NisrUNITED STATES

DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

NAIL INST Of STAND & TECH R.I C

A111D3 3TMSE1

NIST

PUBLICATIONS

NIST Technical Note 1335
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Informatkm Center

Gaithersburg, MD 20899

DATE DUE

Demco Inc 38-233
1^ J :> ' L-- usns

Generating Standard Reference ^7

Electromagnetic Fields in the NIST

Anechoic Chamber,

0.2 to 40 GHz

D.A. Hill

M. Kanda

E.B. Larsen

G.H. Koepke

R.D. Orr

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

\ /

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher. Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

Issued March 1990
National Institute of Standards and Technology Technical Note 1335

Natl. Inst. Stand. Technol., Tech. Note 1335, 48 pages (Mar. 1990)

CODEN;NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1990

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

Page

Abstract 1

1

.

INTRODUCTION 1

2. DESCRIPTION AND EVALUATION OF THE NIST RF ANECHOIC CHAMBER 2

3 . DESCRIPTION AND GAIN OF THE STANDARD TRANSMITTING ANTENNAS 5 3 . 1 Open- Ended Waveguides 5 3 . 2 Pyramidal Horns 6

4. MEASUREMENT OF THE RF POWER DELIVERED TO THE TRANSMITTING ANTENNA 9 4.

1

Present Method 9 4.2 Calibration of the Present RF Power Delivery System 11

5. COMPUTATION AND UNCERTAINTY OF THE CHAMBER FIELD MAGNITUDE 12

6 . PROCEDURE FOR GENERATING A STANDARD FIELD 14

7 . PLANNED IMPROVEMENTS IN THE ANECHOIC CHAMBER SYSTEM 15

7.1 Transmitting Antenna Positioner 15

7 . 2 Advanced Measurement Software 16

7 . 3 Control Charts and Support Software 16

7 . 4 Control Standards 16

7 . 5 Detailed Uncertainty Analysis 16

8 . CONCLUSIONS 17

9 . REFERENCES 17

111

GENERATING STANDARD REFERENCE ELECTROMAGNETIC FIELDS

IN THE NIST ANECHOIC CHAMBER, 0.2 TO 40 GHZ

D.A. Hill, M. Kanda, E.B. Larsen, G.H. Koepke , and R.D. Orr

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, CO 80303

The NIST anechoic chamber is used to generate standard (known)

EM fields for frequencies from 200 MHz to 40 GHz. The

transmitting antennas used are open-ended rectangular waveguides

from 200 to 500 MHz and pyramidal horns from 450 MHz to 40 GHz.

The uncertainty in the electric field is currently estimated to

be ± 1 . dB . A number of changes and additions are planned to

improve the accuracy, repeatability, and efficiency of the

system.

Key words: anechoic chamber; delivered power; near-field gain;

open-ended waveguide; pyramidal horn; standard field.

1 . INTRODUCTION

The National Institute of Standards and Technology (NIST) radio

frequency (rf) anechoic chamber is one of several facilities maintained by

NIST for the purpose of generating a standard (known) electromagnetic (EM)

field. This chamber can be used for calibrating antennas, EM probes, and rf

radiation hazard monitors and for research purposes. The anechoic chamber

provides calculable EM fields over the frequency range of 200 MHz to 40 GHz.

The uncertainty of the standard field is now ± 1 dB. A side-view sketch of

the NIST anechoic chamber is given in figure 1.

There are three main parts to the NIST standard-field system: (1) an anechoic chamber, which is described in Section 2 of this report; (2)

several standard- gain transmitting antennas, described in Section 3; and (3) an rf power delivery system which is discussed in Section 4. The antennas

are used to radiate a beam of EM energy into the anechoic chamber, which is

a shielded metal-clad room covered internally with rf absorbers. Thus,

outside ambient EM energy is greatly attenuated, and energy beamed into the

chamber will be absorbed rather than reflected and scattered from the walls.
The electric field strength at a specific distance from the radiating

antenna is calculated from measurements of the power delivered to the

transmitting antenna and a knowledge of the gain of the antenna as a function of frequency and distance to the field point. The equation used to

calculate the electric field E on the boresight axis of the transmitting

antenna is

E = [r/QP^^^G/(47r)]^/Vd = (30 ?G)'^^^/d, (1)

where E = electric field strength, V/m,

P, T = P = net power delivered to the transmitting antenna, W, del ^ o ' >

»7_ = free -space impedance ~ 120 n, 0,

G = gain of the transmitting antenna at the given frequency and

distance, and

d = distance from the center of the aperture of the transmitting

antenna (horn or OEG) to the on-axis field point, m.

Control charts are being implemented to monitor and evaluate the

overall performance of the field generating system used in the anechoic

chamber, so that quality checks which are made each time the facility is

used will reveal whether the system is functioning properly . Separate

control charts will also be maintained for the rf power delivery system.

These serve two purposes: (1) to check on the proper functioning of the

transmitting system and (2) to develop more accurate values of forward and reverse coupling ratios for the directional coupler used in measuring the

net power delivered to the transmitting antenna.

