next to the 

transducer. It is in this manner that EEDs are normally designed to 

operate and this is what we have designated as the pin-to-pin mode. 

However, there are other modes in which an' EED can be caused to fire e 

which were not planned in the original design. The most common such 

mode is designated pins-to-case. 

In this mode an electrical signal impressed between the pins 

leading to the normal transducer and the case of the EED can cause a 

voltage breakdown or some other disruptive phenomena directly through 

the explosives between the bridgewire posts and the case. This mode 

is frequently overlooked but is of vital interest in the case of irradiation by RF and spurious electrostatic potentials. 

In addition some EEDs contain an additional circuit either to 

permit monitoring of proper connection or to support a redundant transducer. In this case, in a manner similar to the pins-to-case phenomena, 

signals can appear between the two circuits and once again directly 

across the explosive mix. This is the bridge-to-bridge mode. 

Frequently, when a group is considering the possibility of RF 

hazard in connection with an EED the assumption is made that the RF I 

seneitivity of the EED can be characterized as no greater than the dc 

no fire level, or, in some more conservative cases, to be no greater 

than an arbitrarily chosen 6 db below the no fire level. The reasoning 

behind this seems to stem from the concept that the RF probably heats 

up the transducer in the same manner as dc. Were it not for the other 
modes this assumption would be reasonably valid at least over part of 

the frequency spectrum. Experience gained in performing RF sensitivity 

tests on over 75 different EEDs has indicated that up to approximately 

1000 MHz and for RF applied directly to the normal transducer, i.e., 

pin-to-pin, the functioning sensitivity of hot wire type EEDs is no 

greater than the dc constant current sensitivity for long pulses of 10 

seconds or more duration. However, RF signals applied between the pins 

and the case or between dual bridgewires may frequently produce sensitivities much greater than the dc sensitivity over the frequency range. 

Above 1000 MHz, and particularly when pulsed RF signals are applied, 

the sensitivity may be greater e than dc in all modes including through 

the bridgewire. In many cases this sensitivity is increased by considerabely more than the 6 dB safety factor sometimes used, and since 

the pins-to-case and bridge-to-bridge mode have very little to do with 

the normal functioning mode, insensitivity to dc signals in the normal 

firing mode (pin-to-pin) is no protection. Many 1 ampere -1 watt devices are more sensitive in the pins-to-case mode than EEDs designed 

to be considerably more sensitive in the normal functioning mode. 

On the other side of the ledger many EEDs are far less 

sensitive to RF than they are to dc particularly in the pin-to-pin mode. 

In these cases assumption of the dc level as the sensitivity could so 

severely penalize the evaluation of the EED circuits as to make acceptable circuits appear quite hazardous. 

All of these pitfalls can be avoided by adequate RF testing 

of the EEDs. Procedures and equipment are available which permit 

accurate determination of the amount of RF power required to be delivered to an EED in any functioning mode to produce functioning or degrading. 

The expense of the hardware required for such tests is frequently a 

deterrent, but more often than not it is false economy to avoid this 

step. 
Appendix B 

Notes on Multiple Sources 

While the analysis procedures discussed in this monograph 

assume that the magnitude of the incident RF field is already known it 

is frequently necessary for the engineer to make some judgments of this 

field himself. A classical case of this occurs when several RF sources 

exist at or near the same frequency and,while an incident field can be 

calculated for each source, the question arises as to the effect of all 

of the sources combined. 

It can be showri that the worst case average power absorbed 

(P ) by a given load from n sources at the same frequency is given by A 

where P is the power supplied by the qth source with other sources 

9 

quiescent and when the phase angles of the individual sources are 

chosen to maximize the simultaneous absorbed power. The use of the 

above equation as a worst case condition should be limited, however, to 

continuous wave sources of precisely the same frequencies. If the 

frequencies differ by even a small amount and average power (P ) is A 

defined as 

q 

P~ = lit J PI dt 

q+rnq 0 

where PI is the instantaneous simultaneous power, then the worst case 

average power is given by 

where P is defined as before. 

Q 

This equation is a worst case condition for either continuous 

wave or pulsed sources of differing frequency and in addition is independent of the starting times of individual pulses of the pulsed 

sources. 
The technique of combining power densitities for closely 

spaced frequencies and then predicting worst case possible hazard at 

the widely spaced frequency groups so obtained is founded on the 

assumption that the worst case conditions assumed in the pickup analysis will not occur more than once in the entire frequency band of 

interest. Due to the complicated frequency dependence of many of the 

parameters that are worst case approximated we believe this assumption 

to be conservative. The alternative to this technique is to combine 

power densities at all frequencies and use this result throughout the 

frequency range of interest. In our judgement this technique is overly 

conservative. 
Appendix C 

Notes on Field Tests 

One approach to determining the extent of RF vulnerability of 

a system is to place the system in question in the fields of various 

RF transmitters and to observe the effects produced on the system components. This technique has been extensively applied to missile systems 

containing firing circuits terminated in electroexplosive devices (EEDs) 

but the basic technique is applicable to circuits containing components 

other than EEDs. If the data obtained are to be anything more than 

go/no-go infomation for the particular RF field intensity used it is 

necessary to put detectors in plaoc of the components being evaluated; 

these detectors must give an indication of the amount of RF energy 

delivered to the compoTient. In general this approach to hazard determination is particularly appialing because it is direct and appears to 

be a test which closely approximates the actual conditions which would be 

exist in a operational situation. However, in most cases the technique 

falls somewhat short of the ideal. 

