NBSIR 75-804

GENERATION OF STANDARD EM FIELDS FOR

CALIBRATION OF POWER DENSITY METERS

20 kHz to 1000 MHz

M.L. Crawford

Electromagnetic Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Co 80302

January 1975

Final Report

Prepared for

Calibration Coordination Group

Army /Navy /Air Force

U S DEPARTMENT OF COMMERCE. Frederick B Dent. Secretary

NATIONAL BUREAU OF STANDARDS R. chard W Roberta Dnectoi
I

CONTENTS

Page

1 . Introduction 1 -

2. Techniques for Generating Standard Test Fields 3

2.1 Parallel Plate Transmission Lines 3

2.2 Parallel Wire Transmission Line 5 2.3 TEM Transmission Cells 6

2.4 Directive Antennas 7 3. Description, Design, and Evaluation of TEM Cells

(Recommended Technique for Calibrating Power Density

Meters from 20 kHz to 500 MHz) 11

5.1 Mapping the Fields Inside the Cells 15

4. Measurement Procedure 15

5. Error Analysis for Standardization of the Field Inside

the Cells 18

6. Intercompar ison of Parallel Plate Line, TEM Cells,

OEG, and Standard Gain Horns 19

7. Summary and Conclusions 25

References 25
LIST OF FIGURES

Page

Figure i. Typical parallel plate line per MIL-STD-462 26

Figure 2. Block diagram of paral lei -wire field apparatus- 27

Figure 3. TEM cells and associated equipment for calibration of radiation hazard meters 28

Figure 4. Block diagram of system for testing radiation

hazard meters using directive antennas 29

Figure 5. Short backfire antenna with radiation patterns- 30

Figure 6. Four element dipole array antenna 31

Fi gure 7. Radiation hazard probe mounted in front of

open-ended waveguide (OEG) antenna 32

Figure 8. Design for rectangular TEM transmission cell 33

Figure 9. Cross sectional view of rectangular transmission line 34

Figure 10. Time domain ref lectometer trace of distributed

impedance of empty cell 35

Figure 11. Port input VSV.'R of empty cell, b x W = 25 cm x 42 cm 36

Figure 12. Relative electric field distribution inside

cell. Cross sectional cut thru upper half at

center of cell 37

Figure 13. Block diagram of TEM cell measurement system

for RF power density meter testing and calibration (1 MHz to 500 MHz) 38

Figure 14. Block diagram of TEM cell measurement svstem

for RF power density meter testing and calibration (20 kHz to 1 MHz) 39

Figure 15. Polyfoam tower supporting probe in front of

standard gain horn on NBS extrapolation range-- 40

iv
I

GENERATION OE STANDARD EM FIELDS FOR

CALIBRATION OF POKER DENSITY METERS

2(1 kHz to 1000 MHz

TIi is report describes techniques for calibrating power density meters used by the Department

of Defense in measuring high intensity (hazard level)

RF fields in the frequency range 20 kHz to 1000 MHz.

It reports on part of the work sponsored by the

Calibration Coordination Group (CCG) , of the Department of Defense covering the frequency range 20 kHz

to 20 GHz.

Several techniques were considered for producing a standard field including parallel plate

and parallel wire transmission lines, transverse

electromagnetic mode (TEM) transmission cells,

various directive antennas and open ended waveguide

(OEG) . The major emphasis in this report is on

the TEM cells, which are recommended for the frequency range 20 kHz to 500 MHz. Design and evaluation details and an error analysis associated with

the TEM cell measurement system are given. Power

density levels can be established in the cells from

? ? a few pW/cm to 100 mW/cm with uncertainties less

than ± 1 dB.

Limited information is also given describing the

use of OEG, the recommended technique for the frequency

range 500 MHz to 2.6 GHz, and giving the results of

intercompar is ions among parallel plate lines, TEM

ceils, OEG, and standard gain horns.

Key words: Hazard level fields; power density meter

calibration; TEM transmission cells.

1 . Int roduct ion

There is increasing concern among military and civilian

agencies about the effects of non-ionizing (RF) radiation on

equipment and personnel. For example, potentially hazardous

electromagnetic (EM) radiation may occur near radar or radio
*

antennas, or may be produced by electric appliances such as

microwave ovens. There are several types of commercial meters

now being used to survey the strength of EM radiation. This

report describes work performed under the sponsorship of DoD/CCG

to develop and evaluate instrumentation and standard techniques

for generating known RF fields in order to calibrate these

radiation meters.

The objective was to develop optimum procedures for calibrating power density meters used by the DoD for measuring

high- intensity (hazard level), free space RF fields over the

frequency range 20 kHz to 20 GHz. The required power density

levels are approximately 1 to 100 mW/cm or E field strengths

of 60 to 600 Y/m. The desired calibration accuracy is ± 1 dB

.

