ANTENNA DESIGN{ COUPLING 

STUDIES AT MEDIUM 

FOR 

FR EQUE NCY 

IMPROVED 

COAL MINE COMMUNICATION5 

PREPARED FOR 

UN l TED STATES DEPARTMENT 0 F THE I NTEF? I OR 

BUREAU OF MlflES 

TERRYS. CORY, P.E. 

ENGl NEERl NG CONSULTANT 

CEDAR RAPIDS, IOWA 

FI NAL REPORT 

PURCHASE ORDER NO. PO382223 15 DECEMEER 1978 
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 
p

g pg

17. Orlglnator's Key Words - 18. Availability Statement 

MF wireless radio communication; vehicular 

antennas; portable antennas 

I 

19. U.S. Security Classif. of the Repon ; 20. U.S. Security Classif. of This Page 21, No. of Pages 22. Price f 

Unclassified Unclassified I 55 f 
FOREWARD 

This report was prepared by Terry S. Cory, P.E. for the U.S. Bureau of 

Mines under purchase order P038223. The work was initiated via an 

unso l ici ted proposa l under Coal Mine Health and Safety Program. It was 

admi n i stered under the techn i ca I d i rect i on of the 8ruceton Research 

Center with Mr. Robert Chufo acting as the Technical Project Officer. 

The BuMines Contracting Officer was klrs. Chalene Wolper. 

Thi s report i s a summary of the work recently cornp leted; performed 

during the period 15 July 1978 through I Oecomber 1978. The report w3s 

submitted by the author on 15 Decomber 1978. 

Any reference to specific brands, equipmen?, or trade names eithsr 

specifically written or implied does not imply endorsement by the 

Bureau of Mines. No patentable techniques have been developed or 

otherwise used during the course of thf s program, The vehicular an+:~nne 

design may be patentable. 
Tabls of Contents 

Page 

Report Documentation Page---------------- Foreward--------------------------------- 

1.0 Introduction----------------------- 

2.0 SUm~ryIIII--I-I-I-~--------------- 

2.1 Antenna Design 8 Performance------- 

2.1.1 Performance bnslderations--------- 

2.1.2 Prototype Antenna DescrlptIon &---- 

Performance 

3.0 Technical Approach---------------- 

3.1 General Description---------------- 

3.2 Theoretical Baseline------------- 

3.2.1 Antenna Design Algorithm----------- 

3.2.2 Litz vs Sol id Wire Comparisons----- 

3.2.3 Intrinsic Safety Parameters-------- 

3.2.4 Antenna Winding Conficuratlon------ 

Considerations 

3.3 Selection of Design Parameters----- 

4.0 Veh icu l ar Antennz Performance------ 

5.0 Antenna Coupl ing to PJine Wlring---- 

5.1 General Considerations------------- 

5.2 Description of the Technique------- 

5.3 Entry Crossectional Geometry------- 

5.4 Presentation of Results------------ 
Li st of l l l ustrat ions 

Fi gure 

I Vehicular Antenna Prototype------------------------ 

2 Portable Antenna Test Configuration---------------- 

3 Resistance of Li tz Wire Compared to Sol id \dire----- 

for Fi xed Overa l l Diameter and Strand Diameter 

vs Wire-Wire Spacing 

4 Res i stance of Li tz W 1 re Compared to Sol i d W i re----- 

for Fixed Strand Diameter and Wire-Wire Spacing 

vs Overal l Wire Diameter 

5 Resistance of Li tz Wire Compared to Sol id Wi re----- 

for Fi xed Overa l I Dl amterand W i re-W i re Spacl ng 

vs Strand Size 

6 Possible AC Intrinsic Safety Criteria Using ------- 

Bandwidth of Antenna as a Measure Showing Vinimum 

Bandwidth as a Function of Input Power 

7 Winding and Wiring Configuration for the VehicularAntenna Prototype Showi ng the Connection Arrangement 

of Turns to Mi n i mi ze Dl stri buted Ca~aci tance 

8 NIA & Bandwidth Comparisons for 6-1 nch by 3-Foot--- 

Vehicular Antenna 8 1000 KHz vs Solid Wire a~d 

Litz Wire 

9 NIA for Vehicular Antennas 6-Inches Htgh vs-------- 

Length and Number of Turns for 1790/44 Litz Wi re 

for 3 KHz Minimum Bandwidth @ 1000 KHz and Below - 

Optimum for If-Inch Electrostatic Shield 

10 NIA Comparison for 6-1 nch by 3-Foot Vehicular------ 

Antenna Designs for 1700/44 Li tz Wire for 12 KHz 

Minimum Bandwidth @ 1000 KHz and Below - Optimized 

for I+-Inch Electrostatlc Shield 

I I NIA vs Frequency for 6-Inch by 3-Foot Vehicular---- 

Antenna of 12 Turns in a I$-Inch Electrostatic 

Shield with 1700/44 Litz Wire 

12 Unpadded Bandwidth vs Frequency for 6-Inch by------ 

3-Foot Veh icu l at Antenna of 12 Turns in It- I nch 

Electrostatic Shield - With and Without Litz Wire 

13 Cornpari son of Measured and Computed Reactance for-- 

Vehicular Antenna Prototypes Illustrating the 

Effect of Windi ng Reversal 

14 Balanced Tuning Circuits used In Vehicular--------- 

Antenna Prototype Match i ng to a Matched l mpedance 

Level of 200 Ohms 

15 4:l Balanced ( 200 Ohm 1 to Unbanlanced ( 50 Ohm)-- 

Transformer 

16 Tuning Curve for Prototype Vehicular Antenna Tunedto Nominally 520 KHz Showing Frequency Shift Due 

to Measurement over Dielectric and Steel Mounting 

surfaces 

17 Tuni ng Curve for Prototype Vehicular Antenna Tunedto Nominally 920 KHz Showin9 Frequency Shift Due 

to Measurement over Dielectric and Steel Mounting 

Surfaces 
Fi gure 

List of lilustrations (cont,) 

18 Magnetic Field Mapping of a Conducting Line--------------- 

Source in Free Space 

19 Graphical Field Mapping from Conductor Near--------------- 

Roof & Rib Corner Without a Vehicle Present 

20 Graphical Field Mapping from Conductor Near--------------- 

Roof & Ri b Corner With a Veh ic I e Present 

21 Graph i ca I Fie1 d Mapp i ng from Conductor Near--------------- 

Fioor & Rib Corner With a Vehicle Present 

21 Graphical Field Pkpping from Conductor on----------------- 

Floor Close to Vehicle 

Page 
1.0 INTRODUCTION 

This report covers antenna deslgn, analysis, and development work performed 

for the U.S, Bureau of Mines under Purchase Order PO382223 by Terry S. Cory, 

P.E. The anticipated usage of the antennas and accompanying analysis is 

for medium frequency wireless communications in coal mines. 

The work has l nc l uded a comp l ete veh icu l ar antenna des i gn experience 

through the prototype development stage, design analysis and comparative tests 

to be useful in eventual development of optimized portable PAIant~nna 

systems, and analysis of the coupling between vehicular antennas and 

existing mine wiring. 

The vehicular antenna work was an extension of earl ier configuration 

stud! es and pre l i mi nary des i gn ana I ys i s performd under USBM Contract 

H0377013 ( Collins/Rockwell on Wireless Communications for Trackless 

Haulage Vehicles, 

Medium frequency technology In coal nines Is rapidly aporoaching th: point 

where systems can be configured for use in particular minss, A key portion 

of the R&D invloves establishing a viable set of vehicular antcnna 

parameters so that coupling loss into or out of existing mine siring can 

be estimated for the case where the vehicle is in the same entry as the 

wiring, Such a set of parameters has been established and coupling estimates 

have been made as part of this work. Coup l i ng for remotel y located 

transmitters ( and receivers 1 has besn estimated as part of propagation 

rmasuremnt Contracts I40366028 and H0377053. The only part of the vehicular 

antenna problem for which there is no data is that where the antenna is 

si mu1 taneous I y matched to two freauenciss, The eventua l mi ne wi re l ess 

radio systems are expected to be two frequency systems to permit the use 

of repeaters, 

Portable radio systems including antennas at medium frequency have heen 

desianed and have undergone lirclted testing In coal mines. An optirlzed 

portable system probably has not yet been designed; particularly whm 

operation is expected in close proxi mi ty to ex1 st1 ng mine wf ri ng conductors, 

The integral design of the radio transrnltter PA and the antenna matchins/ 

phasl ng network as a system Is necessary for opt1 mum design. Tests of 

the portable equipment available have, however, provided a starting point 

for the general inclusion of portables into a complete system design, The 

analysis performed as part of this work will help to bound the physical 

characteristics of portable antennas and data is given on ths rzw ( untuned 

character of flat-strip winding conflguratlons. 
2.0 SUMMARY 

This program has provided antenna design analysis which has led to the 

design, building, and testing of a prototype mine wireless vehicular 

antenna prototype. The analysis also enabled the building of a portabls 

antenna wiring configuration which was tssted against the RACAL antenna 

and agai nst another portab l e antenna w i ri ng a l ternati ve provi ded by the 

Bureau. The antenna design activity also provided the basis for the 

prediction of antenna coupling into existing mine wiring which was 

performed using graphical field mapping. 

