ARL-TR-7963 ● MAR 2017

US Army Research Laboratory

When to Perform Antenna Measurements in a 

Near-Field Range or a Short Tapered Chamber

by Theodore K Anthony

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ARL-TR-7963 ● MAR 2017

US Army Research Laboratory

When to Perform Antenna Measurements in a 

Near-Field Range or a Short Tapered Chamber

by Theodore K Anthony

Sensors and Electron Devices Directorate, ARL

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When to Perform Antenna Measurements in a Near-Field Range or a Short 

Tapered Chamber

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Theodore K Anthony

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US Army Research Laboratory

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Adelphi, MD 20783-1138

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ARL-TR-7963

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

This study was undertaken to quantify and compare electromagnetic device (i.e., antenna) measurements using the US Army 

Research Laboratory’s (ARL’s) near-field range and tapered anechoic chamber. Our 2 antenna measurement systems obtain 

similar results as reported in a previous ARL technical report, A Comparison of Antenna Measurements in a Near-Field Range 

and a Newly Renovated Short Tapered Chamber, but a decision tree is needed to obtain the results for future antennas under 

test in the most cost-effective manner (man-hours, custom mount, etc.)

15. SUBJECT TERMS

electromagnetic, chamber, near-field range, anechoic chamber, antenna measurement

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Theodore K Anthony

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Contents

List of Figures iv

1. Introduction 1

2. Decision Tree Parameters 1

3. Decision Tree Questions 2

4. Antenna Theory 3

5. Near-Field Range 4

6. Tapered Anechoic Chamber (Far-Field Range) 4

7. Conclusion 7

8. References 8

List of Symbols, Abbreviations, and Acronyms 9

Distribution List 10
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iv

List of Figures

Fig. 1 Fields from a radiating antenna (image courtesy of Nearfield Systems, 

Inc.)........................................................................................................3

Fig. 2 NFR/FFR decision tree ..........................................................................5

Fig. 3 Venn diagram shows the best antenna range given conditions..............6
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1. Introduction

This study was undertaken to quantify and compare electromagnetic (EM) device 

(i.e., antenna) measurements using the US Army Research Laboratory’s (ARL)

near-field range (NFR) and tapered anechoic chamber. Our 2 antenna measurement 

systems obtain similar results, as reported in a previous ARL technical report, A 

Comparison of Antenna Measurements in a Near-Field Range and a Newly 

Renovated Short Tapered Chamber,

1 but a decision tree was requested to obtain the 

results for future antennas under test (AUTs) in the most cost-effective manner 

(man-hours, custom mounting, etc.). The NFR and tapered anechoic chamber are 

the basic resources that the antenna team can use to measure and characterize EM 

fields that are transmitted and/or received by devices (e.g., antennas) to validate 

simulated performance with measured data. Accordingly, it was imperative that a

decision tree mapping out the most cost-effective manner to obtain AUT data be 

established for either the NFR or tapered anechoic chamber.

The NFR can obtain planar, cylindrical, or spherical near-field measurements of 

EM fields, while the tapered anechoic chamber can obtain spherical far-field pattern

measurements of EM fields. Our NFR has a frequency range of 1.2–50 GHz with 

maximum internal chamber dimensions of 25 × 16 × 10 ft, while the tapered 

anechoic chamber has a frequency range of 0.2 –50 GHz with chamber dimensions 

of 20 × 20 × 55 ft.

2. Decision Tree Parameters

Deciding when to use either the NFR or the tapered anechoic chamber, also named 

the far-field range (FFR), is a complex process. Fortunately, the information needed

for this decision can be obtained, but compromises might need to be made for 

speed, resolution, and setup costs. The most important decision parameters of 

interest are the customer’s output requirements. More decision parameters are the 

AUT’s characteristics such as weight, dimensions, and directivity. These 

parameters, along with time frame and funding level, determine if there is a need 

for phi axis AUT custom mounting.

The customer’s output requirements can include frequency range and resolution, 

angular range and resolution, polarization, 3-D and 2-D radiation patterns, and 

boresight realized gain. The customer’s output requirements will navigate most of 

the route on the decision tree. The AUT’s weight, dimensions, and directivity will 

then narrow the decision tree parameters further, taking you to the final turn. 

Finally, a decision has to be made if the data collection process needs to be sped up 

by crafting a phi axis custom mount for the AUT.
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3. Decision Tree Questions

This decision tree process is mostly yes/no answers to questions based on the 

information on hand. The following are a few questions to ask, with the decision 

and reason in parentheses:

• When must you get this data by? (Helps to set priority level, and sets the 

bound for the most that could possibly be done.)