This report describes techniques for use below 1000 MHz, with

emphasis on the TEM (Transverse Electromagnetic Mode) cell

which is recommended for use over the 20 kHz to 500 MHz range.

The companion report will describe in more detail the techniques

which are recommended in the frequency range 500 MHz to 20 GHz.

A number of alternative techniques were evaluated and

compared for ease, accuracy, and cost of duplicating the measurement procedures. These techniques are described briefly in

section 2 of this report and include: (a) producing a uniform

field between two parallel plate conductors or between two

parallel wire lines, (b) generating a uniform field inside

TEM transmission cells or waveguide, (c) producing a calculable

field in front of directive antennas such as a standard gain

horn or open-ended waveguide (OEG) . The advantages and

limitations of the various approaches are given to justify

the choice of the recommended techniques (i.e., TEM cells

and OEG). Sections 3, 4, and 5 describe the TEM cell method

in considerable detail, since it is recommended over a large

portion of the frequency range (20 kHz - 500 MHz). Details

for using OEG over the frequency range (500 MHz - 2.6 GHz)

2
will be given by Bowman in a companion report and are not

repeated here. However, some of the background leading to

the selection of OEG is included, along with results of some

intercompar is ions among parallel plate lines, TEM cells, OEG,

and standard gain horns. These measurements were made at

overlapping frequencies to strengthen the credibility of the

particular approach and provide verification for the accuracy

statements included in the error analysis portion of this

report. These results are contained in section 4 and indicate excellent agreement, well within the prescribed accuracy

limits (± 1 dB) attributable to these particular measurement

techniques

.

2 . Techniques for Generating Standard Test. Fields

2 . 1 Parallel Plate Transmission Lines

This technique has been in use for some time for EM pulse

studies and for susceptibility testing of electronic equipment.

Various authors have presented the technique in detail [1,2],

hence only a brief description will be given here. Essentially

the test field is established between the conducting planes of

a paral lei - strip transmission line (figure 1) which is terminated with its characteristic impedance and driven by a high

power RF source through an impedance matching network. Static

field analysis given in reference [1] shows the TEM mode of

a parallel plate line can simulate a free-space plane electromagnetic wave over a substantial portion of its interior

region.

The impedance of the line, neglecting fringing, is

given approximately by

Z - 37 7 — ohms . ( 1

)

o w

3
where h and w are as shown in figure 1 . The electric field

is given as

E = rr volts/meter. (21 v h

For the line shown, field strengths in excess of 200 V/m can

be obtained using 100 watt generators.

This system can be used at frequencies up to a few

hundred MHz with a fair degree of accuracy if the test item,

or hazard probe, is small (< h/5xw/5). However, as the frequency increases and h approaches A/4, the line radiates

strongly from the open sides. This creates interference

which may interact with the measurement itself, be hazardous

to an operator, or interfere with other experiments being

conducted within transmission range. Also higher order modes

will exist whenever the plate separation exceeds A/4. These

modes distort the test field configuration and limit the

accuracy in determining the known field. The main disadvantages are then the size limitations imposed by the upper

useful test frequency and the lack of shielding to prevent

radiation. The system must also be carefully impedance

matched and the dimensions of the device being tested should

not exceed (h/5xw/5). Otherwise large standing waves can

exist within the test region and the electric field will be

significantly different between the test item and the plates

than indicated by field calculations based upon the RF voltage

measured between the plates. Construction costs are minimal

for building a line capable of testing probes up to approximately 10 cm size, at frequencies up to 100 MHz. Parts plus

labor should not exceed $ 2 , 0 0 0 - $ 3 , 00 0 . A parallel plate line was constructed at NBS and used at

15 MHz to calibrate a dipole transfer standard probe for

intercompar ing with similar calibrations using a TEM transmission cell. The results of this comparison are contained

4

iiiiM m iimnim in i H ii Mil iiMiiirm
in section 6. Errors associated with the parallel plate

transmission line technique are similar to those described

in evaluating the TEM transmission cell and are believed to

be within the desired calibration accuracy of ± 1 dB.

2 . 2 Parallel Wire Transmission Line

This technique is very similar to the parallel plate

line except it utilizes electrically balanced wires in place

of electrically balanced or unbalanced plates. Since typical power density meter probes are small, the spacing between

conductors of a parallel wire transmission line can be kept

small thus making this technique appear feasible for generating

a standard test field [3]. The field midway between the conductors is approximately uniform if the wire diameter is significantly less than the spacing between conductors and is

given by

E s 377 ij

| (3} v ird 1 i

1 where 1^ is the current in the conductors, and d is the half

distance separating the two conductors of the transmission

line. The characteristic impedance of open two-wire line in

air is given as

Z = 120 cosh" 1 — (4)

o a

K J where d is a defined above and a is the conductor diameter.

