E-BOOK
Precision Antenna
Measurement Guide
September 2017

S P O N S O R E D

B Y

Table of Contents

3

Introduction

Pat Hindle

Microwave Journal, Editor

4

Basic Rules for Anechoic Chamber Design,Part One: RF Absorber Approximations

Vince Rodriguez
MI Technologies

10 Basic Rules for Anechoic Chamber Design,Part Two: Compact Ranges and Near Field
Measurements

Vince Rodriguez
MI Technologies

15 Extending
the Quiet Zone Using an RF Lens
on a Conical Tapered Chamber to 18 GHz
V. Rodriguez, ETS-Lindgren L.P.; S. Matitsine, Matsing Pte. Ltd. and Temasek Laboratories, National
University of Singapore; T.T. Chia, DSO National Laboratories and Temasek Laboratories,
National University of
Characterizing and Tuning Antennas Using an
APP NOTE Automated Measurement System and a VNA
Singapore; P. Lagoiski, L. Matytsine and M. Matytsine, Matsing Pte. Ltd.; P.K. Tan, Temasek Laboratories,
National University of Singapore

Application Profile

22 Characterizing
and Tuning Antennas
Using an Automated Measurement

As the number of devices featuring wireless connectivity
grows, ensuring their performance specifications while staying
within regulatory requirements becomes even more important.
Antenna pattern measurement is a critical step in the design
process of antennas and wireless devices. Compact antenna
measurement systems combined with a high performance
VNA are necessary to characterize parameters such as pattern,gain, VSWR, and efficiency. These results are used to validate
simulated designs and identify possible performance issues
before final testing. By using an in-house measurement system,
multiple design revisions can be tested and pre-certified
without the high cost of using an accredited or certified
measurement facility for each test. Other considerations likeportability and low cost are important to engineers in various
environments like defense or education, respectively.

System and a VNA

Copper Mountain Technologies
Diamond Engineering

Application: Characterizing the Antenna

In this example, an unknown 2.4 GHz antenna ischaracterized, with a configuration consisting of: DiamondEngineering DAMS 5000 positioner with an FSM sphericalmount, RF cables, a calibrated reference antenna, a Copper
Making accurate antenna measurements carries a number
Mountain Technologies Planar 804/1 VNA, and a computer (PC)of challenges, some of the most important being dynamic
running the measurement and VNA software. To begin, therange, calibration, and speed. A sufficient dynamic range will
VNA is set to the antenna's frequency range, all RF cable loss
allow the antenna to be measured accurately at both minimum(including positioner) is calibrated out using the VNA’s built inand maximum signal level with a minimal amount of trace
12 term calibration. Once the calibration is complete the AUT
noise. Older network analyzers can reach a reasonable dynamic
is mounted to the positioner’s RF Rotary Joint and the reference
range and reduced noise by reducing the IF bandwidth but
antenna is connected then positioned at the appropriate
the analyzer speed is greatly reduced, a single sweep can takedistance (typically 1 or 3 meters). Now that the antennas aremany seconds to complete. PC-based VNA’s offer the benefit
connected and the VNA is set to transmission (S12 or S21), theof greater initial dynamic range, higher speeds, and direct data
VNA will show the response of the entire link. At this pointtransfer.
the user can note the signal response on the analyzer; if thepolarization is correct and the AUT is at a point of high signal,
The full DAMS setup
the VNA should show a strong profile with little
DAMS Positioner / AUT trace noise. Depending on the type of antenna being
measured, the antenna can be manually positioned
to a point of lowest signal to confirm that the signal
Scalar or Vector* Calibrationis still above the noise floor and the trace noise is(* Requires VNA)
acceptable for testing requirements. After the setup isverified, the DAMS antenna measurement software isused to make a measurement. The software indexes
the antenna to every physical point and executes atransmission sweep on the VNA. Optionally, VSWR
data can be automatically collected at every point.
Precision RF Cables
Once complete, the entire data set can be viewed,
DAMS Antenna
processed and exported.