2. DESCRIPTION AND EVALUATION OF THE NIST RF ANECHOIC CHAMBER

The anechoic chamber shown in figure 1 is constructed within a metalshielded room approximately 4.9 m (16 ft) high by 6.7 m (22 ft) wide by

8.53 m (28 ft) long. It is lined with rf absorbing material on the interior

walls, ceiling, and floor to reduce electromagnetic reflections. The
absorber covers all chamber surfaces except for exhaust fans in two side

walls, an opening in the front wall where the transmitting antenna is

placed, and metal rails on the floor. These precision rails support a rolling equipment cart on which the receiving antenna under test (AUT) is

mounted, together with auxiliary equipment such as an antenna

rotator/positioner, driver motors for moving the cart, and rf absorbers on

the front wall of the cart. There is a gap 0.2m wide in the absorbers on

the floor to accomodate each of the two rails. The AUT is mounted inside

the chamber on this equipment cart, which permits movement of the AUT

parallel to the boresight axis of the standard transmitting antenna. The

transmitting antenna is either a pyramidal horn or a rectangular open-ended

waveguide (OEG) and is mounted on a carriage outside the anechoic chamber,

with its radiating aperture located approximately 0.6 m inside the plane of

the absorber tips on the front wall of the chamber.

The equipment cart can be moved manually or positioned with computer

controlled stepping motors to within ± 2 mm along the 6 m length of the

rails. The receiving antenna is installed in a specially fabricated

dielectric fixture placed on or affixed to the cart. The antenna rotator

can be used to make receiving antenna pattern measurements, but pattern

measurements will not be covered in this report.

The response of the AUT to an established standard field is monitored

by voltage or power readouts from instruments placed on the chamber

equipment cart. The response can be read by computer-controlled acquisition

through an IEEE 488 bus. The data are conveyed through fiber optic lines

which extend from the cart to a computer located in an adjacent room. Fiber

optic lines are used because they are essentially transparent to rf energy

and immune to interference or other pickup from fields within the chamber.

The absorber used is a commercially available carbon- impregnated

urethane foam with pyramidal points, as shown in figure 2. This material

has reasonably good absorption at frequencies down to 200 MHz, especially

for normal incidence, but is much better at higher frequencies. Two

independent evaluations were made of the NIST anechoic chamber [1,2] by

measuring the distortion of an electromagnetic field caused by reflections

within the chamber.
In [1], the relative Insertion loss between a standard- gain antenna and

a probe antenna was measured along the bores Ight axis of the standard- gain

antenna. The measurements were made at 20 frequencies from 175 MHz to 18

GHz and compared with theoretical, Inverse -square curves. The theoretical,

Inverse -square curves were fitted to the measured curves at a distance of

1 m. The difference between the measured and theoretical Insertion loss was assumed to be caused by chamber reflection error, and the ranges of

differences are shown In table 1. A maximum difference range of - 0.6 to

0.5 dB occurred at 229 MHz, but at 18 GHz the variation was only ± 0.04 dB.

Insertion loss curves as a function of antenna separation are shown in

figures 3-7. Standing waves due to chamber reflections are most evident at

the lower frequencies.

Measurements were made of electric field components in the anechoic

chamber [2] using a calibrated isotropic NIST probe having three dipoles

mutually orthogonal (designated EFM-3). These data were taken along the

boreslght axis of the transmitting antenna and also along other parallel

axes. The off boreslght data give an indication of error in standard values

as a function of displacement from the boreslght axis and the deviation from

boreslght field value that would occur at off-axis points on a large

receiving antenna or device under test. The off -axis field reduction is

primarily a function of the pattern of the transmitting antenna, while

standing- wave patterns in field strength are due to multi-path interference

caused by chamber reflections.

In the experiments described in [2], a2mx2m wooden frame was mounted vertically on the chamber receiving cart in a plane perpendicular to

the beam from the transmitting antenna. Locations of the receiving probe on

the frame are shown in figure 8. Typical data of measured field strength,

labeled according to probe position, are given in figure 9, and standard

deviations of the electric field are shown in figure 10. The measured offboresight errors are summarized in table 2 for frequencies from 250 to 1000

MHz. The data indicate that for frequencies below 1000 MHz, a 25 cm displacement from the boreslght axis could cause a field strength error as great as ±0.5 dB . However, the laser alignment system in the anechoic

chamber makes it a simple matter to align a receiving probe within ± 1 cm of

the boreslght axis, over a range in antenna separation distance of 1 to 6 m.
Experimental results in [2] indicate that during a 5 m traverse in distance

an unwanted lateral displacement of ± 1 cm at the receiving probe would

cause a field strength deviation from the boresight value by much less than

0.1 dB.

The experiments described in [1] and [2] were performed to evaluate the

uncertainty in computed values of electric field in the anechoic chamber

caused by chamber reflections and uncertainty in probe position. The data

in [1] show that a typical boresight position uncertainty of ± 1 cm makes a negligible contribution to the field strength uncertainties reported in

table 1. The boresight scans of electric field versus distance given in [2] are consistent with those reported in [1], both in shape and amount of

standing-wave distortion.