The design of vehicular sntennas is dependent on the operatlonal utility 

of particular configurations mounted on vehicles, on optimizing ths 

antenna parameters for achieving near-maximum NIA, on reaui remnts for 

intrinsic safetv, and to a lesser degree on the parameters of mine 

wireless radios, 

The design of portable antennas is dependent on human factors of carry ing 

and accomodation of variations in tuning due to the differences in human 

bodies, and on parameters of mi ne wi reless rad 10s to a sna ter degree 

than with vehicular antennas in addition to achieving hlgh NIA and 

providing i ntrinsic safety, 

The design of vehicular antennas is sufficiently independent of overall 

transcetver subsystem configurations that it is practical to build and 

evaluate such antennas separate from the complete subsystem. Ths deslcn 

of portable antennas cannot so easily be divorced from the radio design 

and, thersfore, should always be considered as part of the design of a 

complete portable subsystem. 

The emphasis of this program has been placed on defining vehicular antenna 

designs, Information from the design analysis is avallable to balp design 

portable antennas; however, the deslgn of an acceptable portable antenna 

was beyond the scope of this program, 

The coupling of mine wireless antsnnas to existing mins wirlng Is dependent 

both on the entry crossectional geometry (contalninqthe conductors and 

the antenna 1 and on tha antenna parameters themsel ves. The graph i ca l f ie l d 

mapping technique has been shown to provide a simple effective means to 

include all variables electromagnetically, 

The departure point for this program has included the composite of tec5,nology 

provided from previous med 1 urn frequency programs as dl scussed I n Secticn 1. 

Assumed as the basei lne for this work has been: 

(1) use of medium frequencies in the 100-1000 KHz frequency rang3 

with particular emphasis on the range 500-1000 KHz which has 

previously been shown to provide the most effective coupling 

to existing mine wiring 

(2) that coupling of carrler current into exlsting mlns wlring 

provides the principal transmlsslon means for wlreless 

commun i cat i ons 
(3) an effective mechanically rugged vehicular antenna design 

employing a multi-turn loop winding inside a rlgld electrostatlc 

shield Is feasible 

(4) an antenna desisn algorithm of proven vat idity Is avai lable to 

trade off the several antenna design parameters Involved in 

optirnizin~ antenna performance 

ANTENNA DES l GPJ 8, PERFOR4ANCE 

2, I , I PERFORPANCE CONS 1 OERAT 1 ONS 

Performance optimization and intrinsic safety are compatible bases for 

overall antenna design. That this Is true was not obvious a+ +be onset of 

the program, 

For a multi-turn antenna of constrained crossectional dimensions, the 

achievable NIA is quasi-independent of the number of turns, when wireresistance-limited, over a substantial range of turns, This range of turns, 

for a gi ven power source voltage, a l so fa l 1 s in the constant gower region 

of the intrinsic safety curve of inductance vs current ( ie, the current 

voltage curve for negligible circuit stored energy 1, The number cf turns 

beyond which the NIA decreases markedly also rou~hly corresponds to the 

number of turns beyond which the methane ignition current becomes constan+- 

energy-limited. The Q of a practical antenna Is ultimately limited by the 

losses in the transforning/tuning circuitry but becomes wire-rsslstancsl i ~i ted as the number of turns i s i ncreased or as the f requnncy i s i ncreased, 

At the design frequency range of interest, the wire resistants plus turnsdependent series tuning loss msistance when used with the maximum allowable 

inductance ( to prevent MIA fa 1 l-of f and energy-l irni ted safety 3erforrrance 

Is just about at the point where this resistance is squat +o 5b0 resistmce 

due to f i xed ( non-turn dependent ) losses, 

The NIA increases with decreasing antenna bandwidth and Increaslrg antenna Q, 

If the choice Is to desian to a fixed bandwidth by elther using fhe "in5erentU 

wire resistance as a limit or by padding an inherently hig?er Q antenna 

w 1 th externa l f 1 xed ser ies resi stance ( non-f requency-dependent 1, the 

external padding technique is always the best, Thls Is because its use gives 

h i gher N I A than the equi va l ent w 1 re-res i stance- l f mi fed des i gn and s l so 

because the bandwidth can be held near-constant over a ranse of frequency, 

If external padding resistance Is used, It also serves as a currsnt-limiting 

resistance for intrinsic safety purposes because its value is nearly the 

same at mf as It i s a t dc; whereas, the wire resistance is much lower at dc 

than it is at mf, 

As the MIA is quasi-independent of turns in the useful design range when 

w i re-res i stance- I i mi ted, the rss i stance 1 eve l of the antenna can be 

arbitrarily chosen. The actual operating resistance level is dictated by 

the desired bandwidth and/or the fixed resistance of the circuit; so to 

maximize NIA for a finished design, the wire resistance 1 eve1 should be 

at the high end of the range ( 1 e, approxi mate ly equa l I ing the f i xed res i stance 

or else the wire resistance plus the turns-dependent tuning rqslstance 

such as from an assumed constant Q capacitor should be equal to the fixed 

resistance 1. Thls Is accomplished by using as many turns as are allowable 

( per above definition 1. 
The above discussion deals with the simultaneous happening of several 

fortuitous conditions. The use of Lltz wire is germaine to this, whgre 

the strand size is made as small as ~ossible ( for vehicular antenna 

prototypes of this program, 1700/44 Lltz was chosen; lo, 1790 strands 

of 844 wire having approximately 6800 circular mils of area per conductor). 

The statement about maximizing NIA by having the wi re resistance qua l fo 

the fixed resistance assumes that the fixed resi stance a l ready is the 

lowest allowable to give the desired ( or required 1 operating bandwidth 

and/or to inhibit excessive current for intrinsically safe operation. A 

near-opt i mum overa I l des 1 gn w 1 l 1 resu l t when : 

(I) The inductance is made as large as is "allowable" and 

the wire rssistance is designed to be less than that 

to give the minlmum bandwidth per (2) below 

(2) The minimum bandwidth ( for comm system power level/detuning 

or intrinsic safety power source reasons > is establ ishsd 

by f i xed ( non-f requency-dependent 1 externa l rss i s*ance. 

Th is estab I i shes the "i nherent" i ntri ns ic safety of the 

antenna. 

I f an argument can be made for intrinsic safety "i nherency" basd on seri2s 

res i stance ( coveri ng both open-ci rcu i t-fau l t arc d i schargs and sudden 

connection of the dc source to the antmna clrcuit arguments 1, then 

blocking capacitors per UL-913 may not be required. Actuslly, a; with the 

vehicular antsnna prototype where an impedance match i nu transformer is used, 

series current-limiting resistance can be added at the " ac 57 ohm level" 

with negl i gi b le perfortrance degradat Ion as is suggested in +he next subsect Ion. 

It should also be pointed out that when considering the antenna winding 

current itself as a current source for a potential short circuit fault, the 

use of the highest allowable inductance and low winding resistmce wil I 

limit the peak ac current if the winding is shorted out as ths time constant 

of the winding is maximized. 

The mi ni mum bandwidth for communication system performance reasons on l y is 

determi ned from both modu l ation bandwidth and detun i ng considerat ions. For 

modulation bandwidth; if the system is SSB the bandwidth could approach 

3 KHz whereas i f the system i s FM with modulation 1 ndex = I, the bandwidth 

could approach 12 KHz. For vehicular antennas operating only with the loop 

in the vertical position, SSB and FM bandwidths of 3-6 and 12-16 KHz 

respecti vely may be feasi bl e. For portab I e operation, the bandwidth must 

be larger due to the detuning of antennas by shape variations when worn 

by different men and by differences in human bodies and in garments. 