The line current 1^ is determined by terminating the line

in its characteristic impedance (approximately 600 ohms),

accurately measuring the resistance R of the termination,

the power P dissipated in R, and using the relation

|I £

I = /P7R. (5)

A block diagram of the line and associated equipment, is shown

in figure 2. The thermopile is used to compare the heating

effect of the RF power in the terminating resistor to an equiva

lent dc power which is measured with the dc voltmeter.

5
This technique suffers from essentially the same disadvantages as the parallel plate line with the added disadvantage of less field unifo^*ity; thus no effort was made

to further develop it for calibrating power density meters.

2 . 3 TEM Transmission Cells

This technique utilizes a transverse electromagnetic

(TEM) transmission cell that operates as a 50ft impedancematched system (figure 3). A calculable, uniform TEM field

is established inside the cell at the test frequency of

interest by coupling RF energy through the cell from a transmitter connected at the cell input port. A SOQ (ref lectionless)

termination is connected at the cell's output port. This technique operates essentially the same as a parallel plate line

except it has the major advantage of not radiating energy

into the surrounding space, i.e., the EM field is contained

inside the cell. It is extremely broadband in frequency,

being limited only by the waveguide multimode frequency associated with the cell size. The cells are inexpensive to

construct, approximately $3,000 per cell and the use of expensive anechoic chambers or shielded enclosures are unnecessary. The cells can be used to establish known field strength

levels from below 1 V/meter to 600 V/meter with uncertainties

less than ±1.0 dB . This technique proved to be the optimum

procedure for establishing standard fields for calibrating

power density meters at frequencies below about 500 MHz.

(Details of this technique are contained in sections 3 through

5.) At frequencies above about 500 MHz, the small physical

size of the cells (required to prevent multimoding) provides

too small a test region for use with typical commercially

available units, and alternative techniques of producing

known fields are required.

6
2 . 4 Directive Antennas

At frequencies above a few hundred megahertz, the most

feasible technique for generating an accurately known cw

calibrating field proved to be by radiation from a directive

antenna. The probe is mounted on a positioner and aligned in

front of the source antenna. Measurements are made by adjusting the input power to the source antenna until the desired

power density meter reading is obtained. The standard test

field level, P^ , is then calculated from the following equation:

E

2 P -G • NZC 7 = P

d

= ~—^2 (watts/m z

) (6)

377 4iTr

where P^ = net power into the source antenna,

Gp = source antenna power gain,

NZC = near-zone correction factor, which is a function

of r

,

r = separation distance between the source antenna

and the probe under test, and

E = standard test field in V/m.

Corrections can be made for the multipath within the chamber by

recording the power density meter reading, P^

nc[ic'

as a function of r while holding the input power of the source

antenna constant. The correction factor ratio, P^^indic CdB) , is then computed as a function of r and averaged to give its

corrected value.

Four alternative source antennas were considered: a) calibrated single or multifeed backfire antennas, b) a 4-element

dipole array with calculable gain, c) pyramidal standard gain

horns, and d) calibrated "short horns" or open-ended waveguides.

Short backfire antennas, shown in figure 5, were considered

because of their relatively high gain [4], CiL 15 dB) , thus

requiring less source power. Two short backfire antennas

patterned after figure 5 were constructed at NBS to operate
centered at approximately 500 MHz and 1.0 GHz, respectively.

Their gain characteristics were determined as a function of

frequency variations about their particular design frequency

and found to be quite dependent upon the length and spacing

of the feed dipole elements. This type of antenna has considerable potential as a gain standard in the VHF-UHF range

but will require additional effort to completely characterize

it. The required measurement separation distance between the

antenna and a hazard meter to be calibrated (based upon a given multiple of a

2 /A where a is the diameter of the back

reflector) is large for this antenna, thus nullifying part

of the advantage achieved by the higher gain.* At present,

the backfire antenna is useful only as a transfer standard

because its on-axis gain cannot be calculated directly.

The four element dipole array over a ground plane (figure

6) on the other hand has considerable gain and can also be

characterized mathematically [5]. Work is presently underway

at NBS to carefully evaluate this antenna between the frequencies 500 MHz to 2000 MHz for potential use in establishing

calculable standard fields. This antenna, however, also has

a large aperture, its gain is quite frequency dependent, and

it is awkward to use because of the weight and size. Both

this antenna and the backfire antenna are more suitable for

generating fields over larger test areas than are required for

hazard probes .

*To establish intense EM fields over a small region for calibrating EM hazard meters with a minimum of required oscillator

power, the main consideration is the aperture size of the

radiator. It is easy to show (and is shown in the companion

report by R.R. Bowman) that for the usual antenna designs and

for a separation distance of a given multiple of a

2 /X, where

a is the larger aperture dimension of the radiator, the field

intensity is a maximum for a given input power when the

aperture area is minimum.