Challenges

25 Practical
Antenna Connection for
Accurate Testing
Clayton Karmel, Pdicta Corp.;
Ben Maxson, Copper Mountain

Measurement Studio

2

GPIB / Ethernet / USB
VNA, Rec. Only or
Source/ReceiverUSB
Controller

Introduction

Antenna measurements are difficult to make due to their multi-directional nature,
their cables/connections to equipment and interfering signals in the environment.  Themeasurement setup, surrounding environment and physical connections all need to becarefully specified and setup, paying close attention to the instruments and materials used.
Anechoic chambers are a key element in the measurement system so the first threearticles in this eBook discuss chamber design including compact ranges and near fieldmeasurements. While there are some articles and books that address anechoic chamber
design, there are few concise articles summarizing their design and simple rules of thumb.
The first article in this eBook covers chamber design and recommends the proper type ofrange for different antenna types and frequencies of operation. Rules of thumb are provided
to select the best approach for the required test or antenna type. The article concentrates onrectangular chambers with simple approximations used for absorber performance to generate
a series of charts that can be used as a guide to specify performance and appropriate
facility size. It also discusses the limitations in using far field chambers, mainly related to theelectrical size of antennas that can be tested.
The second article is part two by the same author and covers compact ranges and nearfield measurements as a solution to the limitations of far field chambers covered in part one.
The third article discusses extending the quiet zone using an RF lens on a conical tapered
chamber so the series of three articles cover a lot of ground on the subject on anechoic
chamber design and technology.
Making accurate antenna measurements has a number of challenges, some of
the most important being dynamic range, calibration, and speed. Compact antenna
measurement systems combined with a high performance VNA are necessary to characterizeparameters such as pattern, gain, VSWR, and efficiency. The fourth article covers antenna
positioning and VNA setup for automated measurements used to validate simulated designs
and identify possible performance issues before final testing.
Connecting RF test and measurement equipment to an antenna under test usually
involves tradeoffs among measurement accuracy, electrical considerations, cost andmechanical ruggedness.  In the final article of this eBook, aspects of the antenna connection
are covered with practical advice for such test and measurement scenarios with a compact
WiFi module example. We hope this eBook will give you the technical knowledge andbackground to help you design test systems and accurately make antenna measurements.

Pat Hindle, Microwave Journal Editor

3

Basic Rules for Anechoic
Chamber Design,Part One: RF Absorber
Approximations
Vince Rodriguez

MI Technologies, Suwanee, Ga.

T

he task of adequately specifying
performance for an indoor anechoic
chamber without driving unnecessary costs or specifying contradictory requirements calls for insight that is not always available to the author of the specification. While there are some articles andbooks1-3 that address anechoic chamber
design, a concise compendium of reference information and rules of thumb on
the subject would be useful. This article
intends to be a helpful tool in that regard.
It starts by recommending the proper types Fig. 1 General geometry of an indoor range - two antennas are located in the range (one for transmitting and one for receiving).

4of range for different antenna types andfrequencies of operation. Rules of thumb
are provided to select the best approachfor the required test or antenna type. The
article concentrates on rectangular chambers. Simple approximations are used forabsorber performance to generate a series of charts that can be used as a guideto specify performance and appropriate
facility size.
The ability to measure an antenna is an
important design requirement in determining if energy is radiating properly and in thedesired direction, as well as how much energy is traveling in undesired directions. To
measure antennas (like many other devices
that are being measured), there is the desire to have the antenna unaffected by its
surrounding environment. This is where theanechoic chamber becomes a viable solution. The anechoic chamber provides anenvironment free of echo or other radiatedsignals to reduce the effects of these undesirable signals.
This article covers applications where anantenna is radiating or receiving a given
signal, and its performance as a function of
direction is being measured.

r/

1000
500
300
200
100
50
30
20
10
5
3
2
1

2

4 6

This equation can be used when
the antenna is under one wavelength in electrical size. From Equation 1, the far field distance can beplotted as a function of the electrical size of the antenna, as shown in
Figure 2.
As a rule of thumb for indoor
ranges, the far field illumination
techniques are better suited forantenna sizes under 10l. This rule
is related to the electrical antenna
size. Frequency of operation adds
another factor that will influence
the type of range. An antenna with

8 10 12 14 16 18 20
D/s Fig. 2 The far field distance plotted
related to the wavelength.