Insufficient information is available to select a portable antenna minimum 

bandwidth for communication purposes. 
2,l. 2 PROTOTYPE ANTENNA DESCR I TI ON 8 PERFORMANCE 

The vehicular antenna prototype(s) built and tested durlng this program 

is shown in Figure I, The specifications for this antenna include: 

ELECTR I CALArea 

Turns 12 ea, #1700/44 Litz 

d i ameter 0.1 15 inch 

strand dlamter 0.002 inch 

area 6800 circ rnl l s 

spaced 0,250 inch 

Lo 127,3 uh 

L @ 500 KHz I90 uh 

Bandwidth 

520 KHz model 14 KHz , Q = 37,l 

920 KHz model 20 KHz , Q = 46.0 

NIA ( 20 watts ) 

520 KHz model 1.8 

920 KHz model 1 ,5 

Center frequency varlable + 5: vla external capacl tor 

Detun i ng 

520 KHz rmdel 7 Ydz steel vs dielectric munclng 

10 KHz vertical vs 45 degrss origntation 

920 KHz model 10 KHz steel vs dlelsctric mounting 

10 KHz vertical vs 45 degres orientation 

Maximum Power ( I irni ted by use of 350 VDC trimmer capacitors) 

520 KHz made1 46 watts 

920 KHz mdel 9 watts 

1 nput 1 rnpedance 50 ohms nominal 

Intrinsic safety none Inherent in prototypes 

except a1 l-metal enclosure 

MECHAN I CALPaxi mum 1 ength 40 inches ( includlnp circuitry enclosure 

Pilaxi mum w i dth 12,3 inches w/ antenna horlzon+al 

10 inches w/ antenna vertical 

Maximum height 8 inches w/ antenna vertlca l 

2.2 inches w/ antenna hori tonta l 

Electrosi-atlc shield 14 inch diarne+er nominal 

Element orientaion vertical 

45 degree 

hor i zonta I 
L .- I-.:. . i 

L VEH I CULAR AIITEII'IA PRDTOTYPE 

SHOWI-J W lTH LOOP 1i.I THE 45 DECREE 

Oii I ErlTAT 1011 

VEH I CtJLAR A: ITC'!'IF( PROT3TYPE 

SHON:I WIT11 LOOP 1'1 TtiE HORIZ0:ITAL 

OR I El ITAT I 0' I 

FIGURE 1 

VEH I CULAR AtlTErll.JA f.i?OTOTYF3L 
The vehicular antenna munts to a horizontal steel surface on the vehicle 

via 4 ea magnets located in the corners of the muntl ng frame. The antenna 

may be positioned in any of the three principal orientations by pushing the 

rear housing against the mounting frame ( against the spring tension 1, 

rotating to the desired position, and releasing the element alot~ing it to 

seat in one of the three detent positions. 

The mechanical configuration of the vehicular antenna has been made as close 

to that expected to be useful as possible. The basic antenna winding has 

been designed to be as close to optimum,providing that proper aftention is 

paid to the matching circuitry, as posslble. The prototype antennas have 

been provided with BNC connectors to fscil itate evaluation. Each of +he 

two prototypes provided have been tuned to a different single! frequency; 

one to 520 KHz, nomi na l and the other to 920 KHz nomi na I. The w i nd i ng 

sense of the turns is important to obtain minimum distrlbufed capacitance 

and highest resonant frequency. The wlnding sence of one of the antennas 

was inadvertantly reversed in manufacturs, resulting in a stlgLtly higher 

reactance. This antenna was tuned to 520 KHz whers the d i f ference was 

negl i gi b l e. To accomdate a two-f requency system p l an, the match i ng c i rcui try 

may eventr~a 1 l y have to be md i fled so the antenna i s tuned si mu1 tansour l y 

at two frequencies. 

No attempt has been mde to ~rovide inherent intrinsic safsty via fixed 

externa l ser i es res i stance. The part i cu l ar Q of the f i n i shed prototypes 

is estab l i shed by the tun1 ng cl rcui try and the transformer package as 

mounted in the enclosure. The unloaded Q of the antenna(s1 is ap~roximately 

400 @ 500 KHz. The tuning and matching circuitry used components whlch 

were available. The antenna(s1 is tuned with a balanced capacitive L-section 

to 200 ohms and then transformed to 50 ohms using a 4:l ferrite transformer 

suppl ied by the Bureau. Fixed si lvered mica capacitors in +ha L-section 

were rated at 1000 VDC; however, the a i r variabl e tri mmers were rated at 

only 350 VDC and thus provide the limlting condition for power handling. 

The voltage handling could be Improved to at least 1000 volts by using only 

f i xed capacitor comb i nations or el se by subst i tut l ng vaccuurn var i ab l es for 

the air variable trimmers. 

The antenna winding is protected from the prlmary power source via the 

transformer winding whlch operates at a 50 ohm level on the primary at ac. 

If used with a radio driven from a 12 volt source, for example, a series 

current-limiting resistor of 1.2 ohms wpuld limit the currsnf to 10 amps. 

A current-l lmiting resistor of up to 5 ohms could be added to the antenna 

winding ( 2.5 ohms to each balanced side 1 with l lttle performance 

degradation. In addition, the antenna is com~letely enclosed in metal except 

in the region of the feed gap in the electrostatic shield which is plastlc. 

This should provide some i nherent protection against methane i gnitlon if 

the enclosure is made gas-ti aht. 
The performance considerations presented early in this section are 

predicated on having an antenna of fixed dimensions which Is not 

degraded by variations in mounting or by proximity to lossy objects 

when used. These considerations are, thus, more appropriate for 

vehicular antennas than for portable antennas, Knowledge of these 

considerations logically leads to a design procedure which will 

give a near-optimum antenna of either type, however, 

A portable antenna has different and subjectively more severe design 

requirements than a vehicular antenna. To lend insight into the overall 

portable antenna problem, it wi ll be useful to list several of these 

requirsments, These include: 

(1) The antenna must be flexible or at least semi-flexible, 

The antenna, to be operat iona l l y useful , must bs "worn" 

by the operator in "bandol ier" fashion ( over one shoulder 

and under the opposlte arm ) to provlde a non-restrict1 ve 

"hands-of f" user capabi I ity, 

(2) The antenna tuning and matching circuitry should be 

provided integral to the radio enclosure to minimize bulk, 

potentia l damage, and to maxi mi ze current trensfsr. 

(3) The antenna should be tuned for placement on a person's body. 

The antenna will be deQfed automatically by placement on 

a person's body ( equivalent to added series resistance) 

(4) According to (3)) the antenna must accomodate a "detuning" 

bandwidth range due to shape changes when worn. 

The approach to portable antenna design in this program has been t o t r y 

and establ ish an upper bound on I'JIA performance for the raw unmatched 

antenna based on variations in antenna design parameters which can be 

handled with the antenna design algorithm; then to compare the effect 

on the raw antenna of "wearing" a conf i auration approxi mating this upper 

bound. This has been accornpllshed by using the antenna design algorithm 

to trade off antenna wlre spacing, wire slze, and number of turns; 

selecting a set of parameters; and running comparative tests of this 

configuration with other configurations, A configuration was chosen which 

represents the probable largest size of a f tat strip conf iguratlon using 

equally spaced turns, The large size included the greatest length, width, 

and wire size conceivable to be feasible for such an antenna. The testing 

was conducted by BuMlnes comparing this configuration, first, with a 

si mi I ar spaced turn conf 1 guratlon of sma l l er slze ( and using stranded 

wlre instead of the Litz wlre used in the upper-limit conflqur~tlon and, 

second, with the RACAL antenna. 

Previous experience with portable antennas has been with the rigid 

el liptical shaped antennas built by ECAM and by Collins whereby a "bunch" 

of stranded wires were qlaced Inside an approximate 3/4-inch diameter 

plastic tube of 0,217 rn area, These antennas both had a self-contained 
matching network to 50 ohms, The Col l ins antenna used 7 turns of #I8 

wire and at 520 KHz, the antenna Q was 44 and the bandwidth was 12 KHz. 

The NIA was approximately 2.5 for an assumed 20 watts input. 

This type of antenna has been since deemed fmpractical for use In coal 

mines because of its rigid configuration and because the coax connecting 

the antenna to the radio was always getting in the way. The Collins 

antenna, by virtue of the "bunchedt' rather than spaced turns was 

w i re-res i stance l i mi ted and unsu l ted for ef f i c i ent operation above I FlHz 

( resistance i ncreases from 10 ohms at I MHz to 75 ohms at 1.5 MHz 1. 