8
Well characterized (on axis gain calibration <_ 0 . 3 dB)

pyramidal standard gain horns are commercially available at

frequencies above a few hundred megahertz. However, they also

have the disadvantage of being large and cumbersome at frequencies below about 2.6 GHz, and lend themselves to generating

fields over larger test areas than required by hazard probes.

Their size and subsequent large a

2 /A measurement distance required to minimize the near-field correction error make indoor

measurements difficult if not impossible. Standard fields were

generated at 500, 750, and 1000 MHz using standard gain horns

on a large outdoor extrapolation range for comparison with

the TEM cells and OEG radiators. Results of these measurements

are contained in section 6. For routine hazard probe calibrations, however, at frequencies below 2.6 GHz, use of standard

gain horns is not considered practical.

The OEG (figure 7) is significantly smaller than any of

the other three types of antennas considered, and was selected

as the preferred option for calibrating small probes over the

500 MHz to 2.6 GHz frequency range. There are several reasons

for this: (1) the small aperture area means less oscillator

power is required; (2) the front - to -back pattern ratio is

satisfactory despite the low directivity of OEG; (3) the

entire frequency range from 2.6 GHz to below 500 MHz is

covered by commercially available rectangular waveguides with

precisely scaled dimensions, permitting gain measurements for

one frequency band to provide the basis for calibrations

throughout this frequency range; (4) along the radiation axis,

at least, the backscatter from fields incident on OEG is quite

small; (5) while the reflection coefficient of OEG is fairly

large, it is small enough (< 2.1 VSWR) to be connected directly

to many commercial travel ing -wave -tube (TWT) amplifiers or

oscillator without causing damage to the oscillator;^ (6) the

*We recommend using isolators in the circuit to protect the

oscillator against possible accidental damage, even though

the antenna does have a relatively low VSWR.

9
gain and the near-field corrections for OEG are smooth and

almost linear functions of frequency; (7) the relatively

small size of the OEG simplifies its mounting, alignment,

and handling.

Complete, high-accuracy measurements of the gain of OEG

(using V,"R4 50) , were made for this project. Further, assertions (2), (4) , (5), and (6) above, which are not obvious,

have been experimentally established. Details of these tests,

along with formulas and graphs for the gain and for approximate

near-zone corrections for waveguide sizes VvR1500 , KR975,

KR650, and WR450 will be given by Bowman. His report will also

present an error analysis and give the measurement procedure for using OEG and standard gain horn radiators to establish

standard test fields and hence are not repeated here.

The relatively low directivity of OEG causes a strong illumination of the surrounding test chamber which then produces a relatively large amount of backscatter in the test chamber.

However, calculations indicate that this effect will be more

than compensated by the fact that the probe under test can be

brought much closer to low-directivity radiators than to highdirectivity radiators. This was verified by moving the probe

along the radiation axis and observing the response perturbations due to multipath interference (MPI) . Despite the

small dimensions of our test chamber (about 5.5 by 5.5 by

5 meters) and the fact that our radiation absorbing material

is less than the best available, the MPI perturbations were

less than ± 0.2 dB for all frequencies when using the OEG

radiators. These measurements were made for an EM hazard

meter that had a response pattern that was essentially isotropic. Therefore, unless a probe is being calibrated that

does not have its maximum response pattern directed to\\ard

the radiator, these MPI perturbations should represent the

maximum that would be observed on any high quality indoor

range with equal or larger dimensions than 3.5 i by 5 . 5 m by

5 m long. 10
3 . Description, Design and Evaluation of the TEM Cells

(Recommended techniques for calibrating power density meters

from 20 kHz to 500 MHz .

)

The cells (figure 8) consist of a section of "rectangular

coaxial" transmission line tapered at each end to adapt to

standard coaxial connectors. The line and tapered transitions

have a nominal characteristic impedance of 50 ohms (real) along

their length to insure minimum voltage standing wave ratios

(VSWR) * The electromagnetic (EM) field is developed inside

the cell when RF energy is coupled to the line from a transmitter connected at the cell input port. A matched 50 ohm

termination is connected to the output port. The fields inside the cell were monitored using special electric and magnetic

field probes designed and constructed by NBS [6].

Experience with the cells show that at frequencies for

which only the principal wave (TEM mode) propagates through

the cell, a reasonably uniform electric field can be generated.

The major design considerations are to:

1. maximize usable test cross sectional area,

2. maximize upper useful frequency limit,

3. minimize cell impedance mismatch or voltage standing

wave ratio,

4. maximize uniformity of EM field inside the cell.

The cells were designed using experimental modeling* and

the approximate equation for the characteristic impedance of

shielded strip line [7]

Z = 94,15

n (OHMS). (7)

w/b . L

f

1 -t/b 0 .0885 e

*Work is in progress jointly at NBS and the University of

Colorado to de\r elop the mathematical formulation to provide

a more exact theoretical description of the TEM cell.