TABLE IFREQUENCY RANGES AND ANTENNA SIZES FOR THE DIFFERENT INDOOR ANTENNAMEASUREMENT APPROACHES
Indoor Ranges

Antenna Size in Wavelengths

Frequency

Far Field
Illumination

Near Field

Compact Range

100 MHz<2

>2

Not ideal

500 MHz<2

>2

Not ideal

1 GHz

<5

>5

>5

2 GHz

<10

>10

>10

≥ 4 GHz

<10

>10

>10

RANGE TYPE SELECTION
The general range geometry
is shown in Figure 1. There areseveral methods of measuring theradiation patterns of antennas indoors: far field illumination, nearfield measurements and compact
range. While they all present pros
and cons, there is not a single solution that is ideal for all types ofantennas and situations. The typeof range most suitable for a given
type of antenna is driven by two
parameters: frequency and electrical size of the antenna under
test (AUT). The far field condition
given by the following equation
drives the selection:
2D2
(1)
λ
The parameters mentioned
above are embedded into the far
field equation. D is the largest
physical dimension of the antenna.
Wavelength is l, which is related to
the frequency of operation on theantenna. For smaller antennas thefar field range length, r, can be approximated by:4r≥r ≈ 10λ(2)

a size of 10l will have a far field
distance of 200l, making the test
distance 20 times the size of the antenna. At some microwave frequencies this may be a test distance of
200 inches (5 m) so an indoor range
may be easy to implement. However, note a 20l antenna will havea test distance that is 800l.
For example, consider an 18 inch
dish used by a popular satellite TV
service. This satellite service operates at 18.55 GHz. The dish antenna is 28.29l in size. The far field is
at approximately 1600l or 25.86 m(84.84 ft). Clearly, for such an electrically large antenna, a far field illumination approach indoors is not
economically feasible. For this antenna, a compact range or a nearfield approach is more suitable.
Conversely, a 10l antenna at 300
MHz, which is 10 m in size, would
be extremely difficult to manipulate
at a test distance of 200 m. For thiscase, the best solution would be an
outdoor range.
In general, for frequencies below 100 MHz, an outdoor range is abetter approach. Current absorbertechnology does not support some5indoor measurements at those low
frequencies. Indoor ranges can bebuilt, but the antenna size should
be kept less than 2l; which limits
the far field distance to 8l (24 m).
This distance is close to the 10l
given by Equation 2. Table 1 provides an approximate guide forthe different antenna sizes and frequencies of operation.
The values in Table 1 are general guidelines. Spherical near field(SNF) ranges can test antennas as
small as l/2. But for such a small antenna it may be a better approach
to use a far field illumination range
as it relates to the typical electricalsize of the AUT.
When creating an anechoic
chamber, the goal is to obtain avolume in the chamber where any
reflected energy from the walls of
the range (ceiling and floor) will bemuch lower than any of the featuresof interest on the radiation pattern.
This volume is known as the quiet
zone (QZ). Figure 1 shows that as
one antenna transmits, it illuminates the receive antenna and all
the walls and surfaces of the range.
The energy incident onto these surfaces will be reflected towards theQZ.1,3 The level of reflected energy
must be a given number of decibels below the direct path between
the transmitting and receiving antennas.
As the antenna being measured
is rotated (see Figure 3), its main
beam will illuminate different surfaces of the chamber. The range
antenna will measure the level offield radiated by the AUT along
the direct path between the twoantennas. However, the range antenna will also receive the reflectedenergy from the walls, ceiling andfloor. If the reflected energy levelis higher than the energy radiatedalong the direct path between the
two antennas, then the radiation
pattern in that direction cannot
be measured accurately. In Figure
3, the measuring antenna, (alsoknown as range antenna or source
antenna) is pointing at a null, but it
is also receiving the reflected signal from the wall that is illuminatedby the main beam of the AUT. The
range antenna is receiving the reflected signal in a direction of 30°.
In that 30° direction, the gain ofAntenna Being
Tested (Transmitting)

Measuring
Antenna (Receiving)