The choice for evaluation during this program was the flat sirip consisting 

of equal l y spaced turns due to the expected I rnprovemn+ in raw antennz 

performance ( hi oher sesonant frequency, hi gher Q, lower wi r3 resl stance, 

amd lower inductance 1. The range of practical antenna lengt5s for 

portable antennas was estimated to be frgm 5 feet ( 3rea of about 0.15 m2 1 

to about 6.2 feet ( area of about 0.23 rn . The wire configura+i~n bui It 

for testing ( i I l ustrated in Figure 2 together with the comparab l e PuMi nes 

sma l ler conf iguration) conformed to the fol lowing: 

ELECTR l CAL 

Area 

Turns 

2 0.23 m ( approx. for el l iptical 

shape computation) 

6ea #1700/44 Litz ( same as vehicular) 

spaced 0.67 inches 

P;lea s u red Measured Computed 

(bench 1 (on body) 

Q 500 KHz 310 

1000 KHz 430 

250 

200 755 Lltz 

213 solid 

Bandwidth 500 KHz 1612 2000 

1000 KHz 2496 5714 1325 LItz 

4691 sol id 

MECHANICAL 

Maxi mum I ength 

libxi mum width 

Construction 

74 inches 

3.5 inches ( winding) 

windlng skin-packed I polyethy!ene ) 

to 6-inch wide strip of fibreglass cloth 
-

FIBREGLASS CLOTH WITH POLYETHYLENE 

SKINPACKED COATING TO ENCAPSULATE uuuu 74 INCH 

ANTENNA CONDUCTORS 

Fl GURE 2 

PORTABLE ANTD.lt.lA TEST CON F I GU
Six turns were used to provide cornpatability with exisTing designs for 

comoarison purposes as the raw antenna data only was to be compared. 

The test design was not optimized in the sense previously discussed in 

this section. For a raw antenna, assuminc a perfectly lossless natcning 

basis, the highest NIA ( and highest Q 1 occurs for 4-5 turns; so, 6 

turns are adequate to enter the re Iati ve ly f I at quasl-turns-independent 

region in NIA at the low turns end of the region. 

The comparative testin9 showed little if no advantage of the test design 

vs the smaller-in-crossection Butlines configuration when placed on the 

body ( comparative Q's of 200 vs 160 1. There Is a question of whether 

or not a final loaded Q of as great as 100 is feasible for portable 

antennas due to potential detuning variations, 

There was substantial diffsrence between the resonant frequency of 

a1 l three antennas tuned on the bench compared with tuning on the body. 

For all three cases, the frequency shift ( for the saEe tuning capacitance 

varied from 20 KHz @ 500 KHz to about 50 KHz 8 iOO9 KHz, We know that 

tuning on the bench is not a real lstic condition, however, and comparative 

tuning on several different bodies would have been better. Cot I Ins found, 

for an antenna Q of 44 I? 520 KHz, that carefu l tuning of their antenna 

in air ( antenna suspended from the ceiling 1 did net differ significantly 

in frequency shift from tuning on a person's body. A truly optimized 

antenna may not permit this latitude, however. 

As can be seen, considerably more work needs to be done to derive optimum 

portable antenna performance characteristics, The work acconpl ished on 

portable antenna design and analysis during this program has shown: 

(I) there is probably no advantage in using antennas of lar~e 

crossectlon ( size of wire and wire spacing ) 

(2) there is probably no advantase in using Litz wire 

(3) it would be very difficult to build an antenna with bandwidth 

of less than 6 KHz t? I VHz even if this low a bandwidth 

were proven to be feasible 

(4) at least 5-6 turns are required on a portable antenna to enter 

the quas1-maxlmum NIA range. 

To complete a design for a portable antenna, the fol lowing steps should be 

taken : 

(I) determine the radio PA output source impedance level and 

the practical maximum drive current level 

(2) determine the Q ( equivalent added series resistants ) which 

can be provided using high quality tunlng compcnents for 

a matching circuit i~plementable In the radio package 

(3) determine the minimum allowable bandwldth from detuning and 

modul&+ion considerations ( 12 KHz appears to be feasible 

from the previous Collins work 1 
(4) choose an antenna crossectional slze including overall width 

and wire size 

(5) use as many turns as possible while keeping within both the 

NIA-flat range and the constant power portion of the intrinsic 

safety curve(s>( fhis mans trading off turns vs total 

equivalent series resistance in the PA-tunins circuit-ant, rnnna 

path ) 

(6) pad with additicnal current-l irnitlng resistance to obtain 

the amp l i f i er current ti mi t and/or the mi n l mum bandwidth 

Note: The optimum design will probably be wire-resktance limited due to 

the added equivalant wire resistance caused by losses In tb human 

body, Any padding will probably facilitate only intrinsicly safe 

operation and, if used, will result in some devrease In performance. 

The resulting design mzy still have an tllA comparable to a vehicular 

type design of the same size, however, as the impedance level will 

be lower than the 50 ohms dktated by use of coaxial cable connecting 

separated transmitter and antenna 
3.0 TECHN I CA L APPROACH 

3.1 GENERAL DESCRIPTION 

The departure point for this work is based on results obtained during 

the previ ous l y referenced Contract H03770 13 ( WI re less Commun i cat i ons 

for Track less Haul age Vehlc les 1. Thi s work led to an antenna design 

algorithm for use in antenna parameter studies, Preliminary encineering 

model testing resulted in the dsf in ition of a vehicular antsnna 

conf i guratlon and revealed severa l important results, 

The antenna design algorithm will be delineated in the next subsection 

together with some new work which allows the incorporatlon of Litz wire, 

The results of this previous testing incl ude: 

(1) use of turns spaced as far apart as possible rssults in the 

lowest impedance, highest 3 raw antenna configuraticn for 

a given number of turns 

(2) a multi-turn winding may be snclosed within an elsctros+atic 

shield with only a single circumferential gap and still acnielve 

the mu lti-turn effect 

(3) If the turns are symmetrically located within %he slectrost~tic 

shield and balanced-fed, then the shield has a ~sgligible 

effect on antenna Inductance 

(4) If the electrostatic shleld is constructed-of non-ferrous 

metal and if the antenna winding is disolaced sl Tghtly from 

a ferrous mounting surface, then the winding resistance Is 

comparable to that in absence of the shield and "he mounting 

surf ace 

(5) If the Individual turns are connected so that ad-iscent electrical 

turns are not physically side-by-side, then ths distributed 

winding capacltance is rninirnlzed and the winding resonant 

frequency l s maxi mi zed 

These results led to definlng the vehicular loop configuratio~ i l lus+ra+ed 

in Section 3.2.4 where the electrostatic shield provides a rlc~ld rugged 

enclosure suited for use on mine vehicles. The engineering mdsl te.tsd 

during the previous program employed 7 turns within a 3/4-inch ID s+l~ld. 

Ana lysi s at that time suggested that up to 13 turns within a I -i ncn ID 

( I 1 /4-l nch OD shield wou Id probabl y work better, The mountlng conceat 

for the vehicular antenna was that of using a magnetically mountable frame 

for the antenna. 

The results lended further insight toward the use of parallel spaced turns 

arranged in a flat strip for portable antenna use. 
3.2 THEORET l CAL BASEL l NE 

3.2.1 ANTENNA DES l GN ALGORITHM 

During the Wireless Communication for Trackless Haulage Vehicles program 

( USBM Contract with Collins, H0377013 1 an algorithm was developed to 

compute the impedance and >JIA of multi-turn loop antennas. Although the 

algorithm was originally intended for use with spaced turns In a singlelayer solenoid configuration ( suited for rnan-carried antenna analysis ), 

it was found to be applicable for use with spaced turns in a circular 

configuration enclosed by an elsctrostatic shield ( sui'ted for vehicular 

antenna analysis 1 orovidlng that the winding configuration was chosen 

for minimum distributed capacitance and providing that the winding was 

balanced-fed with the electrostatic shield being placed at ground potential. 

The algorithm was programmed for the HP-67 calculator so that parametric 

antenna data could be computed. The algorithm is given again in this 

subsection, 

The antenna inductance, L , Is given by 

where, L is the low frequency Inductance given by 0 

2s s 

= 0.02339 n2 [(sl + s21 log I2 

Lo lo no 

microhenries 

sl' s2 are the side lengths in inches 

g is the diagonal in inches 

n Is the number of turns 

D is the wire separation in inches 

fr is the self resonant frequency given by 

C is the distributed capacitance of on3 turn 

and is 
1 is the length of one turn in meters 

d is the wire diameter in inches 

0 is the effective wire separation In inches I 

0 is greater than D by about 17% I 

The antenna resistance, R , is given by 

where, for copper, 

R is the low-frequency resistance given by 0 

f+ = n 3 in inches 

The NIA product for matched conditions into 50 ohms is given by 

50 PIIA = NA lolT 

where, lo is the input current into 50 ohms 

(exp, 0,632 amps for 20 watts 

The antenna resistance is that for solid or stranded copper wire. The use 

of Litz wlre has been considered during thls program to reduce the wlre 

resistance component of the total antenna series resistance including losses 

external to the winding, The characteri stlcs of Litz wire are considered 

in the next subsection, 
3.2.2. LIT2 VS SOLID W.I2E COMPARISONS 

Litz wire consists of a number of separately insulated strands woven 

or bunched together so that each strand tends to take all possible 

positions in the crossection of the entire conductor. The purpose of 

Litz wire is to reduce skin-effect losses by distrlbutlng ths flow of 

radio frequency current more uniformly over the wire crossection. 