11
In eq. (7), is the relative dielectric constant of the

medium between the conductors, C

1 ^ is the fringing capacitance

in pf/cm, and w, b, and t are as shown in figure 9.

The problem of converting the shielded strip line into

a "rectangular coaxial" line is primarily one of determining

experimentally the value of C

1 ^. This was done using a time

domain ref lectometer (TDR) to evaluate a small scale model of

the cells. C'^ was found to be approximately equal to 0.053

pf/cm. Dimensions for b were determined from the design

criterion that no more than 1/3* of the dimension, b/2, be

exceeded by the radiation hazard probe or meter to meet

design considerations 1, 3, and 4. Once b and C

' ^ are

determined and an available metal thickness t selected, w can

be calculated from (7), assuming approximately 51 ohms, for

the line characteristic impedance. (Fifty-one ohms was chosen

to allow for some impedance loading effect when inserting the

meter inside the cell.) Table 1 gives the dimensions for

constructing the cells with specified upper frequency limits.

The TDR was then used to make refinements by trimming w until

the proper characteristic impedance was obtained.

Table 1. TEM Cell Dimensions

Plate

Upper SeparaUseful tion

Frequency b w t C '

(MH z) cm cm cm pf/cm

100 90 108.15 .157 .053

300 30 36.05 .157 .053

500 18 21.63 .157 .053

*The one third factor is considered a maximum. The impedance

loading effect from inserting the meter should not exceed a few ohms if a reasonable VSWR and EM field perturbation is

to be maintained.

12
Figure 10 gives a typical TDR trace of the distributed

impedance along the length of a cell and figure 11 shows

the VSWR as seen at the cell's input and output ports.

The cell distributed impedance as determined by the TDR is

given to a very good approximation by

Z Q

- /L7F . (8)

This condition exists since the cell is essentially lossless

at the frequencies of operation.

3 . 1 Mapping the Fields Inside the Cells

Measurements were made using a calibrated short dipole to

probe the electric field inside the empty cells. The variations

in relative field strength vs. position were determined in the

longitudinal and transverse directions within the cells. The

electric field, E, is essentially vertically polarized in the

region near the center of the cells and gradually becomes horizontally polarized as one moves in the horizontal direction

toward the gap at the side. Both vertical and horizontal

components of E were measured at each point to determine the

7~2 J total electric field E = /E" + E rT (E and E TT are in phase) . v H

v v Hv j The electric field distribution is shown in figure 12. The

electric field in the test regions shown is primarily vertically

polarized (E^ >> E^) . The relative field distribution is

independent of the magnitude of the test field used and the

frequency so long as the frequency is less than the first

order TE mode (TE-,q) cut-off frequency given approximately

by the following equation in rectangular waveguide [8]

The equation for determining the cut-off frequency for a specific higher order mode in general is given approximately

13
by

s c/bWn2

. c m,n 2bw

For eqs. (9) and (10), c = propagation velocity of light

within the cell medium in meters/sec. b and !\

r are as shown

on figures 8 and 9, and m and n are integers specifying the

mode. Due to the symmetry of the empty cell, the TE^ mode

is not excited. However, as shown in the example of figure 11,

the TE-^ and/or TNL^ mode can exist contributing to the

resonance phenomena displayed by the high VSWR spike. If

higher order modes are allowed to propagate, the field configuration, which is the vector sum of each contributing

mode, no longer has the simple pattern shown in figure 12.

Thus higher order modes greatly complicate interpretation

of the measured results of the cell and limit the useful

upper frequency.

Variations in the electric field strength for the empty

cells are less than 0.5 dB for the cross section of figure 9

over the area typically occupied by the meter. Inserting the

meter shorts out part of the electric field due to the metal

in the sensors and case and increases the indicated field

strength in proportion to the percentage cross section occupied.

For calibrating small probes or meters (less than 1/5 of b/2)

the effect is small and is included as part of the field nonuniformity error treated in section 4. However, for larger

probes or meters (> 1/5) this increase in field strength for

a constant test input power cannot be neglected and must be

taken into account when determining the absolute test field

ins ide the cell.

Leakage around or through the cell's access door is

negligible if the door's dimension is kept small compared to

the test frequency wavelength and is located in the side wall

as high above or below the center septum as possible (where the

current flow is a minimum)

.

14
4 . Measurement Procedure

A block diagram of a typical system for making power

density meter calibrations using the cell is shown in figure

13. The meter and/or just the probe, is mounted inside the

cell centered midway between the top of the cell and the

center septum in the desired orientation. The orientation,

size, and type (shielded or unshielded, high resistance or

metallic, balanced or unbalanced) of interconnecting leads

between the hazard meter and its sensor or probe can have

a large effect upon the meter's calibration. Tests can be

performed with the leads oriented for minimum or maximum

field coupling while making the measurements, thus providing

data for evaluating the lead contributions to the overall

meter calibration accuracy. Minimum lead interaction with

the test field is achieved by orienting them in a direction

perpendicular to the E field and/or by properly shielding

them. Maximum interaction is achieved by aligning them with

the polarization of the test field.