The coefficients in this equation are functions of the thickness.
When the thickness of the absorber is such that 0.25  t  2, thecoefficients of Equation 4 are given by the following polynominals:
2A−+
A11 (t)(t) == 1.5252
1.5252− 4.8243t
4.8243t+ 6.9479t
6.9479t 2
3
4
−
(4a)
−3.8332t
3.8332t 3 ++ 0.7333t
0.7333t 4(4a)2A
A 22 (t)(t) ==−−0.07540.0754 ++ 40.24782t
0.24782t −− 0.3984t
0.3984t 2+(4b)+0.22850.2285 −− 0.0442t
0.0442t 4
(4b)
2
2
A
−
0.00502t+
0.00938t
A 33 (t)(t) == 0.0016
0.0016−0.00502t+
0.00938t3
4−(4c)−0.00577t
0.00577t 3 ++ 0.001155t
0.001155t 4
(4c)
−5
−5
A
⋅⋅ 10
A 44 (t)
(t) =
=−
−1.58
1.58
10−5 ++ 4.91
4.91−5⋅⋅ 10
103 −5 tt−4 2−−1.0151.015 ⋅⋅ 10
10−5−44tt 2 ++ 6.58
6.58 ⋅⋅ 10
10−5 tt 3−5 t 4
−
1.35
⋅
10
(4d)
−1.35 ⋅ 10 t
(4d)s Fig. 3 An indoor range showing one of the reflected paths and the direct path
between the AUT and the source antennas.

the range is lower than in the direct path (boresight) to the AUT.
The reflected energy is a numberof dB lower, for example, 20 dB.
Let us assume that the gain in the
30° direction is 10 dB lower thanthe boresight. The signal received
by the antenna on that direction
will be -30 dB compared to theenergy received when the main
beam of the AUT was pointing
to the range antenna. If the nullis less than -30 dB, the measured
pattern will have errors.5
RF ABSORBER
A key design item for an anechoic chamber is the RF absorber.
The absorber treatment must be
such that the reflected energy has
a small or negligible effect on themeasured data. A typical RF absorber is a lossy material shaped toallow for incoming electromagnetic
waves to penetrate with minimalreflections. Once the electromagnetic (EM) energy travels inside thematerial, the RF energy transforms
into thermal energy and dissipates
into the surrounding air.6 The
electrical thickness of the material
determines how much energy isabsorbed. The reflection level at
normal incidence can be approximated by the following equation:
R o (t) = −13.374 ln(t) − 26.515(3)where t is the thickness in wavelengths. The equation is valid for0.25  t  20. This approximation
can be used to get a conservativereflectivity value of an absorber of agiven thickness. Most manufacturers provide the information in theirdatasheets.
Figure 1 shows that some of
the absorber in the range is not
located in the normal incident
wave direction, but rather in an
oblique incidence. For oblique
incidence, the main reflectivityof the absorber is in the bi-static
direction. Backscattering occurs
when the distance between thetips of the pyramids is l. 7 Hemming 1 provides plots that show
the estimated bi-static reflectivityof absorber at oblique incidence.
A series of polynomial approximations, together with Equation
3, provide a general descriptionof the performance of pyramidal absorbers of different thicknesses and at different angles ofincidence. These are conservative approximations. That leaves
a margin of error to account forthings like lights, doors, positioning equipment and edge diffractions from treatment discontinuities.
The absorber performance in dB
is given by the following polynomial:Rθ (t, θ) = R o (t) + A1 (t) ⋅θ + A 2 (t) ⋅θ2+ A 3 (t) ⋅θ3 + A 4 (t) ⋅θ4 + A 5 (t) ⋅θ5 (4)
6

−8
−7
A
⋅⋅ 10
A 55 (t)(t) == 5.84
5.84
10−8 −− 1.78
1.78−7⋅⋅ 10
103 −7 tt
−7 2
+
+4.02
4.02 ⋅⋅ 10
10−8−74tt 2 −− 2.712.71 ⋅⋅ 10
10−7 tt 3−8 t 4+5.7⋅
10+5.7 ⋅ 10 t

(4e)
(4e)

When the thickness of the treatment is such that 2  t  20,then the coefficients are given by
the set of polynominals:
A1 (t) = 0.1751 + 0.149t −0.0119t 2 + 0.00028t 3(4f)

A 2 (t) = −0.0105 − 0.00824t +0.0007t 2 − 1.61 ⋅ 10−5 t 3(4g)

A 3 (t) = 0.00029 + 0.000123t−1.13 ⋅ 10−5 t 2 + 2.57 ⋅ 10−7 t 3(4h)

A 4 (t) = −1.69 ⋅ 10−6 − 4.77 ⋅ 10−7 t+5.08 ⋅ 10−810−8 t 2 − 1.14 ⋅ 10−9 t 3 (4i)A 5 (t) = 0(4 j)

The domain of Equation 4 is limited by those angles of incidence
where 0°    85°and where=0° is normal incidence. Additionally, the domain is limited by
the domain of the coefficient polynomials. Hence Equation 4 is valid
when 0.25l  t  20l. The rangeof Equation 4 should also be limited to -55  R(dB)  0. For an absorber thickness larger than 20l,
the reflectivity can be approximated using the results for a 20l thickabsorber. Figure 4 shows the bistatic performance as a function ofangle for a series of different electrical thickness of the absorber.