The use of Litz wi re may or may not have a practlca l advantap over 

normal solid or stranded wire for mf antenna designs, depending on the 

overall loss resistance In the complete matched antenna and the magnitude 

of the component of this loss reslstance attributable to wire r5sistance. 

The objective of using Litz wire assumes that the wire resistance 

component wi l l be si sn i f icant and therefore ( in at least the prototype 

development) is to minimize the dependancy of this total series an+enna 

circuit loss resistance on the frequency dependent wlre resistance 

component. This offers the possibility of obtaining near constant 

antenna bandwidth over a range of frequencies. This bandwidth is, In 

principle, obtained by adding a fixed non-frequency-dependent resistance 

at the antenna terminals. 

The resistance of Litz wire is frequency dependent with the resistance 

being a function of the overall wire diameter, d, the strand diamtsr, dS, 

and the wire-wire spacing, D. A low frequencies, the use of Lit? wire 

results in a significant reduction in the low-frequency ac resistance, R . At higher frequencies, typically I - 3 MHz, the advantage disappears and 0 

Litz wire can actually have a greater resistance than solid wire. This 

latter effect is primarily a function of the strand diameter, d , and 

operation at h l gher frequencies requ i res a sma l l er strand d T ame?er and, 

hence, more strands for a fixed desired wire diameter. 

Terman gives an expression for the ratlo of R to Rdc for Litz wl re. 

Fo antenna design, we are most interested in th8 ratio RoLITZ / R L~~ 

so that the R LITZ can be used wl th the loop antenna des~ gn al gor~?f!m. 

The author hag never seen curves of this ratlo plotted in the I iterature, 

so these curves as applicable to antenna design were developed as part 

of this program and should serve as a standard reference for the future. 

The appropriate formul ations for R 

0 ITZ / Rdc and R / R will be 

derived and then ratioed to obtain \he above measur~S8blF~ tz $!?re 

advantage for.antenna design. 

For a sol id wire, 

Kbm 

where A for multi layered coi Is = i( 1 

D 

with Kb/D = 5 typically and with m 7 3 

layers typically, so that A = &(I51 = 56.25 

G is a proximity factor for multi layer 

coils varying nearly linearly with x 

( x = 0.1087 d fl for copper) for larger 
vat ues of x so that G Is approximately 

equal to 0,0439 d f. 

H is the ratio by which Rdc of an isolated 

wire is multiplied to give Rat and = 

approximately 0.0962 d fi for copper. 

therefore, d 

Ro~~~l~ / Rdc = 0.092 d fi + 2.469 d fi ( ,, 1 

For Litz wire, d 

2 

Ro~~~ / Rdc =I+n G( 

d D 

4 where, G = approx. x /64 = 0.873 x 10" d4 f2 7 s 

The ratio of these quantities has been evaluated for a range of parametars 

useful in antenna desi gn and the results are given as Figures 3 through 5 . 
RESISTA!JCE OF LIT2 WIRE COMPARED TO SOLID WIRE 

FOR F l XED O\/ERALL D I AMFTER AflD STRAIJD D I AMEISER 

VS W I RE-W I RE SPAC I I.IG 

1703/44 LlTZ dS = 0.002 IN 

d = 0,115 IF4 

FREQUENCY - KHz 
Fl GURE 4 

RES l STANCE OF L l TZ W I RE COPPARED 

SOL 

APJE'f 

OVE 

1000 

FREQUENCY - KHz 
FIGURE 5 

RES I STANCE OF L l TZ W l RE COtlPARED 

TO SOLID WlRE FOR FIXED OVERALL 

Dl APETER AND W I RE-W I RE SPAC I f3G 

VS STRAND S IZE 

D = 0,250 IN 

d = 0, 1 15 1!~1 

= 6800 C l RC MI LLS 

WlRE 

= 0,905 

FREQUENCY - KHz 
3,2,3 INTRINSIC SAFETY PARAMETERS 

l ntri ns ic safety requi rements are geared toward prevent i ng potenti a l l y 

dangerous dc sources from producing arcs which might explode a methaneair mixture under fault conditions in coal mines, The curves for minimum 

voltage-current combinations, minimum ignition current in inductive 

circuits, and minimum ignition voltage in capacitive circuits apply 

strictly to dc excitation of circuits. The ac excitation of antenna 

circuits at radio frequencies are not explicitly covered by the curves. 

The proceedure for inductive cI rcui t protect1 on now be1 ng appl ied 

involves isolating the antenna circuit from potential dc sources 

through the use of blocking capacitors, The requirsments for these 

blocking capacitors come from UL-913 whereby two capacitors are pl~cgd 

in serles and then tested for voltage breakdown with an ac voltage -bevel 

equal to twice the working ( potential fault 1 voltage + 1900 volts rms. 

It is reasonab l e to expect that arcs from ac sources at rad lo freauenci es 

under fault conditions could explode a methane-air mixture in a circuit 

not intrinsicly safe at dc. It Is also reasonable to assert that dc 

ci rcui t performance represents the worst case; thus, a ci rcui t which i st 

"dc intrinsical ly safe" wi l l also be "ac intrinsical iy safe". 

It is probably to the designer's advantage to take any steps hward 

making an antenna "dc i ntrinsical ly safe", over and above any Slockl ng 

isolation, if this can be acconplished without serious perf~rnance loss. 

This could be termed the "inherent safety factor" of the antenna. 

Toward evaluating ac intrinsic safety, it may be worthwhile to define 

operating power levels for which the antenna would be intrinsicly safe 

if the dc and ac .parameters were the same and the dc curves could b9 

appl ied. 

It Is possible to at least partially~otect a loop antenna without loss 

of performance so as to achieve some inherent safety, ThIs is possible 

because the NIA of a loop antenna of restricted dimensions is relatively 

constant over a range of numbers of turns and because thsre are ac losses 

in the antenna other than those encountered in the wire reslst?nce, 

Specifically: 

(I) impedance level; the antenna low-frequency wire reslstance 

should be as high as is practical wi thout reduclng YIA in 

order to minimize the peak current which could flow in the 

circuit. ThTs is accomplished by using as many turns as 

possible, 

(2) added series loss; the antenna matching circuitry losses will 

produce an equivalent series antenna loss reslstance in addltion 

to the w i re res i stance, A l urnped current- l i mi t i ng rss i stor of 

value comparable to the above loss reslstance can be added 

without reducl ng the fllA sign i f icant ly. The l umped resi st~nce 

will retain its value at dc whereas the wire resistance wlll not. 
It is better, in designing to a fixed allowable wire resistance, 

to use Litz wire and then add a l umped current-l imi ti ng 

resi stor than to use sol i d wl re or stranded wi rs havi na the 

same resistance as the Li tz p l us the I umped res i stor. 

Assuming the dc curves apoly, the current in the loop as a function of 

input power is given by 

In terms of stored energy, 

where, P is the input Tower 

L is the inductance 

(BW) is the 3 dB bandwidth 

so that for a fixed maximum energy level, the allowable bandwidth is a 

function only of the input power. If for a fixed input power and bandwidth 

the current is to be minimized, then the current is inversely proportional 

to inductance, so the inductance should be as large as possible while stlll 

maintaining the bandwidth. 

The minimum ignition current curve Is given for 24 volts and essentially 

plots a constant energy level, which deternines the bandwidth power 

relationship for that voltage level. For other voltage levels ( as defined 

on the voltage-current curve 1, the constant energy level varles. If the 

antenna parameters are fixed, as in any particular desi gn, th9n the 

power cannot be independent of energy level and must follow tho voltagecurrent curve. 

The inductance of the vehicular antenna @ I MHz will be shown In the next 

section to be approximately 190 uh . To illustrate possible power-bandwidth 

limits for "ac intrinsic safety*(", calculatTons were made assuming 

(0 a constant energy of 0.5 millijoule 

(2) energy-power as defined from voltage-current curve 

and antenna parameters 

(3) inherent intrinsic safety provided by 5 and 10 ohm 

current-l imi ti ng resistors 

The results of these calculations are given in Figure 6 . This figure shows 

the voltage-current derived condition to be the most stringent in restricting 

the minimum bandwidth for higher power levels for a 50 ohm matched antsnna. 

Using this condition as the basls, a IO-ohm current limiting resistor ( for 

example provides inherent protection UD to a power level of about II watts. 
FIGURE 6 

POSSIBLE AC INTRINSIC SAFETY CRITERIA 

US I F,IG BANDW I DTH OF ANTEFINA AS A F4EASUf 

SHOW 1 NG MI N 1 FlUM BANDW l DTH AS A FUNCTI ( 

OF INPUT POWER 

LL 1 JOULE 

5 10 15 

INPUT POWER - WATTS 

-28- 
3,2,4 ANTENNA WIhlDING CONFIGURATION CONSIDERATIONS 

The generic configuration of the vehicular antenna winding is i I lustratsd 

in Figure 7 This shows both the winding and the electrostatic shield. 