• Does the AUT only transmit through its own source? (If yes, the FFR is best 

suited for this measurement since the NFR would be harder to convert for 

this type of measurement.)

• Are any of the frequencies of interest below 1.2 GHz? (If yes, only the FFR 

[0.2–50 GHz] can measure below 1.2 GHz. The NFR measures from 1.2 to 

50 GHz.)

• Do you only need boresight realized gain? (If yes, only the FFR can measure 

just the boresight realized gain. The NFR requires measuring a great portion 

of the AUT’s radiation surface energy [3-D] to calculate the AUT’s gain.)

• Is fMAX[GHz] > 19.685/(D[feet])2

, where D is the diagonal across the AUT’s 

aperture? (If yes, only the NFR can measure this AUT since this is 

considered a near-field measurement.)

• Is the AUT’s front to back ratio worse than 10 dB? (If yes, only the FFR 

can measure this AUT since the NFR’s back plate will corrupt the data 

without reflection mitigation.)

• What are the AUT’s dimensions and weight? (The FFR is more 

accommodating than the NFR in that it has more space to maneuver large 

AUTs and the ability to spin over 5000 N on its turntable.)

• Would the AUT’s torque on the phi axis plate be greater than 60 N-m? (If 

yes, the FFR is best suited for this measurement since the NFR cannot 

withstand as much torque as the FFR.)

• Do you need 3-D radiation patterns? (If yes, a phi axis custom mount for 

the AUT needs to be crafted for the NFR, or a Styrofoam cylinder or sphere 

needs to surround the AUT under test centered along the AUT’s phi axis for 

the FFR.)

• Is there going to be a phi axis custom mounting for the AUT? (If no, the 

FFR is best suited for this measurement since the NFR cannot cover 330°

without a phi axis custom mount.)
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• Do you only need 2-D radiation patterns? (If yes, only the FFR can measure 

just the 2-D radiation patterns. The NFR requires measuring a great portion 

of the AUT’s radiation surface energy [3-D] to calculate the AUT’s 2-D 

radiation patterns.)

4. Antenna Theory

For the purposes of testing in the NFR, the antenna must be situated in the radiating 

near-field for proper data acquisition. The antenna has a reactive near-field from 0 

to λ with a fairly flat field distribution, where objects in this region can possibly 

cause unwanted coupling to the antenna. As such, near-field systems cannot 

measure inside this region due to the unknown coupling effects to an AUT. The 

radiating near-field is from λ to 2D2

/λ with a fairly smooth field distribution, where 

D is the largest dimension across the aperture.3 Near-field systems measure inside 

this region. The far-field region is next, extending beyond 2D2

/λ, where most 

antenna chamber measurements are done. Figure 1 shows the fields from a radiating 

antenna.

Fig. 1 Fields from a radiating antenna (image courtesy of Nearfield Systems, Inc.2)

The theory behind near-field measurements was developed at the National Institute 

of Science and Technology (NIST) in the 1970s by the Technical University of 

Denmark and NIST. NIST has validated the mathematical calculation of far-field 

patterns based on amplitude and phase samples in the radiating near-field.
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5. Near-Field Range

The near-field planar measurement system is best suited for characterizing highly 

directive antennas or arrays because less than a hemisphere (spanning 180° × 180°) 

of energy can be measured with a planar scan. The AUT is positioned

3–5 wavelengths away from the planar scanner, hampering any wideband 

characterization without relocating the AUT. In addition, the hemisphere 

transitions to a narrower cone measurement as the test frequency lowers, thereby 

reducing the angular span of the measurement. The planar NFR uses waveguide 

probe antennas (7 antennas cover 1.1–18 GHz) with no AUT slide or removable 

mast and can measure mid- and small-sized AUTs. A single planar scan requires 

about 1 h of acquisition time for each waveguide probe measurement. No 2-D 

far-field patterns can be produced until a full planar near-field measurement is 

completed.

The near-field spherical measurement system can be used for characterizing 

antennas or arrays and can measure EM fields spanning 330° × 180° of a sphere on 

a single setup. For this measurement the AUT should be placed above the azimuth 

positioner’s center of rotation while centered along the phi positioner’s center of 

rotation. The spherical NFR uses waveguide probe antennas (7 antennas cover 

1.1–18 GHz) with no AUT slide or removable mast and can measure mid- and

small-sized AUTs. A single spherical scan can require 2–72 h depending on 

wavelength and antenna positioning. Two-dimensional far-field patterns cannot be 

produced until a full spherical near-field measurement is completed. Therefore, 

spherical near-field measurements are better than planar for this comparison study 

with the tapered anechoic chamber.