Bi-static Reflectivity of Pyramidal Absorber0

Thickness in 
0.25
0.5
1
1.2
1.4
1.5
1.6
1.8
2
2.5
3
4
5
6
7
8
9
10
15
20

–5
–10
–15

Reflectivity (dB)

–20
–25
–30
–35
–40
–45
–50
–55

0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
Angle of Incidence (°)

s Fig. 4 Estimated reflectivity of RF absorber as a function of angle of incidence.
1 Equation 42 Equation 4
1 Computed TL Approximation
2 Computed TL Approximation
1 Manufacturer A Specs
2 Manufacturer A Specs
0–5

Reflectivity (dB)

–10
–15
–20
–25
–30
–35
–40
–45
–50
–55

0

10 20 30 40 50 60 70 80
Angle of Incidence (°)

s Fig. 5 Comparison of bi-static reflectivity from a computational approach,manufacturer’s specifications and Equation 4.

Figure 5 shows a comparison ofcomputed results using the method in reference 8, a given manufacturer specifications and the results from Equation 4 for a materialof thickness equal to l and 2l. If
the results of the polynomials presented here are compared to thosefrom numerical computations, thepolynomials appear to provide aconservative number for the re-flectivity — higher by about 10 dB.
The manufacturer specifications
were only provided from 45° to 80°
and normal incidence. Computed
results were obtained only at a few
angles. For the 1l thick absorber,
the different methods follow similar trends, with the polynomials
providing the most conservative
number. There is a large difference
at 35° between the computed andthe polynomial results. However,that null in the reflectivity may
shift depending on the material on
which the absorber is mounted.9 In
general, the polynomials are a safe
approximation for the performanceof RF materials at different angles
of incidence.
The largest typical absorber size
currently available is 72” (1.82 m).
This size provides a frequency limit
for the use of indoor ranges. At 100
MHz, the thickness of this absorberis 1.64 with a normal incidence
performance at about -33 dB. In an
indoor range lined with this material, pattern features -20 dB fromthe peak will be difficult to measure accurately. There are hybrid
absorbers merging ferrite tiles andlossy substrate pyramids of wedges that operate down to 30 MHz
or even 20 MHz. These are more
7

suitable for EMC applications as
their normal incidence absorption
is typically limited between 25 and35 dB.
RECTANGULAR FAR FIELDCHAMBERS
Sizing the range begins withrectangular far field ranges thathave a test distance is determinedby Equation 1. It is common to findsources stating the rules of thumbfor sizing a rectangular anechoic
chamber for far field illumination.
Generally, the width and height ofan anechoic chamber should be
three times the diameter of theminimum sphere that contains thelargest antenna being tested. It
is important to check that a minimum spacing of 2l between the
AUT and the tips of the absorberis maintained to avoid loading of
the AUT. The far field distance isgiven by:r(5)
= 2n 2λWhere n is the number of wavelengths in size of the AUT. The QZ
must be large enough to encompass the AUT. Hence the QZ is
nl. Figure 6 shows a typical rectangular range geometry. Fromthe geometry, an equation for the
distance x can be derived. The
distance x is the distance from therange centerline to the absorber
tips.
x
(6)
= n 2 ⋅ cot θλEquation 6 gives the distance interms of wavelengths. In Figure 3,
the value of  can be chosen for adesired reflectivity. The curves in
Figure 4 will also provide a value
for the thickness of the absorber.
Hence, if the AUT has features thatneed to be measured in the -25
dB level, the bi-static reflectivityof the absorber must exceed thatlevel. Absorber 2l in thickness will
exceed -25 dB up to angles of 50°.
Hence the width of the chamber is()