3. DESCRIPTION AND GAIN OF THE STANDARD TRANSMITTING ANTENNAS

Two types of antennas are used in the NIST anechoic chamber as standard

transmitting antennas in the 0.2-40 GHz frequency range: (1) open-ended

rectangular waveguides (OEGs) at frequencies from 200 to 500 MHz and (2)

pyramidal standard-gain horns from 450 MHz to 40 GHz. Both antenna types

radiate a calculable, linearly polarized electric field on boresight.

3.1 Open-Ended Waveguides

An OEG of width w and height h is shown in figure 11. The approximate

far-field gain, G, of an OEG having a two-to-one aspect ratio (w = 2h) is

calculated at NIST from the equation [3]

G = 21.6 Fw, (2)

where F is the frequency in GHz and w is the waveguide width (longer side)

in meters. A list of the physical dimensions of the OEGs used at NIST is

given in table 3. Two different OEGs are used to cover the frequency range

200 to 500 MHz.
The approximate gain expression in (2) was obtained by fitting an empirical equation [3] to data determined experimentally using a small

indoor antenna range at NIST [4] . The gain of a WR-430 OEG was measured in

the frequency range of 1.7 to 2.6 GHz using two identical OEGs as a transmitter and receiver pair. A graph of the gain calculated from (2), for

a transmitting WR-2100 OEG at 500 MHz, versus the gain obtained from field

strength measurements using a calibrated short-dipole probe is shown in

figure 12. The experimentally determined gain relationship of (2) is

accurate at all distances greater than 1 m, except for cyclical variations

caused by multi-path interference in the anechoic chamber. In general, (2)

is accurate within ± 0.5 dB if the distance d from the OEG aperture to the

field point is greater than 2w [3]. The frequency range where (2) is valid

is the range for which the waveguide size is appropriate and only the TE^

mode is propagating. We also assumed that corrections have been made for

the small insertion loss of the waveguide between the power measurement

point and the radiating aperture.

Possible near-zone corrections for the gain of OEGs have been discussed

in [4] and [5]. However, the near-zone corrections for OEGs are much less

than those of pyramidal horns because the electrical size of the aperture is

smaller. Thus the far- field gain as given by (2) can be used without near- field correction for d > 2w.

New replacement OEGs at NIST are longer than those previously used.

The extra length stabilizes the OEGs mechanically as they extend over the

end of the transmitting antenna positioner table and gives the TE^ „ mode a

greater distance to form before reaching the radiating aperture.

3 . 2 Pyramidal Horns

The geometry of a pyramidal horn is shown in figure 13. The width and

height of the rectangular aperture are a and b, and the slant lengths are i

and £ . Jull's expression for the near-field gain G is [6,7] n
G = ^

R^R„. (3)

where A is the free-space wavelength, and R and R^ include the gain

reduction due to the E and H plane flare of the horn as well as the effect

of finite range. The gain reduction factors are

^ _ C^Cw) + S^(w)

^E " 2 w

(4)

^ _ 7r^{rC(u) - C(v)]^ + {S(u) - S(v)l^)

4(u - v)

where w = h/(2\2')^^'^

£

v' " ;<^V^>^^^ * a/(2Xt^)^^^

,,E

^^

d + 2„'

..H^

d + i„'

hi H

and d is the on-axis distance from the aperture plane. The Fresnel

Integrals C and S are defined as

C(w) - jS(w) = J exp(-J7rt /2) dt

.

(5)

In our computer program for (3) -(5), we use the approximations of Boersma

[8] to compute the Fresnel integrals, and we have checked our results with

Jull's curves and tables for R and R^ [6,7]. Larsen and Ries [3] have

derived polynomial fits for R and R^ that are easy to compute, and they are

accurate for most cases of practical interest.
A comparison of the theoretical gain and the gain values determined

experimentally in the anechoic chamber, as a function of distance, are shown

in figure 14 for a frequency of 500 MHz and in figure 15 at 1000 MHz [3].

These figures show that there is less than 0.8 dB difference between the

theoretical and measured gains for the separation distances used in

calibrations at NIST, which range from 1 to 4 m from the horn aperture. The

greatest difference encountered (0.8 dB) was for distances between 1 and 2 m from the large horn at 500 MHz. At other distances and frequencies the

differences were generally less than 0.5 dB . Some of this difference is due

to an estimated ± 0.5 dB uncertainty in measurement of the E- field with a A/2 standard dipole. The uncertainty due to calculating G from (3) is

approximately ± 0.4 dB for the distances and frequencies used in the NIST

chamber. An additional uncertainty of ± 0.4 dB is caused by in-phase and

out-of -phase reflections from the imperfect absorber and other objects.