The procedure for calibrating the meter from 1 MHz to the

multimode frequency is as follows. The generator output is

adjusted until the desired hazard meter reading is obtained.

The test field is then calculated from the equation,

/PRn v a~ E„ = . (11)

where P^ is the net power flowing through the cell, R

c is

the measured characteristic impedance of the cell at the

test location (approximately 51ft for the example of figure 10),

and d is the separation distance between the upper wall and

the center plate or septum.

P^ is determined from the power meter readings on the

sidearms of the calibrated coupler using the following

equat ion

P = CR. • P. - CRD

• P r (12)

n f mc R ref
where CR^ and CR^ are the forward and reverse coupling ratios

of the bi-directional coupler and P^

nc

and P

re £ are the indicated incident and reflected coupler sidearn power meter

readings. is invariant with position along the length of

the cell since the ceil is essentially7 lossless. If R is to

c be obtained from complex impedance measurements made at the

cell's input or output port, a Smith chart would be needed to

transform the measured impedance plane to the center (location

of the item under test) of the cell. From this the real or

resistive component, equivalent to R can be found. The

C- separation distance, d, can be determined by a simple measurement, in meters, using a centimeter scale or caliper at

the point of interest. The calibration factor, CF, for the

meter is then given as E P,

CF = g—3 or CF =

p (15)

indie p indie

where E . or P , is the level of the standard test field (V/m)

or power density (mW/cm ) and E- -,^ c or P^

n(^c

i- 5 the indicated meter reading in volts/meter or mW/cm4- . P^ can be

calculated from E _ by the equation

P, = E

2

r /3770 (mW/cm2

) . (14)

Measurements below 1 MHz are made using a voltage monitor tee and RF voltmeter (figure 14) because directional

couplers are not readily available at these lower frequencies.

Directional couplers and power meters are used above 1 MHz

because of the ability to monitor impedance variations in the

system when installing the meter inside the cell. When using

the system of figure 14, the input voltage to the cell is

monitored directly and eq. (11) becomes approximately;

V

E

y

= ^, (15)

16
where V is the measured input voltage to the cell and is

approximately equivalent to V~ c

= /P^R^. assuming the cell length

is short compared to the test frequency wavelength. The error

analysis then involves evaluating the uncertainty in measuring

V" c as compared to and , and at the frequencies indicated

for these tests, the errors would be about equivalent.

Swept measurements can be made for a fixed orientation

of the meter and/or its probe using the system block diagrams

of figures 13 or 14 assuming the circuit components remain

matched over the desired frequency range and have acceptable

frequency bandwidth characteristics.

An estimated cost for implementing the TEM cell measurement technique is given in Table 2.

Table 2. Measurement System Estimated Cost

* Block Block

Diagram Diagram

Fig. 13 Fig. 14

TEM Cell 3 ,500 3, 500

100 W RF Generator 10,000 10,000

Low-Pass Filters 1 ,200 1 ,200

Signal Sampler 100 100

Frequency Counter 1 ,600 1 ,600

Monitor Tee 100

Bi-directional Coupler 600

RF Voltmeter 700

Power Meters 3 , 000

100 W, 50ft Termination 300 300

$20,300 $17,500

Much of this equipment is already available in DoD labs thus

reducing the initial capital expense considerably. The system

of figure 14 could be used over the complete frequency range

20 Kc to 500 MHz using appropriate filters, etc., but with

some loss in accuracy at the higher frequencies.
5. Error Analysis for Standardization of the Field Inside . the Cells

A brief discussion of the sources of error in establishing

the standard test field inside the cell is given below. The

A.E total fractional error, <5E = —f— in determining the absolute

v E

y

field strength, E , inside the cell is given as

<5E v

* |(|5R| + |SP|) + | fid | + |5E| (16)

AP

n AR Ad where 5P = —p—, <5R = , 6d = — and <5E is the error due

n c to the nonuniformity of E^ determined experimentally by

mapping the field distribution in the test region of the

cell. Equation (14) was derived by substituting

P' = |P +AP |, R' = |R +AR I, and d' = |d+Ad| in*o eq. (11)

n

1 n n

1 ' c

1 c c1 1 1 n v > and adding 5E (obtained from E field mapping) . Higher order

terms contributing to small errors were then dropped in

the derivation to arrive at eq. (16).