2x = 2n 2 (0.84) + 4 λ(7)Where the added 4 accounts forthe 2l thickness of the absorber.
If a different thickness of absorber


x
n
2n2

s Fig. 6 Geometry of a far field range.
is used, Equation 7 will change. In
general, the chamber width can be
written as()

W = 2n 2 cot(θ) + 2t λ

(8)Parameters u and t must be chosen to obtain the required reflectivity. It is important to keep a minimum 2l spacing from the QZ to theabsorber. The length of the rectangular far field chamber is mainly
given by the far field distance and
the QZ size plus the absorber thickness. Added space should be included for the range antenna andthe absorber behind it.
The total chamber or range
length (L) is given by:
L = 2n 2 + n + 2 + t + K λ
(9)

(

)

where K is a factor large enough toinclude the source antenna, the 2l
spacing, and the absorber behindthe source. It should be noted, thatthese equations provide a minimum requirement. Work must be
performed inside the chamber —mounting and connecting the antenna, switching range antennas,
etc. The space should be checked
to allow for people to performthese tasks inside the anechoic
range.
Expected chamber sizes can be
examined by entering values into
previous equations. It will be assumed that the source antenna is
the directive with a sufficient frontto-back ratio. The absorber behindthe source antenna will be one
wavelength in thickness and thefactor K will be set to 4.
In Figure 7, the width and lengthof a series of rectangular chambers
have been plotted versus the lowest frequency of operation. In addition the electrical size of the AUTat the lowest frequency is indicatedby the value of n for each chamber
size plotted.
If a chamber is designed for anantenna of a given nl at the lowest frequency, that same chamberis large enough for testing antennas of the same electrical size athigher frequencies. Similarly, as thefrequency is lowered, the chamber
size must increase. At 500 MHz, achamber for a 2l sized antenna is
about 10 × 5 m. If the antenna size
was increased to 4, the chamber
would need to be 18 × 10 m. Tapered anechoic chambers should
be used at these lower frequencies.1,10-12 The geometry of thetaper chamber uses the specularreflection off the side walls for the

Width and Length of a Rectangular Chamber
Far Field Ilumination.  = 60 and K = 4.
Absorber Thickness is 2
Qz Zone Size is n
50

Width and Length (meters)

45

LENGTH
WIDTH

n=2

40
35

n = 1030
25
20

n = 1010

n = 10n=2

5
0n = 10

n=5

15

0.1

0.2 0.3 0.5 0.7 1

2

3 4 5 6 7 8 10

n = 1020

Lowest Frequency of Use (GHz)s Fig. 7 Rectangular far field chambers for different lowest frequencies of operationand different largest size antennas at their lowest frequencies.

8

AUT illumination rather than reducing its level as it is done in the rectangular chamber. This leads to aphysically smaller chamber.
The height of the chambershould be the same as the width.
By doing this, the reflections fromthe ceiling and floor will be similarin level. This is important since thereflected energy from the ceilingand floor will be similar, and theeffects of the range on polarization dependant parameters suchas cross polarization and axial ratio
will be minimized.
Equations 8 and 9 provide agood idea of the space requirements for an indoor range. In mostcases, a chamber size can be adjusted. For example, the absorberon the ceiling and floor can be increased in thickness to maintain thereflectivity at more oblique angles
of incidence (larger u). Chebyshev
arrangements13 of the absorberlayout can also be used to improvereflectivity.
Figure 3 also reveals anotherclue to improve reflectivity. The reflected ray arrives at the range antenna at an angle at which the gainof the antenna is lower than in theboresight direction. Using higher
directivity antennas as sources reduces the amount of energy received from the side walls, ceilingand floor. Hence, shorter absorbers
reduce the chamber size.
CONCLUSIONPart one of this two-part series
has dealt mainly with approximations for bi-static reflectivity of RFabsorbers and the rectangular design of rectangular RF anechoic
chambers. The polynomial equations include a “margin of safety” intheir results. This helps in accounting for secondary bounces andedge diffractions as well as light
fixtures, vents, doors and other disruptions of the absorber treatment.
Part two will provide equations forcompact ranges and near field tofar field ranges.
ACKNOWLEDGMENT
The author will like to thankZhong Chen for providing the computed results based on the NIST algorithm.8 n