More recently, near-field gain measurements from 18 to 40 GHz have been

made in the NIST anechoic chamber using pairs of pyramidal horn antennas

[9]. The gain as determined by the two-antenna method is [10]

¥ (V^t)'^'- ^^)

where d is the distance between the horn apertures , P is the net power

delivered to the transmitting horn, and P is the received power. Normally

(6) is used for the far-field gain, but here we also use (6) for the near- field gain. In figures 16-18, we show the theoretical and measured near- field gains as a function of distance d for frequencies of 18 , 26.5, and 40

GHz [9]. The agreement between theory and measurements is better than in

figures 14 and 15 for two reasons: (1) the two-antenna method eliminates

the uncertainty caused by the A/2 receiving dipole, and (2) the chamber

reflections are very small at frequencies above 18 GHz [1]. The

disadvantage of the two -antenna method is that the near -field gain is not

quite the same as Jull's near- field gain [6,7] which is defined in terms of

the on-axis electric field. However, the two quantities have been shown to

agree quite well for separation distances of interest in generating standard

fields [91.
4. MEASUREMENT OF THE RF POWER DELIVERED TO THE TRANSMITTING ANTENNA

In this section we describe the method currently used to determine the

net power delivered to a transmitting antenna in the NIST anechoic chamber.

A somewhat different method [10] has been proposed for possible future use,

but this proposed method will not be covered here.

4.1 Present Method

The net power delivered to the transmitting antenna is now determined

using a dual-directional coupler attached directly to the transmitting

antenna, as shown in figure 19. The net power is calculated from power

meter readings made at the two side arms of the directional coupler and from

coupling ratios determined by previous calibrations using the equation

P, , = P. - P ^_ (7) del mc rf1

where P .. = net power delivered to the transmitting antenna,

P. = incident power at the antenna input terminal, and

mc ^ '^

P J-, = reflected power at the antenna input terminal.

In (7), the two quantities, ^ P. and P ^^ , are determined from

mc rfl

measurements with 50-fi power meters, using the two equations

(C /lO + A/10)

P. = P. , 10 "^ °

, (8) mc fwd

(C yio)

P .^ = P 10 ^"^

, (9) rfl rev

where P = power meter reading of forward power at side arm 1 of the

directional coupler, port 1 in figure 19,
P = power meter readine of reverse power at side arm 2 of the rev ^ b f

directional coupler, port 2 in figure 19,

C- = forward coupling ratio (in dB) of the directional coupler with

respect to the output port of the coupler in figure 19,

A = insertion loss (in dB) of the calibrated forward attenuator

pad, if used, and

C = reverse coupling ratio (in dB) of the directional coupler with

respect to the output port of the coupler.

The 50-r2 power meters are calibrated by the NIST power calibration services.

C^ in (8) must be measured in terms of the ratio of the power reading

at side arm 1 (port 1) to the power accepted by a SO-fi power meter connected

at the output (port 4) of the directional coupler. Accurate measurements of

power when using the NIST-calibrated power meters require that correction

factors be applied to the power meter by means of a dial setting on the

front panel of the power meter. This correction factor makes allowance for:

(1) the measured efficiency of the bolometer mount of the power sensor, (2)

any mismatch error in the power meter sensor caused by its impedance

deviating from 50 Q, and 3) errors associated with the dc portion of the

power meter. The NIST calibration of this overall correction factor, in the

frequency range of . 2 to 2 GHz, has a total uncertainty of ± 0.5 %. The

value of the total correction factor varies from about 90 to 100 %,

depending on the signal frequency (and power to a minor degree) . After the

appropriate correction factor has been set on the power meter panel, the

indicated power is correct to ± 0.5 %. When using the NIST standard gain

transmitting antennas, which have a maximum standing wave ratio of 1.5, it

is possible to measure P, , with an uncertainty of ± 0.6 %. The additional

del

0.1 % in uncertainty is caused by taking the difference between the larger

value of P. and the relatively small value of P ^^ . mc -^ rfl

10
The NIST measurements of the two coupling ratios, C^ . and C , ^ ^ fwd rev

also have an uncertainty of ± 0.6 % associated with their calibrations. The

values of the measured coupling ratios vary slightly with signal

frequency; so a look-up table giving coupling ratios versus frequency is

used for the computer calculations of net delivered power. The frequencies

listed in the look-up tables are in increments of 50 MHz from 0.2 to 2 GHz

and in increments of 100 MHz from 2 to 40 GHz. Linear interpolation is used

for signal frequencies which are between two values included in the table.

This approach yields interpolated coupling ratio values having an added

uncertainty^ of ± 0.5 %. Therefore, the measurements of P^ , and P have a fwd rev maximum total uncertainty of ± 1.7 %. This represents the sum of 0.6 % for

the power meter measurements, 0.6 % for the coupling ratio calibrations, and

0.5 % for possible errors caused by interpolation in the tables of measured

coupling ratios. The overall ± 1.7 % uncertainty corresponds to an uncertainty in power delivered to the transmitting antenna (P .. ) of

approximately ± 0.07 dB.

The above expressions for P , , assume that the directional coupler is

del

ideal. That is, coupling of rf power occurs only between ports 3 and 1, 4

and 2, and 3 and 4 shown in figure 19. Such a coupler is ideal in the sense

that there is complete isolation between ports 1 and 4, 2 and 3, and 1 and

2.

4.2 Calibration of the Present RF Power Delivery System

A dedicated dual-directional coupler shown in figure 19 is periodically

calibrated by one of three different and independent methods at NIST: (1) a six-port power measurement system, which is the present NIST standard for

measurement of network parameters, (2) a commercial automatic network

analyzer which has been calibrated at NIST, or (3) a procedure in which the

coupling ratios are measured with calibrated power meters in our laboratory.