Among the major problems in using this technique is the extension of information received on one system to other systems of the 

same type. The number of more or less uncontrollable variables makes 

any generalized correlation very difficult. For example, determination 1 

of the actual RF field incident on a given circuit at the time of test 

can be very difficult since envirohnental factors can greatly influence 

localized field strengths and variation of circuit orientation and circuit design from test vehicle to test vehicle can present a problem. 

Frequently, this situation ie handled by using an arbitrary safety 

factor in connection with the resulte; in a sense, "worst casing" the 

field tests. 
A more definitive problemgrows out of the same problem that 

was discussed in the notes on EED sen~itivity; i.e., the detectors used 

in the field tests consider only the pin-to-pin mode and are calibrated 

on the basis of dc sensitivity and with dc signals only. 
places as possible. There are many specification documents that go 

into considerable detail on the proper design of EED circuits. However, 

in actual practice the designer often finds it difficult to comply with 

all of the requirements. The physical layout and the complexity of 

the system often force him to break the shields. Furthermore the wiring 

philosophy is often in conflict with other philosophies. The primary 

example of this is the multiple point grounds versus the single point 

ground philosophy. And yet the single point ground combined with discontinuous shields frequently leads to large pins-to-case RF pickup 

problems. 

The third solution is the use of RF filters. Ideally these 

filters should be broadband, very light and small and should not affect' 

the EED's dc firing characteristics. In addition, the filter should 

in no way compromise system reliability. The optimum solution would 

14 6 
have the filter as an iniLgral part of the EED; however this is not 

very common. It is more common to mount the filter separately but 

close to the EED. When this is done the wiring between the filter and 

the EED must be completely shielded. 

While RF filters are a valid solution to the problem, the 

designer must be certain that the filter is capable of accomplishing 

the task he desires. To do this he must be certain of the information 

on the filters. The parameter of major interest is attenuation as a 

function of frequency. Frequently filter values are quoted in terms 

of insertion loss which has meaning only in the specific measuring - system used. What is worse, occasionally attenuation, or true loss, 

is used interchangeably with the term insertion loss. If the designer 

is uncertain of how published data was obtained and is unable to find 

out, he should make certain the proper tests are performed. 

6kE FRANKUN INSTITUTE RESEARCH LABORATORIES 

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APPENDIX C 

RAW DATA FOR THE 

TRANSMISSION LINE MEASUREMENTS 
LISTIIZG FOR ALL TRA?iSNISSION LISE HAi4 DATA PAGE 1 

LOCATIOIV FREQUEbCY PICKUP(FIV1 Y Z CUHHEfiT 

t F:-UJZ) AMPS 

m60 

m60 

048 

m4a 

m48 

m48 

048 

m48 

1 moo 

1,00 

1,00 

1.00 

1 moo 

1 moo 
LLSTItiG FOR ALL TRANSMISSIO!I LIKE RAW DATA PAGE 2 

LOCATION CUHHENT 

AMOS 
LISTIPJG FIJI? ALL THAI~SYISS~O.~ LIiJE RAvt DATA PAGE 3 
LISTING FUR ALL TRAbSYISSIOid LIhE RAd DATA 

LOCATION 
LISTItvG FOR ALL TRAtjS;4ISSIOrq LIhE RAd DATA PAGE 5 

CURRENT 

AMPS 
APPENDIX D 

PLOTS OF THE NORMALIZED DATA 

FOR THE LOOP MEASUREMENTS 
MINE DATA FROM PROJECT C5490--.FWEPJCY= ,173 MHZ 

FOR ~~0.0 AXIS OF con ALONG ME DRIFT 

03 t .0 1.5 2.0 2.S 3.0 

DISTANCE CZ) FROM CENTER OF THE COIL IN METERS 

FOR ALL NINES; PICKUP NORMALIZED TO 10.0 AMPS DRIVE CURRENT 

MINE DATA FROM PROJECT C549@-FREWENCY= .530 MHZ 

FOR Xe0.0 AXIS OF COIL ALONG THE DRIFT 

500 d 

- 

C( 

. 

- 

1 

DISTANCE CZ> FROM CENTER OF THE COIL IN METERS 

FOR ALL NINES; PICKUP NORMALIZED TO t0.B AMPS DRIVE CURRENT 

MINE DATA FROM PROJECT CS4QB-FREQUENCY= 1.000 MHZ, 

FOR ~4.0 AXIS OF COIL ALONG THE DRIFT 


MINE DATA FROM PROJECT C~~~~--FREWENCYP 6.000 MHZ 

FOR ~~0.0 AXIS OF con ACROSS THE DRIFT 
0.5 1.0 1.5 2.0 2.5 3.0 

DISTANCE CZ) FROM CENTER OF THE WIL IN METERS 

FOR ALL NINES; PICKUP NORMALIZED TO 10.0 AMPS DRIVE CURRENT 
MINE DATA FROM PROJECT CS490--FREQUENCY= 10.000 MHZ 

FOR X4.0 AXIS OF COIL ACROSS THE DRIFT 

MINE DATA FROM PROJECT C5490--FREQUENCY= .I73 MHZ 

FOR Z=0.0 COIL AXIS ACROSS THE DRIFT 
FOR