The w i nd i ng crossect i on has two i rnportant aspects; the wi rs connect ion 

arrangement and the symmetry of the ci rcu l arly arranged spaced turns for 

balanced excitation, The overall winding acts as a near-perfect transformer, 

transforming the current in each winding turn into current flowing on the 

outside of the electrostatic shisld considered as a sinsle turn when 

transmitting. The net current flowing in the spaced turns excitss a 

current flow i ng on the inside of the el ectrostat lc shield wh l ch flows 

"around the corner" at the feed gap and on the outside of the shl eld as 

a single turn, This is truly low-irpedance current coupling. On receiving, 

the converse happens; however, the open circuit voltage induced across 

the gap due to an incident electric field does not excite current :flow 

in the internal windings. 

The connection of the winding turns in relation to one another is made is 

a manner which distributes the turn-turn ca~acitance so that nc adjacent 

turns are connected togsther consecutivel\{ and the adjacent turn is 

several turns away capacitlvely. Also, ++e entire windlna pot?n+ial is 

never across two closely spaced turns 5ut is across turns sp?ccd et laasf 

a thi rd of the way around the ci rcu lar arrangenent. The connsc+i~n 

arrangemnt shown in the fisure is the one used for the prototype 

vehicular antennas to be described in the next section. The balance is 

1 I l ustrated by the + and - sians attached to the turns. For balanced 

excitation, there are as many + turns as - turns wlth symmetric potential 

so the distributed differential mode coupling to the inside of the 

electrostatic shield as appeard across the windinq concels out. There 

is, thus, no increase in distributed capacitance due to the electrostatic 

shield. 

The generic configuration of the man-carried antenna is shown In Figure 2 

This is a flat spaced turn antenna of fixed total width so that an 

l ncrease i n the number of turns decreases the turn-turn spacing. Severa l 

connection arrangements of the windins turns are possible. As a final 

matched and evaluated antenna was not delivered to the Bureau for the 

man-carr i ed conf i gurat ion, no preferred conf i gurat ion arrangement for 

the turns has been established. 
Fl GURE 7 

W I ND I NG AIJD W I RI fJG CONF I GURAT I OfJ FOR THE VEH l CULAR ANTENNA PROTOTYPE SHOW l NG 

THE CONNECT l Ot4 ARRANGEMENT OF TURNS TO t.1 I t4 l fit I ZE D I STR l BlJTED CAPAC I TAIJCE 

- 

CIRCUMFEREN

6 l NCHES 

ELECTROSTAT I C SH l EL0 

BALANCED FEED LINE 
3.3 SELECTION OF DESIGN PARAMETERS 

This section covers the use of the antenna design algorithm to investigate 

parameters for the vehicular antenna, with the subsequent selection of a 

part icu l at set of parameters for the prototypes. 

The first winding parameter to be selected was the wire size within the' 

constraint of providing a variable number of turns to fit within a 

nominal I$-inch copper electrostatic shield ( ID = 1.265 inch ) allowing 

for a small spacing between the wires and the inside wal 1 of the shield 

tube. The effective maximum diameter of the antenna windins crossection, 

center-center, was chosen to be I-inch. The tubing size itself was 

chosen as a compromise to provide a reasonableanount of antenna area for 

a maximum antenna height of about 8 inches when in the vertical position. 

The result was the selection of 6-inches center-center in vertical height 

between the winding crossectional centers with a standard tubing diameter 

of 13 inch es. Based on the Trackless Haulase engineering prototype data, 

the number of turns was expected to be less than 20 with a Guess of 

roughly 12 turns at maximum diameter as being nominal, The wire-wire 

spacing range was bounded to be within 0.125 - 0.50 Inches; governed 

by the number of equally spaced turns In circumference around a I-inch 

circle. 

With the nominal spacing range chosen, the wire size range was bounded 

to lie roughly between $-inch as a maximum and a quarter of that as the 

mi nirnum. Parametric data for the ratio of the resistance of Lltz wi re 

to sol id wire was run for a range of Litz strand diameters; keeping 

the resistance ratio less than unity up to at least I !!Hz. The bounding 

range of strand diameters was p laced to be from #46 to 636 wire. Thls 

parametric data is given in the previous subsection as Figures 3, 4, and 

5. The wire size was chosen as being 0.115-inch in diameter corres~onding 

to 6800 clrcular mils and consisting of 1700 strands of if44 wire. 

The remaining major antenna parameters to be chosen were the antsnna 

length and the number of turns ( which also dictates the wire-wire 

spacing 1. These parameters were investiagted in terms of antenna NIA 

and bandwidth, assuming I amp into a 50 ohm antenna as a reference 

( 50 watts 1. The bandwidth range for boundlng purposes was taken tg 

lie between 3 KHz as an absolute minimum for SSB modulation and 12 KHz 

as a minimum for FM modulation. It was recognized that these bandwidths 

could be used for analysis evaluation purposes even though the achiwable 

"loaded" bandwidth In a practical design was, as yet, unknown. 

Parametric data was run over a range of turns from 6 to 20 for antenna 

lenths from 2 to 4 feet.at a frequency of 1000 KHz ( the optimum frequency 

as determi ned from the previous mf measurements for coup I i ng i nto 

existing mine wiring 1. This data is shown plotted In Figures 8 and 9. 

These curves clearly show that fllA is fairly constant over a range of 

turns for a constrained antenna crossection ( wire-wire spaclng decreases 

linearly with increase in turns and then rapldly drops off as the 

antenna resonant frequency is approached, At this point in the analysis, 

an antenna length of 3 feet was chosen. 
Figures 8 and also show bandwidth conditlons for the raw antenna. For 

the 3-foot antenna, the use of 12 turns ( for example provides Just over 

a 3 KHz bandwidth for Litz wire compared to a 12 KHz bandwidth for solid 

w i re. A ca l cu l at ion was made to determl ne i f the NI A were hi gher for a 

12 KHz bandwidth antenna l imi ted by natura I wi re resi stance ( Lit? ) or 

limited by fixed resistance padding of the above lower bandwidth design. 

For this calculation, to lncrease the natural bandwidth to 12 KHz, tke 

nominal I-inch diameter of the spaced turns winding was decreased to 

&-inch with the nominal wire-wire spaclng decreasing from 0.250 inc? to 

0.125 inch. Figure 9 shows that the padded YIA Is always higher. 

Recognl zi ng the need to use as many turns as possible to keg? the 

irrpedance level hi ah i n order to counteract losses in the antenna matchi no 

circuitry, the number of turns was chosen to be 12. This is the number 

of turns for which a 3-foot antenna and a 4-foot antenna deliver equal 

performance. 

With these parameters chosen, NIA and bandwidth for the raw antenna 

were computed as a function of frequency from 100 KHz to 2000 KHz. These 

resu l ts are g i ven i n Fi uures I 1 and 12. Fi gure I i shows the effect of 

using fixed resistance padding to achieve the bounding minlmum bandwidths 

of 3 and 12 KHz. Using this padding, the NIA Is seen to be essentislly 

frequency independent. This is an inherent property of an inductive 

ci rcui t. 

These raw ( unmatched 1 antenna results wi l l serve as a basis of compari son 

for the prototype performance actually achieved as discussed in Section 4.0. 
FOR 

000 

ICH 

VS 

6 B 10 12 14 16 18 

# OF TURNS 

-33- 
FIGURE 3 

RES 1 ST 

TO GIV 

N I A FOR VEHl CULAR ANTENFJAS 6-INCHES H l GH VS 

LENGTH AND NUMBER OF TURNS FOR 1 700/44 L ITZ 

W l RE FOR .3 KHz 141 N I P.1UF.I BANDW 1 DTH @ I000 KHz 

AND BELOW - OPT l MUM FOR 1 $ l NCH ELECTRO 

STATIC SHIELD 
FIGURE 10 

N I A COMPAR 1 SOFl FOR 6- I NCH BY 3-FOOT 

VEH l CULAR AtJTEtJI..IA DES I GNS FOR 1 700/44 

LlTZ WIRE FOR 12 KHz MIPJIMUM BANDWIDTH 

@ 1000 KHz AND BELOW - OPTIMIZED FOR I$ 

INCH ELECTROSTATIC SHIELD 

PADDED 
FtGURE I I 

N LA VS FREQUENCY FOR 6-1 NCH BY 3-FOOT 

VEHf CULAR ANTEI4NA OF [2 TURNS IN A I & 

f NCH ELECTROSTAT f C SH l ELD W I TH 1 700144 

LiTZ WIRE 

SOLID WIRE 

UFIPADDED 

Ir)OO 

FREQUENCY - KHZ 
FIGURE 12 

UNPADDED BANDW I DTH VS FREQUENCY FOR 6- I NCH 

BY 3-FOOT VEH l CULAR ANTENNA OF I2 TURNS 1 N 

I& INCH ELECTROSTATIC SHIELD - WITH AND 

WITHOUT LIT2 WIRE 

FREQUENCY - WZ 
4-0 VEHICULAR ANTENNA PERFORMANCE 

This section presents selected deslgn det?tls of the vehicular antenna 

prototypes and measured performance characteristics, Conclusions are 

also drawn between the measured results obtained with the prototypss 

and increased performance achievable with additional design effort in 

the i mpedance match i ng area. 