Over a period of two years, three sets of coupling ratio measurements were

made in 20 MHz increments from 0.2 to 2.0 GHz. These measurements show less

than ± 0.06 dB variation (at any given frequency) between the sets of data.

11
Recently, three sets of coupling ratios were measured from 0.2 to 2.0 GHz,

and again the sets differed by less than 0.1 dB.

The periodic NIST calibrations of the power meters and directional

couplers constitute the required calibrations for the present rf power

delivery system.

5. COMPUTATION AND UNCERTAINTY OF THE CHAMBER FIELD MAGNITUDE

Using (1) and the estimated uncertainties for the three quantities

appearing on the right side of the equation, the uncertainty in the computed

value for the chamber E-field can be calculated. These three uncertainties

are discussed separately in the following paragraphs.

The uncertainty in P, ^ can be derived from (7) -(9) written in the form

del

[C /lO + A/10] [C v/10]

P, = P^ 10

^""^

- P 10

""^^

, (10) del fwd rev

where P^ , and P are the readings of the power meters on ports 1 and 2 of fwd rev tor r

the dual-directional coupler in figure 19. When these power meters are calibrated with reference to the NIST primary rf power standard, the

measured power has an uncertainty of ± 0.6 %. In the present NIST power

delivery method, a high-quality network analyzer is used to measure the two

coupling° ratios, C^ , and C , with an uncertainty of ± 0.6 %. As

fwd rev -^

discussed in Section 4.2, an additional possible uncertainty of ± 0.5 % comes from using interpolated values of coupling ratios at frequencies not

previously measured. The maximum total possible uncertainty is the sum of

these separate uncertainties, or ± 1.7 %, which is approximately ± 0.1 dB

.

At the present time, the theoretical (computed) gain values being used

in (1) are calculated using (2) for OEGs and (3) for pyramidal horns. For

both types of transmitting antennas, the uncertainty of the E-field

magnitude in the anechoic chamber, as computed from (1), is ± 0.8 dB. As

discussed in Section 3, about half of this uncertainty is caused by

multipath interference in the NIST anechoic chamber, and the other half is

12
caused by inaccuracy in determining the effective gain of the transmitting

antennas at the near-zone distances used. For purposes of error analysis,

we combine the chamber multipath with uncertainty in the free -space antenna

gain to obtain the effective uncertainty of ± 0.8 dB for the near- field

antenna gain in the anechoic chamber environment.

The total error equation (in dB) relating to (1) is

U = 10 log^^Qd + ^) + 10 log^Qd + ^) +20 log^^Qd + ^), (11)

where U = worst-case uncertainty in the standard E- field generated in the

NIST anechoic chamber, ± dB

,

AP~r = fractional uncertainty in power delivered to the transmitting

antenna

,

~AG ^ . ^ . . . j~ . . . = fractional uncertainty in near-zone gam of the transmitting

antenna in the anechoic chamber, and

~~: = fractional uncertainty in measuring the field point distance, for

the closest distance used.

The worst -case overall uncertainty in the field strength generated in the

anechoic chamber, as calculated from (11) is the sum of ± 0.1 dB for

measuring the power P, ± 0.8 dB for calculating the antenna gain G in the

anechoic chamber environment, and ± 0.1 dB for measuring the distance d for

a total uncertainty of ± 1.0 dB

.

The major contributor to possible error is uncertainty in G. Research,

both theoretical and experimental, has begun to reduce the uncertainty in

both G and d. This involves extensive improvements to the anechoic chamber,

including new absorbing material and precise antenna positioners under

computer control. This will permit the rapid measurements required for a thorough evaluation of the chamber performance so that accurate corrections

13
can be applied to the theoretically calculated values of field strength for

any frequency and field point location used.

6. PROCEDURE FOR GENERATING A STANDARD FIELD

The boresight axis of the transmitting antenna in the anechoic chamber

is defined by a horizontal laser beam which has been aligned parallel to the

direction of motion of the receiving probe cart, as shown in figure 1. The

laser is located in the chamber anteroom behind the rolling carriage that

supports the transmitting antennas.

To achieve coincidence between the laser alignment beam and the axis of

the transmitting antenna, the transmitting antenna is placed on the table

located on top of the rolling carriage. The table is then rolled to its

maximum forward position into the chamber doorway. The antenna is placed on

this table with its aperture 60 cm beyond the plane of the chamber absorber

tips. For both OEGs and horns, this aperture plane is far enough inside the

chamber so that there are only negligible reflections from the table

undercarriage, chamber doorway, and steps into the chamber. The height of

the antenna table is then adjusted until the laser beam and boresight axis

of the transmitting antenna are at the desired height near the center line.

The procedure for an OEG is to align it with the side of the antenna

positioner table while the OEG is centered on the laser beam. The OEG is

adjusted until its axis is horizontal and in line with the chamber axis. In

this way the receiving antenna on the cart can traverse the entire length of

the anechoic chamber without becoming misaligned with respect to the laser

beam and boresight axis of the transmitting antenna. For the pyramidal

horns, the chamber alignment procedure is similar to that described above

for OEGs.