6. Tapered Anechoic Chamber (Far-Field Range)

ARL’s tapered anechoic chamber spherical measurement system can be used for 

characterizing antennas or arrays and can measure EM fields spanning 360° × 180°

of a sphere. It can measure a single point to determine realized gain, angular sweeps 

for 2-D pattern cuts, circular polarization, and 3-D patterns. The AUT should be 

placed above the azimuth positioner’s center of rotation while centered along the 

phi positioner’s center of rotation. The chamber uses decade calibration antennas 

(2 antennas cover 0.2–18 GHz) and has an AUT slide with removable mast,

allowing measurement of large and small AUTs. A single 3-D spherical scan will 

take 3 h for 1–18 GHz, but a 2-D cut plane takes about 5 min. Figure 2, a decision 

tree, and Fig. 3, a Venn diagram, map out the most cost-effective manner to obtain 

AUT data from either the NFR or FFR.
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Yes

No

Yes

No

Yes

No

Yes

No Yes

No Yes

No

Yes

No

Yes

No

Yes

No

Fig. 2 NFR/FFR decision tree

01

02

03

04

01. Frequencies of interest 

below 1.2 GHz

02. AUT only transmits (Tx) 

through its own source

03. Only need boresightrealized gain or 2-D 

patterns

04. AUT’s front-to-back ratio 

worse than 10 dB

05. AUT too big and/or heavy 

for NFR

06. AUT’s torque on the phi 

axis plate greater than 60 

N-m

07. Phi axis custom mounting 

for the AUT

08. Need 3-D radiation 

patterns

09. FMAX[GHz] <

19.685/(D[feet])2

, where 

D is diagonal across 

AUT’s aperture

05

06

07

08

09

FFR

NFR
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Fig. 3 Venn diagram shows the best antenna range given conditions.

FFR

• Faster setup and data acquisition

• Frequencies of interest below 1.2 GHz

• AUT only transmits (Tx) through its own source

• Only need boresight realized gain or 2-D patterns

• AUT’s front-to-back ratio worse than 10 dB

• AUT too big and/or heavy for NFR

• AUT’s torque on the phi axis plate greater than 60 N-m

• Phi axis custom mounting for the AUT

• Need 3-D radiation patterns

• FMAX[GHz] >19.685/(D[feet])2

, where D is diagonal across AUT’s 

aperture

NFR
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7. Conclusion

This study was initially motivated to further validate our EM device measurements.

Then a decision tree, mapping out the most cost-effective manner to obtain AUT 

data from either the NFR or FFR, was requested. Both systems measure patterns 

well, but each system has its own unique capabilities. Pattern and gain 

measurements are done quickest over a wide frequency range in the tapered 

anechoic chamber. The tapered anechoic chamber can also be used to find an 

antenna’s phase centers. The NFR is best suited to produce far-field patterns from 

measured near-fields of antenna arrays and finding EM leakage with its NSI 

software tools. ARL’s planar near-field measurement system requires the AUT to 

have a gain greater than 15 dBi, while a spherical near-field measurement only 

requires the AUT minimize radiation toward the phi positioner’s metal mounting 

plate. 

The initial study proved that the tapered anechoic chamber and NFR do provide 

comparable results, which further validates our EM device measurements. In 

addition, the highly detailed antenna range decision tree was successfully 

developed, clarifying when to use either the NFR or FFR for a given AUT.
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8. References

1. Anthony TK. A comparison of antenna measurements in a near-field range and 

a newly renovated short tapered chamber. Adelphi (MD): Army Research 

Laboratory (US); 2016 Sept. Report No.: ARL-TR-7828.

2. Gray D. How to choose an antenna range configuration. Torrance (CA): NSI, 

Inc.; 2002 [accessed 2016 Sep]. www.nearfield.com. 

3. Balanis CA. Antenna theory: analysis and design. Hoboken (NJ): Wiley 

Interscience; 2005. p. 167–170.
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List of Symbols, Abbreviations, and Acronyms

2-D 2-dimensional

3-D 3-dimensional

ARL US Army Research Laboratory

AUT antenna under test

EM electromagnetic

FFR far-field range

NFR near-field range

NIST National Institute of Science and Technology
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