Two vehicular antenna prototypes were built, Originally, these prototy~es 

were to diffsr in bandwidth; one to be as near 3 KHz as possible and +he 

other to be as near 12 KHz as possible. Later in the program, it was 

decided to build them in an essentfally identical manner' tuning them 

to different frequencies of nominally 520 and 920 KHz. 

These antennas were wound with the connection arrangement illustrated 

in Figure 7, The turns were laid out and attached to a thin piece cf 

styrofoarn as individual wires ( broken between turns I and 4 1 then 

folded into a circle and pulled through the tubing, The connection 

arrangement provided for a sl i ght overlap between turns 66 1 and 4&7 

when pu l led through "corners" of the tub i ng hav i ng restricted d i meter. 

The winding turns were then connected tosether in the junction box 

attached to the tuning ( see Figure I 1. Unfortunately, one of the 

antennas was inadvertently connected in a reverse arrannercent to teat 

shown in Fi~ure 7which resulted in higher distributed capacitance. It 

was decided to make this the 520 KHz unit where the lower frequency 

minimized the effect of this inadvertsnt mistake, The raw antmn? rqxtance 

is shown plotted in Figure 13 along with the computed rsactance. 

The antenna matching was accomplished using a balanced capacitive L-section; 

transformi ng the antenna i mpedance to 200 ~hms ba l 3nced. A ferri te l4 : I 

balun transformer ( provided by the Bureau was used to down-tr~rsform 

from 200 ohms balanced to 50 ohms unbalanced for the coaxial input, The 

capacitive L-section schematics are given in Figure 14. The ferrft~ bdun 

is illustrated in Figure 15, The L-sectlon components were Sraedbo3rdgd 

on a small piece of printed circuit board, The L-section components were 

chosen from those locally available. The limiting component type frgm 3 

power handling standpoint was +he trimmer capacitor, These are referred 

to in the figures as air variable; actually, they were of the layered 

type having strips of mica between metal plates and air, Increasing 

screw pressure provldes increased capacltance with decreased spaclng 

between the layers, The power handling properties could have been 

improved if vaccuum variables had been available, The L-sections ~rovide 

for variable tuning via one of the series trimmer capacitors which is 

outboarded and available for external adjustment by remving the screw 

cap on the top junction box port. Wider adjustment range Is possible by 

opening the tuning enc losure and adjusting the other serles and shunt 

trimmers, No special provisions were made in the prototypes for intrinsic 

safety . 
Tuning curves for the prototype antennas were obtained for both 

dielectric and ferrous metal surface mountlnos. Also, spot frequency 

shift data was gathered at the nominal tune frequencies for each an+enna 

when the loop orientation was chanced from vertical to the 45-degres 

position, The tuning curves are presented as Figure 16 and 17, The 

detuni ng data and the "loaded antenna bandwidths of 14 KHz @ 520 KHz 

and 20 KHz @ 920 KHz are summarized in Section 2.1.2. 

The analysis of the prototype antenna performance is presented in Tabls I 

where the various antenna losses are partitioned. The prototyps 

antenna bandwidths are lirited by the ferrite transformer and tuning 

capacitor losses. These losses could be minlrnized by additional design 

selection of higher Q capacitors and through the use of hieher frequency 

ferrite in the transformer. It is estimated that proper design selection 

could result in antenna QTs in the 100-150 range which would give 

unpadded bandwidths in the 3 - 5 KHz range using the same prototype 

antenna windings. This would permit the use of fixed paddins resistance 

to adjust the bandwidth to the desired level to accomdate detuning 

and type of modulation. The increase In measured bandwidth with operating 

frequency was due to the frequency dependence of the matching circui? 

losses. Minimizing these losses and providing fixed resistlve padding 

would provide near-constant bandwidth over the 500 Wz - l00Q KHz ranse. 

Eventually, a two-frequency matching scheme will be required for these 

antennas, A posslbl2 mans for acconplishina this would be to separst~lv 

transform the series resonant resistance of t5e antenna ( occurrinc at 

a lower frequency selectable by adjusting the 50-ohmd transformer prirary 

impedance level for the L-section match with a conventional transformer. 

Switching between the transformer Inputs may be required when changing 

frequencies, This would circumvent the probable high clrculating current 

of a multi-pole network to provide two frequencies on a strictly passive 

match i ng bas i s. 

FIGURE 14 

BALANCED TUNING CIRCUITS USED IN VEHICULAR ANTENNA PROTOTYPE MATCHING 

TO A MATCHED I WEDANCE LEVEL OF 200 OHMS 

520 KHz C l RCU l T 

BALANCED 

BALANCED TRANS FORMER OUTPUT ( B 1 100 PF (C) BALANCED ANTENVIA 1 NPUT 

(A) tf35-680PFAIRVARlABLETRlMMER 359VDC FlOMINALRATING 

(6) 70 - 480 PF A I R VARIABLE TRI MFAER 350 VDC NOMI NAL RAT1 NG 

(C) S 1 LVERED MI CA F I XED CAPAC l TORS 1000 VDC NOMINAL RATING 
Dl ELECTRIC 

2 

FIGURE 16 

TUN l NG CURVE FOR PROTOTYPE VEH l CULAR 

ANTENNA TUIJED TO F4OIJII NALLY 520 KHz SHOW l NG 

FREQUENCY SH l FT DUE TO MEASUREMENT OVER 

Dl ELECTRI C AND STEEL MOUfJT I NG SURFACES 

S

PROTOTYPE WOUND THE REVERSE OF THAT 

PER SECTION 3,2,4 

TABLE I 

PERFORMANCE D l AGNOS l S FOR VEH l CULAR PROTOTYPE 

ANTENNA ( WOUND PER SECTION 3,2,4 AS OBTAINED 

DUR l NG PREL l MI NARY TEST l NG @ 476 KHZ 

ANTENNA TUNED AND TRANSFORMED 

WITH TUN1 NG CWiT IN ENCLOSURE 

- -- 

ANTENNA TUNED AND TRANS FORMED 

W l TH TUN l NG CKRT NOT ENCLOSED 

ANTENNA TUNED BUT P4OT TRANSFORMED 

WITH TUNING CKRT NOT ENCLOSED 

ANTENNA NOT TUNED( RAW ANTENNA) 

TUN l NG CAPAC I TORS ( 100 VOLT 

SILVERED MICA) 

TRANS FORMER 

ENCLOSURE 

110 3.63 OHMS OF LOSS 

4.23 GHMS OF LOSS 

2.42 OHblS OF LOSS 
5,0 ANTENNA COUPLl NG TO f,?I NE W l Rl NG 

5,l GENERAL CONS I DE!?AT I ONS 

Antenna coupling to mine wiring is a function of antenna range away 

from the conductors, the antenna deslgn parameters, and detalls of 

the nine topology including location of conductors relatlve to the 

crossectional geometry of entries and crosscuts, 

Previous estimates of coupling ( made by the author have evolved 

from rnf propagation studies with antennas located remotely fro^ the 

conductors based on both measurerents and computations, above ground 

measurements with a larse looa close to conducfors lying jus+ above 

ground level, and calculated estlrnates ln close proxl~ity to conductors 

which neglect geometry effects, While the remote estimates ars 

probably satisfactory for system design purposes, the ~revious conductor 

proximity estimates have not included geometrical effects or a set 

of representative antenna parameters, 

The coupling Is defined as the ratio of the current in the an"?nne 

to the net mnof i lar current in the conductor ensemb l e ( or visa versa, 

depending on which is assumed to be the transmit current, 

The previous conductor proxi mi ty coup l i ng estimates place tLle coup l i ng 

at -25 to -30 dB wlth the suogestion that the coupling varies ~nly 

slowly wlth range away from the conductors as lcng es the antenna is 

in the same entry as the conductors. 