Once an OEG or horn has been aligned properly, its boresight axis will

be parallel to the direction of motion of the cart. The AUT mounted on the

cart in the anechoic chamber is also centered and aligned with respect to

the laser beam. The only remaining adjustment to be made is the positioning

of the chamber cart at the desired field-point distance.

14
All instrumentation for the anechoic chamber system is turned on and

allowed to stabilize while the transmitting and receiving antennas are being

positioned and aligned. Each power meter sensor is calibrated immediately

before each set of measurements by connecting it to the internal 1 mW, 50-

MHz source built into the power meter.

In the present NIST method of chamber operation, the rf power delivery

system is connected to the transmitting antenna by attaching the calibrated

directional coupler directly to the antenna. This arrangement puts port 4

of the 4-port system at the coupler -antenna connection as shown in figure

19.

When the transmitter equipment has warmed up and stabilized, the

desired field can be established according to the flowchart given in figure

20. The sequence shown there provides the required reading on the forwardpower meter in order that the transmitting antenna create the desired field.

The output of the rf signal generator is then adjusted until the required

reading is attained. In this forward-power determination for a given

measurement setup, the only experimental data required is the magnitude of

the reflection coefficient at the transmitting antenna input connector.

This is determined by setting an arbitrary power level, at the desired

frequency, and measuring the forward and reflected power at the side arms of

the directional coupler. The power level must be sufficient to avoid noise

and zero drift in the power meter indication, but below a power level which

could damage the power sensors.

7. PLANNED IMPROVEMENTS IN THE ANECHOIC CHAMBER SYSTEM

7.1 Transmitting Antenna Positioner

We are now developing a new transmitting antenna positioner. The new positioner and alignment procedure will ensure that the boresight axis of

the OEG or the horn is parallel to the direction of cart motion. The new positioner will also improve the repeatability and accuracy of the

transmitting antenna position.

15
7 . 2 Advanced Measurement Software

The basic measurement sequence has been programmed. There are several

algorithms that will greatly enhance the overall quality of measurements in

the chamber. These include monitoring and leveling the output power and/or

measured receiving probe output during the course of a measurement, scanning

the boresight direction and determining the optimum location for the

calibration, an automated method of applying the system calibration values

to reduce the chance for operator errors, and the addition of system

monitoring to prevent damage to the sensors and instruments.

7 . 3 Control Charts and Support Software

We are currently developing general control chart software that will

help us monitor the measurements and check standard data and the system

calibration data. Eventually every component of the system will have a control chart available.

7.4 Control Standards

We are planning to produce additional 3 cm and 8 mm open- circuit dipole

probes for use as check standards in the chamber. A 15 cm dipole probe is

currently in use for frequencies up to 1.0 GHz. A repeatable technique

needs to be developed for mounting the check standard in the chamber such

that it can be positioned at the calibration location, but can be easily

removed and will not interfere with the AUT

.

7.5 Detailed Uncertainty Analysis

There is a need to reexamine the uncertainty for chamber measurements.

The current ± 1 dB might be optimistic for frequencies below 500 MHz, but is

probably conservative for frequencies from 18 to 40 GHz [9]. More antenna

16
gain measurements are needed for frequencies below 18 GHz , particularly at

the boundaries of the various frequency bands

.

8. CONCLUSIONS

The NIST anechoic chamber is used to generate standard (known) EM

fields for frequencies from 200 MHz to 40 GHz. The present uncertainty of

the standard field is the sum of ± 0.1 dB for measuring the power, ± 0.8 dB

for calculating the near- field gain of the transmitting antenna, and

± 0.1 dB for measuring the distance from the transmitting antenna for a total uncertainty of ± 1.0 dB. Of the ± 0.8 dB uncertainty in the near- field gain, about half (± 0.4 dB) is due to multipath interference in the

NIST anechoic chamber, and the other half (±0.4 dB) is due to inaccuracy in

calculating the gain of the transmitting antenna at the near- field distances

used in the chamber.

Some of the planned improvements in the system are a new transmitting

antenna positioner, advanced measurement software, control charts and

support software, control standards, and a detailed error analysis over the

entire frequency range from 200 MHz to 40 GHz. Some measurements between

200 and 500 MHz with OEG transmitting antennas indicate that the uncertainty

due to multipath reflections is dependent on the field point location in the

chamber, and this needs to be better understood to achieve ± 0.8 dB

uncertainty in the near-field antenna gain. On the other hand, measurements

from 18 to 40 GHz with pyramidal horns [9] indicate that the chamber

reflections are very small and that the uncertainty in the near- field gain

is only about ± 0.3 dB in this frequency range.

9 . REFERENCES

[1] Fitzgerrell, R.G. Using free-space transmission loss for evaluating

anechoic chamber performance. IEEE Trans. Electromag. Compat., EMC-24:

356-358; 1982.

[2] Kanda, M. ; Wyss , J.C. Evaluation of off-axis measurements performed in an anechoic chamber. Nat. Bur. Stand. (U.S.) Tech. Note 1305; 1986.