The error 6P is due to uncertainties in the coupler

calibration, the absolute measurement of RF power on the

sidearms of the coupler, and impedance mismatches between

the cell, coupler, RF source, and cell termination. If a precision calibrated coupler and power meter are used and

the cell and its termination are impedance matched

(VSWR £ 1.05), 5P should be less than ± 5

?s. The error, 6R in determining R^ is a function of (1) the

measurement accuracy of the TDR, and (2) the impedance loading

of the meter inside the cell. If the meter occupies a small

portion of the cell (_< 1/5 of any dimension, b/2, w, or L) , 5R will be small (<_ 3%) , and is typically not corrected for

in the calculation of E . For larger meters (occupying up to

1/5 of b/2, w, or L) , however, the impedance loading effect

18
must be measured and used to correct R

c when using eq. (11) to

calculate E . 6R for these cases can be much larger but

typically would be less than 10% if the hazard probe or meter

is centered inside the cell. Exceeding the 1/3 load factor

is not recommended.

Determining 5E is more difficult. Introducing the meter

inside the cell perturbs the electric field distribution as

described in the section on field mapping. This loading

factor (increase in E) is determined using the small calibrated probes referred to earlier. Because of the difficulty

in accurately determining the interaction between the meter

and the cell,* the probes are used only to determine error

limits. If the size of the meter is less than the l/5th factor

6E is less than 6°s . Larger meters necessitate measuring the

field distribution around the meter and probe and a higher

estimate of 6^ is required. The sources of errors are summarized in Table 3.

6 . Intercompar ison of Parallel Plate Line, TEM Cells,

PEG and Standard Gain Horns

An Intercompar ison of the standardized test fields

developed between a parallel plate transmission line and

inside a TEM cell was carried out using a special NBS dipole

probe as a transfer standard. The results obtained at 15 MHz

using the two techniques are given in Table 4. Agreement

between absolute field strengths (generated by the cell and

*The presence of the meter in a free space field also perturbs

the field but no correction is made since the relationship

between its indicated response and the standardized level of

the field without the meter present is what is desired. The

boundary conditions of the cell, however, modify this relationship and result in an increased E field over what would otherwise be computed from eq. (11). Additional work is required

to better establish this relationship.

19
Table 3. Summary of Measurement Errors

Source of Error

a. Absolute measurement of incident RF power on the side arm

of coupler.

b. Coupler calibration.

6P, total error in determination of RF power passing thru cell.

c. Cell impedance, 5R.

d. Cell plate separation, Sd

e. Non-uniformity of electric

field inside cell, 6E.

Maximum Field Strength

Error

5E

v

*

|(|3| + |S|) +

Percent Uncertainty

± 3.0

± 2.0

± 5.0

± 3.0

± 1.0

± 6.0

±11 .0

<± 1.0 dB

Ul t |6|

20
the parallel plate line) required to produce the same, probe

output voltage is well within the uncertainties attributed to

the different techniques.

Table 4. Intercompar ison of NBS TEM Cells* with Parallel Plate

Line at 15 MHz Using 10 cm Dipole Probe

NBS

0.6 m x 1.0 m Parallel

Probe Output Cell Plate Line

Voltage E v (V/m) E

y

(V/m)

0.1 21.3 27.0

0.2 34 . 5 32.0

0.3 47.2 47.0

0.5 71.0 71-5

1.0 127.0 131

Intercompar isons were also made with a calibrated probe to

determine agreement in the test field between cells of different sizes and shapes. Table 5 gives the results of these

measurements

.

Table 5. Intercompar ison of Field Strength in Different Sizes

and Shapes of TEM Cells as a Function of Frequency*

(2 cm reference probe output level set at 100 mv)

Freq

.

MHz

0.6 m x 1.0 m

Cell

E v (V/m)

0.5 m x 0.5 m

Cell

E v

(V/m)

0.3 m x 0.5 m

Cell

E v (V/m)

0 . 2 m x Cell

E (V/ v

v

65 26 .

1

24 . 5 25.6

100 24 . 1 24 . 5 24.8

150 23.0 22 .5 23.8

200 24 . 6 22 . 2 24 . 5 22 . 0

250 20 . 3 23.2 22 . 6

500 24 .4 23 . 5 400 21 .5 22.4

500 20.0 22 . 0

600 ' 23.2

700 17 . 5

0.3m

*Cell dimensions are given in terms of b x W of figure 9.

21
Finally, intercomparisons of field strength were made

using TEM transmission cells, open ended waveguide radiators,

and standard gain horn radiators to establish standard test

fields in the frequency range 500 MHz to 1000 MHz. Figure 15

is a photograph of the polyfoam tower used to support the hazard

probe in front of a standard gain horn on the NBS outdoor

extrapolation range. Results obtained by calibrating a 2 cm

dipole transfer standard are summarized in Table 6. These

measurements indicate that the TEM cell method should be replaced by the OEG or standard gain horn technique above approximately 500 MHz. In general, we recommend use of OEG between

500 MHz and 2.6 GHz because less transmitter power is required

(for reasons alluded to earlier in this report and explained

by Bowman [1]). Agreement between the three techniques is well

within the combined error limits assigned to the particular

measurement methods

.