Analytically derived coupllng estimates are difficult, if not irpractical, 

to perform due to the complex geometry of the entry crosssctlr,ns with 

conductors and because of the number of geometry related varlab I es 

involved. As an alternative, the coupling can be estimated using graphical 

quasi-static field mapping. Thls techniaue has been proven over thc 

years to yield results of sufficient accuracy for engineering ourposes 

while permitting the inclusion of complicated geometries. As example of 

this is the coupling when an antenna is vehicular mounted conpared to 

the coupling when the antenna Is isolated or man carried. 

The graphical field mapping technique, to be described more fully in ths 

next subsection, provides a unique solution of LaPlacers eq~atlon for thf 

qeometry involved and depends only on the orthogonallty betwsen eluipotential 

and flux I ines which are plotted so as to form "curvi l inear squares". The 

approach to be used in this work considers the static field to be set up 

by a net codlrectional transmitted current in the line source conductor 

ensemble and the receiving antenna to be small so as not to dlsturb t5e 

field shape significantly, Uslng the field configuration for +he conductor 

ensemble ( as opposed to that from the an?-enna 3s a transmitter) make 

the problem a two-dTmensional one as the field configur?tion 1s presumed 

to be invariant wlth iongltudinal distance down the entry, 
The computational technique involves determining the magnetlc fisld strength 

at one or more desired locations on the field map proportional -to current 

i n the conductors, us1 ng the field strength and antenna geomstry to compute 

an open circuit voltage at the antenna ( also proportional tc conductor 

current 1, and then to use the antenna irnoedance Information +o compute 

the current driven in the loop for a given open circuit voltage. The 

antenna impedance and bandwidth data used in the computations to follow 

are those determined from the vehicular antenna prototype development. 

5.2 DESCRIPTION OF THE TECHNIQUE 

The magnetlc flux per unit length in a two-dimensional problem is relat~d 

to the conductor curren+ as 

so that the magnetic fie l d strength is gl ven by 

I I A6 

where~eis the angular widtn of th? flux fu5e 

Ar Is the differential rans3 of t3e 

curvi l Inear square 

I, is the net mnofilsr ccnductor current. 

This relationship is illustrated in Figure showing the maoping of the flux 

and equipotentla l l lnes away from the fami l iar isolated l ine source. In 

this particular case, as ~eand At become vanishingly small, the srid shape 

becomes a perfect square and 

Ar+a$, the arc length subtended by A+ 

a0 = Ae I 

then - = - L - - 

A 1 A r 

r 

so that H = - , the known result 

2Kr 
F l GURE 

MAGNETIC FIELD I\.IAPPING OF A CONDUCTING LINE SOURCE It4 FREE SPACE 
For each case, the flux and equipotential lines are drawn to form 

"curvilinear squares" and are subdivided with sufficient granularity to 

define the field at one or more desired points. The flux lines represent 

equal 1 y spaced angular dl vl sions close to the conductor ensenb l s, so 

that 

ae = 

number of l Ines/360 deg rotatlon around the conductor 

Ar is read in meters off the scaled mapping graph. 

The loop open circuit voltage Is given by 

)Vocl= UU(NA)H 

where (NAI is the loop area-turns product 

Assuming matched conditions, 

lvoc l -= 11 ,I, the loop current, = lvocI 'A 

where, R is the loop tuned resistance L 

X Is the loop reactance L 

QA is the antenna Q 

so that the complete expression for tha coupllna ratio is 
5.3 EEITRY CROSSECTIONAL GEOMETRY 

Fkny geometries are posslble. Generally, if the conductors zrs close to 

the antenna and ant obscured by a vehlcle the coupling toss wilt 59 mini~urn. 

The worst cases are those with the conductor ensembles loca4ed el+%r 

against the rib ( on roof or floor or near a vehlcle with an antenna 

on it such that the vehlcl e "masks1' the dl rect path from ths cnnductor(s) 

to the antenna. These worst cases are not pathelogical worst cases, but 

are those commonly occurring and, so, are those which will b3 used In 

the analysis to follow. 

Four cases were deQermined to be representative of these s14u~ti~ns; 

particularly If more than one ant~nna location were chosen for svalu~+lon 

per situation. 

These situations include: 

(I Conductor ensemble near roof 8 rib corner w/o a ve+,icIe present 

Antennas assumed to be locatsd respectively I and 2 

meters away from the opposi $e rl b and ha l fway be$ween 

the roof and floor 

Note: the resu l ts are essent l a l l y the sam for conductors 

located near the floor & rib corner 

(2) Conductor ensemble near roof & rib cornsr with v~hicle gr3sent 

Antennas located alternatively on the near-side ard 

far-side of the vehlcle with respect to the conduc+or(s) 

(3) Conductor ensemble near floor 8 rib corner wi4h vehicle oresent 

Antennas located per (2) 

(4) Conductor ensemble on flooe close to the vehicls 

Antennas located per (2) 

A standard scale was used for the enTry crossectlon and for the vebtcle 

dimensions and location. The entry was chosen to be 2 meters high and 5 meters 

wide. The vehlcle height was taken to be 2/3 meter and the vskicle width was 

taken to be 2 meters. The vehlcle was assumed to be located I mgter away from 

the rl b oppos l te the conductor(s), 

5,4 PRESENTATlON OF RESULTS 

The four above cases along with the graphical field mappings are alvsn in 

Figures through .Oneach fl~urearegiventheappro~riatecomputsd 

couplings and coupling comparisons @ I MHz, 

Antenna data used In the analysis Included: 

2 (NA) = 1,67 meter turns 

B 500 KHz, XL = j456 ohms, QA = 41 

@ I MHz , XL = JlOOO ohms,QA = 55 
Q~ 

The coupling data is given for I MHz only as the ratio is nearly 

x~ 

the same ( 1.7 dB l ess @ 50r) KHz than 8 1 P1lHz 1 for f requencles between 

500 KHz and I P,IHz. 

From these results, the following concluslons may be drawn: 

(I) Based on the antenna data, the coupling is nearly fiat with 

frequency in the 500 - 1000 KHz range ( less than 2 dB 

difference 1. 

2) The antenna rounted Qn the vehicle is withln 2 2 dE in 

coupling compared to the case w/o vehicle rrountlng 

(a) vehicle presence increases the coup l lng for hi g+-muntsd 

conductors. 

(b) Vehicle presence decreases the coupling for low-munted 

conductors ( on floor 1, 

(3) Assuming the antenna is mounted on the bat+wy box mar one 

side of the vehicle, the antenna lf mounted on fhe conductor 

side produces 3-11 dB mre coupllng than If the ant-nna were 

mounted on the side opposite the conductor(s) 

(a) 3 dB for conductor on flocr against rib 

(b) 8 dB for conductor near roof and against rl b 

(c) I I dB for conductor on f l oor and 4 meter away from 

the vehicle, 

(4) For veh icl ular mounted antennas w/parameters of protot\jpe 

for the assumptlons of (3) 

(a) Coupling is -16 to -20 dB when antenna Is on conductor 

side of the vehicle 

(b) Coupling is approximatsly -25 dB when arfsrlna Is on 

opposite slde of vehicle from conduct~r(s) 

(c) Coupling is approximately -30 dB when anterns is on 

opposite slde of the vehicle from conduc+or(s) but 

with conductor(s) on floor near vehicle 

(5) For man-carried antennas symmetrically located bstweev roof 

a nd floor, coup i l ng is approximately -25 to -2g dB 

respecti vei y I to 2 meters away from opposite ri b for e i ther 

roof-mounted or floor-mounted conductor(s). 
F l GURE 

GRAPHICAL F 1 ELD MAPPI NG FROF4 CONDUCTOR NEAR ROOF & RIB CORNER WITHOUT 

A VEH ICLE PRESEiJT 

(3 I MI-IZ 

COUPLING FROIVI ANTENNA 

COIIPL I NG FROPI ANTEf\Ji.IA 
F I GURE 

GRAPHICAL FIELD MAPPING FROI.1 CONDUCTOR NEAR ROOF & RIB CORNER WITH 

A VEHICLE PRESENT 

I--- 2M -1 8 1 MHZ 

COUPLING FROM ANTEN

COUPLING FROM ANTEN

FOR ANTENNA POSITIO

VEHICLE PRESENT, RA
F 1 GURE 

GRAPH l CAL F l ELD MAPP 1 NG FROM CONDUCTOR ElEAR FLOOR & R I B CORNER W 1 TH 

A VEHICLE PRESENT 

COUPL I PIC; FROM ANTEN

COUPL l NG FROM ANTEN

FOR Al4TEilF-JA POS l T l O

VEHICLE PRESENT, RA
F l GURE 

GRAF't-1 I CAL Fl ELD PllAPP I NG FROM CONDUCTOR 0:J FLOOR CLOSE TO VEH l CLE 

d I PWZ 

COlJPL I EJG FFiOF4 ANTEFlNA 

COUPLING FROM ANTENNA