17
3] Larsen, E.B.; Ries , F.X. Design and calibration of the NBS isotropic

electric-field monitor (EFM-5), 0.2 to 1000 MHz . Nat. Bur. Stand.

(U.S.) Tech. Note 1033; 1981.

4] Bowman, R.R. Calibration techniques for electromagnetic hazard meters:

500 MHz to 20 GHz. Nat. Bur. Stand. (U.S.) NBSIR 75-805; 1975.

5] Wu, D.I.; Kanda, M. Comparison of theoretical and experimental data

for the near field of an open-ended rectangular waveguide. IEEE Trans.

Electromag. Compat., 31: 353-358; 1989.

6] Jull, E.V. Finite -range gain of sectoral and pyramidal horns.

Electronics Letters, 6: 680-681; 1970.

7] Jull, E.V. Aperture Antennas and Diffraction Theory. Stevenage: Peter

Peregrinus Ltd.; 1981.

8] Boersma, J. Computation of Fresnel integrals. Math. Comp . , 14: 380;

1960.

9] Hill, D.A. ; Ehret, R.L. Near-field gain of pyramidal horns from 18 to

40 GHz. Nat. Inst. Stand. Tech. (U.S.) NISTIR 89-3924; 1989.

10] Kanda, M. ; Orr, R.D. A radio -frequency power delivery system:

procedures for error analysis and self -calibration. Nat. Bur. Stand.

(U.S.) Tech. Note 1083; 1985.

18
Table 1 . Anechoic chamber reflection errors for 1 to 3 m separation

distances. *

Frequency

MHz

175 OEG

229

301

394

517

517 HORN

677

888

1164

1527

2000

2450

3950

4000

5000

6100

7600

9400

li700

14500

18000

Source

Antenna

Probe Dipole Length

cm or OEG Size

WR 3600 30

WR 3600 30

WR 3600 30

WR 2100 15

WR 2100 15

SA 12-0.5 15

SA 12-0.5 15

SA 12-0.75 15

NARDA 646 10

NARDA 646 10

NARDA 645 10

NARDA 645 3.3

MICROLAB S638A 3.3

MICROU^B H638A WR 187

MICROIAB H638A WR 187

SA 12-5.8 WR 137

SA 12-5.8 WR 137

DBG-520-20 WR 90

DBG-520-20 WR 90

SA 12-12 WR 62

SA 12-12 WR 62

Range of Error Uncertainty

dB dB

-0.6, +0.1 ±0.10

-0.6, +0.5 ±0.14

-0.5, +0.2 ±0.10

-0.4, +0.2 ±0.08

-0.3, +0.2 ±0.08

-0.3, +0.1 ±0.06

-0.3, +0.1 ±0.06

-0.2, +0.1 ±0.06

-0.2, +0.2 ±0.06

-0.2, +0.0 ±0.04

±0.1 ±0.04

±0.1 ±0.04

±0.1 ±0.04

±0.04 ±0.03

±0.12 ±0.04

±0.04 ±0.03

±0.04 ±0.03

±0.05 ±0.03

±0.04 ±0.03

±0.04 ±0.03

±0.04 ±0.03

Certain commercial equipment, instruments, or materials are identified in

this paper in order to adequately specify the experimental procedure.

Such identification does not imply recommendation or endorsement by the

National Institute of Standards and Technology, nor does it imply that

the materials or equipment identified are necessarily the best available

for the purpose.

19
Table 2. Error due to off -axis alignment,

Offset distance from center line (m) 0.25

(dB)

0.50

(dB)

0.75

(dB)

1.00

(dB)

Frequency Source Antenna

(MHz) type

250 OEG

600 Horn

700 Horn

800 Horn

900 Horn

1000 Horn

±0.5 ±1.2 ±2.0

±0.8

±2.5

±2.5

±2.5

±2.5

±3.0

Table 3. Dimensions and frequencies of OEG antennas

1. WR-3600 Open-Ended Guide

Frequency = 200 to 300 MHz

Aperture = 91.44 cm x 45.72 cm (36 in x 18 in)

2. WR-2100 Open-Ended Guide

Frequency = 300 to 500 MHz

Aperture = 53.34 cm x 26.67 cm (21 in x 10.5 in)

20
--' /WW/v

EHP/VHP-26 ABSORBER ON *^^^

METAL CLAD PLYWOOD WALLS <^

REFLECTION FROM *^^^>

REAR WALL k;:;^ ^ PROSE

DIRECT /

OEG PATH /

OR

HORN REFLECTX>j\ FROM \\

wZ2i

DRATOF?Y,^ SOURCE \\SCATTERING -^\\

FROM c^F^^^____\S^K^-. ^MA'

Pl

3 I

4.9 m

8.53 m

Figure 1. A side view of the NIST-Boulder anechoic chamber,

21
ao^

Figure 2. Carbon- impregnated urethane foam which lines the walls, ceiling,

and floor of the anechoic chamber.

22
2 3 SEPARATKDN, m

Figure 3. Relative insertion loss between source and probe antenna versus separation distance with free-space transmission loss curve fitted at 1 m; frequency = 175 MHz; OEG source antenna.