Table 6. Int ercompar ison of Output Voltage From 2 cm Dipole Probe

Vs. Frequency Using TEM Cells, Open End Waveguide, and

Standard Gain Horns (Standardized Test Field = 1 mw/cm 2

) Output of Probe, Volts

15 dB

Freq 0.2 m x 0.3 m Open End Standard

MHz TEM Cell Waveguide Gain Horn 2

500 .368 .342 .350

6 0 0 .353

750 *.340 .330 .362

1000 -- .373 .400

^Corrected 2.1 dB for multimode propagation effect

22
7 . Summary and Conclusions

The objective of the work described in this report was

to develop optimum techniques for calibrating power density

meters used by the DoD for measuring high intensity (hazard

level) RF fields, particularly in the frequency range 20 kHz

and 1000 MHz. A number of alternatives for producing standard EM fields were evaluated from which TEM cells and OEG were

selected. Considered were initial cost, compatibility of calibration system with instrumentation, minimum danger to personnel,

ease of calibration procedures, etc. The TEM cells are extremely broadband (dc to 500 MHz) and are sel f- shielding

which eliminates hazards to personnel or equipment and interference to the same. They are inexpensive to build and have

potential for application for other forms of testing such as evaluating electronic equipment susceptibility and radiated

emissions, and EM pulsed calibrations. Details sufficient

for construction of the cells are included in the report

along with procedures for implementing them into the calibration system.

In conclusion, a few comments are in order concerning

extended use of the TEM cells.

1) Probes or meters of any size could be tested using

a TEM cell modeled from Table 1 and figure 8 to meet the

criteria that the size of the test object should be preferably less than L/5 x w/5 x b/10. Thus small objects

could be tested at higher frequencies in small cells, and

large objects could be tested at lower frequencies in

large cells. The procedure for testing large objects

is the same as for testing in small cells but a higher

power signal source and termination (50ft) would be

needed.

23
2) The upper useful frequency for the cell is reduced

20 to 30 percent from the cut of f /mult imode frequency

given in Table 1 to allow for loading effects of the

meter inserted in the cell and to provide sufficient

higher order mode suppression or attenuation.

The author wishes to express his appreciation to W.E.

Jessen for his assistance and to R.C. Baird for his suggestions

and guidance on this project.
References

[1] Musil, V.P., Generating high- int ens ity electromagnetic

fields for radiated-susceptibility test, IEEE EMC

Symposium Record, Seattle, Wash., pp. 185-194 (July 1968).

[2] Roseberry, B.E., and Schulz, R.B., A paral lei -strip line

for testing RF susceptibility, IEEE Transactions on EMC,

Vol. EMC -7 , pp. 142-150 (June 1965).

[5] Lawton, R.A., New standard of electric field strength,

IEEE Transactions on Inst. § Meas . , Vol. IM-19, No. 1,

pp. 46-51 (Feb. 1970).

[4] Ehrenspeck, H.W., The short -backfire antenna, Proc . of

IEEE, Vol. 53, No. 8, pp. 1138-1140 (Aug. 1964).

[5] Cottony, H.V., Methods for accurate measurement of antenna

gain, NBS Report (unpublished).

[6] Greene, F.M., Design and calibration of E and H field

probes for HF band application, Proc. DoD Electromagnetic

Research Workshop, Washington, D.C., pp. 50-76 (Jan. 1971).

[7] Skaggs, G.A., High frequency exposure chamber for radiobiological research, NRL Memorandum Report 2218 (Feb. 1971).

[8] Ramo , S., and Whinnery, J.R., Fields and Waves in Modern

Radio, p. 367 (Second Edition, Fifth Printing, Nov. 1960).

IS.
26
27
FIGURE 3. TEM CELLS AND ASSOCIATED EQUIPMENT

FOR CALIBRATION OF RADIATION HAZARD METERS.

28
X

osw

CO

PSwHWs

COQ«<N

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29
FIGURE 5. SHORT BACKFIRE ANTENNA WITH RADIATION

PATTERNS

.

50
1

FIGURE 6. FOUR ELEMENT DIPOLE ARRAY ANTENNA.

31
FIGURE 7. RADIATION HAZARD PROBE MOUNTED IN FRONT

OPEN-ENDED WAVEGUIDE (OEG) ANTENNA.

52
rrO

33
— w —

b - 0.6 W

t)

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S EPTUM

figure 9 . Cross Sectional View of Rectangular Transmission Line

34
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35

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39
FIGURE 15. POLY FOAM TOWER SUPPORTING PROBE IN FRONT

OF STANDARD GAIN HORN ON NBS EXTRAPOLATION RANGE.

40
f. ,,6434 j


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MIDDLE TOWN PA JUNE 90