23
2 3 SEPARATION, m

Figure 4. Relative insertion loss between source and probe antenna versus

separation distance with free -space transmission loss curve fitted at 1 m; frequency = 517 MHz; OEG source antenna.

24
2 3 SEPARATION, m

Figure 5

.

Relative insertion loss between source and probe antenna versus separation distance with free-space transmission loss curve fitted at 1 m; frequency = 517 MHz; horn source antenna.

25
2 3 SEPARATION, m

Figure 6. Relative insertion loss between source and probe antenna versus

separation distance with free-space transmission loss curve fitted at 1 m; frequency = 2450 MHz; horn source antenna.

26
SEPARATION, m

Figure 7. Relative insertion loss between source and probe antenna versus separation distance with free-space transmission loss curve fitted at 1 m; frequency = 11 700 MHz; horn source antenna.

Note: Difference between measured and calculated data point on 1-

dB/div curve at 1-m separation is caused by cart inertia.

27
s i r

•

•

•

63

62

61

f. • • • • 1 V

43 42 41

•

•

•

21

22 >.

23^

>• 0.25 m

ft. i E

Probe Position on Frame

Figure 8. Frame used for holding probe on cart. Positions 1 through 9 are separated by 1 m Intervals. The numbered positions (21-24, 41-

44, and 61-64) are each separated by 25 cm.

28
20 600 MHz

^ 15 E

10 ^

....•

2 ^^ ' ^v

,.,•..-•«... .^ "* "^ "*•—• X.^. •••>

3 4 5

Distance from Horn lis)

Figure 9. Field strength versus distance from horn for 1 m off -axis

position at 600 MHz.

29
3.0

CO

o

>Q

U

<o T3C

(TJ +->

2.0 "

0.0

/

0.00 0.25 0.50 0.75

Off-Set from On-Axis (m)

1.00

Figure 10. Standard deviation of the electric field versus offset from

center line at 800 MHz. In each case, the worst case standard

deviation is used for distances more than 2 m from the launching

horn.

30
Figure 11. Geometry for an open-ended rectangular waveguide

31
IS'

14

13

12-

a

IB ' Z g. tCASUREC CAINS

1 <U 8'

• ^ \

. • . . • * •

•

• < _ Z

1- ° Z

< 5

•

• THEOHtnCAL CAIN,

-^MEASU^L CAINS

2-

1

•

2 3 DISTANCE FROM OEG APERTURE. METERS

Figure 12. WR-2100 OEG gain vs distance at 500 MHz

32
Figure 13. Geometry for a pyramidal horn.

33
le

is-t14

13

12 •

11 • g 10 +

8-

76-

5

4

3

2-

1

TVeORETICAL

2 3 DISTANCE FROM HORN APERTURE. METERS

Figure 14. Pyramidal horn gain vs distance at 500 MHz. Horn dimensions

a = 122.5 cm, b = 90.75 cm, i„ = 142.0 cm, i^ = 121.3 cm.

ri L

34
IB

15

14 •

13

12 +

11 m IB" D

2

8-

7

e

5

4

3

2

1 •

, MOSUREO CAINE 12 3 4 DISTANCE FROM HORN APERTURE, METERS

Figure 15. Pyramidal horn gain vs distance at 1000 MHz. Horn dimensions

a = 78 cm, b = 61.18 cm, i„ = 94.3 cm, i^

H E

11.2 cm.

35
1 1 r "I 1 1

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38
rf generator

System Boundary

Power Meters

Antenna

Figure 19. System for measuring rf power delivered to an antenna.

39
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40
NIST-114A

(REV. 3-89) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATASHEET

1 . PUBUCATION OR REPORT NUMBER

NIST/TN-1335

PERFORMING ORGANIZATION REPORT NUMBER

PUBUCATION DATE March 1990

4. TITLE AND SUBTITLE Generating Standard Reference Electromagnetic Fields in the NISI Anechoic Chamber,

0.2 to 40 GHz

5. AUTHOR(S)

D.A. Hill, M. Kanda, E.B. Larsen, G.H. Koepke, and R.D. Orr

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY eA» THcnsBunQ7MB-ge699 Boulder, Colorado 80303-3328

7. CONTRACT/GRANT NUMBER

8. TYPEOFREPORT AND PERIOD COVERED

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, FIPS SOFTWARE SUMMARY, IS ATTACHED.

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

The NIST anechoic chamber is used to generate standard (known) EM fields for

frequencies from 200 MHz to 40 GHz. The transmitting antennas used are openended rectangular waveguides from 200 to 500 MHz and pyramidal horns from 450

MHz to 40 GHz, The uncertainty in the electric field is currently estimated

to be +1.0 dB. A number of changes and additions are planned to improve the

accuracy, repeatability, and efficiency of the system.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

anechoic chamber; delivered power; near-field gain; open-ended waveguide;

pyramidal horn; standard field

13. AVAILABILITY UNLIMITED FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). X

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD. VA 22161.

14. NUMBER OF PRINTED PAGES

-4 8

IS. PRICE

ELECTRONIC FORM

v> U.S. GOVERNMENT PRINTING OFFICE: 1990—775-300/25.102 REGION NO. 6



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