References

1.		L. Hemming, “Electromagnetic
Anechoic Chambers: A Fundamental Design and Specification
Guide,” IEEE Press/Wiley Interscience: Piscataway, N.J., 2002.
2.		G. Sanchez and P. Connor, “How
Much is a dB Worth?” 23rd Annual
Symposium of the Antenna Measurement Techniques Association(AMTA), Denver, Colo., October
2001.
3.		J. Hansen and V. Rodriguez, “Evaluate Antenna Measurement Methods,” Microwave and RF, October
2010, pp. 62–67.
4.		W. Stutzma and G Thiele, “Antenna Theory and Design,” 2nd Ed.,
Wiley 1997 ANSI/IEEE STD 1491979 149-1979 - IEEE Standard
Test Procedures for Antennas,
1979, reaffirmed 2008.
5.		D. Wayne, J. Fordham and J.
McKenna, “Effects of a Non-Ideal
Plane Wave on Compact Range
Measurements,” 36th Annual Antenna Measurement Techniques
Association Symposium (AMTA),
Tucson, Ariz., October 2014.
6.		V. Rodriguez, G. d’Abreu and K.
Liu, “Measurements of the Power
Handling of RF Absorber Materials: Creation of a Medium Power
Absorber by Mechanical Means,”
31st Annual Antenna Measurement Techniques Association
Symposium (AMTA), Salt Lake
City, Utah, October 2009.
7.		W. Sun and C. Balanis, “Analysis
and Design of Periodic Absorbers
by Finite-Difference FrequencyDomain Method,” Report No.
TRC-EM-WS-9301 Telecommuni-

cations Research Center, ArizonaState University, Tempe, Ariz.,
1993.
8.		E. Kuester and C. Holloway, “A
Low-Frequency Model for Wedge
or Pyramid Absorber Arrays-I: Theory,” IEEE Transactions on Electromagnetic Compatibility, Vol. 36,
No. 4, November 1994.
9.		V. Rodriguez, “A Study of the Effect of Placing Microwave Pyramidal Absorber on Top of Ferrite Tile
Absorber on the Performance of
the Ferrite Absorber,” 19th Annual
Review of Progress in Computational Electromagnetics (ACES)
Symposium, Monterey, Calif.,
March 2003.
10.		
W. Emerson and H. Sefton,“An Improved Design for Indoor
Ranges,” Proceedings of the IEEE,
Vol. 53, No. 8, pp. 107921081,
1965.
11.		 H. King, F. Shiukuro andJ. Wong, “Characteristics of aTapered Anechoic Chamber,”IEEE Transactions on Antennas
Propagation, Vol. 15, No. 3, pp.
4882490, 1967.
12.		
V. Rodriguez, “Using Tapered
Chambers to Test Antennas,”
Evaluation Engineering, Vol. 43,
No. 5, pp. 62268, 2004.
13.		
J-R J. Gau, D. Burnside and
M. Gilreath, “Chebyshev Multilevel Absorber Design Concept,”
IEEE Transactions on Antennas
Propagation, Vol. 45, No. 8, pp.
128621293, 1997.
Vince Rodriguez attended the Universityof Mississippi, in Oxford, Mississippi,
where he obtained his B.S.E.E. in 1994.

Following graduation, Rodriguez joined
the department of Electrical Engineering
at the University of Mississippi as a
research assistant. During that time, heearned his M.S. and Ph.D. (both degrees
in engineering science with emphasis
in electromagnetics) in 1996 and 1999
respectively. Rodriguez joined EMC Test
Systems (now ETS-Lindgren) as an RF
and Electromagnetics engineer in 2000.
He was the principal RF engineer for theanechoic chamber at the Brazilian Institutefor Space Research (INPE), the largest
chamber in Latin America and the only
fully automatic, EMC and satellite testing
chamber. In November 2014, Rodriguez
joined MI Technologies in Suwanee, Ga.
as a senior applications engineer bringing
his expertise on numerical modeling, RFabsorber and anechoic range design to thedevelopment of solutions for antenna, RCS
and radome testing facility design.
Rodriguez is the author of more than
50 publications, including journal andconference papers as well as book chapters.
He holds patents for a hybrid RF absorberand a dual ridge horn antenna. Rodriguez
is a senior member of the IEEE andseveral of its technical societies. Among theIEEE technical societies, he is a memberof the EMC Society, where he served as
distinguished lecturer from 2012 to 2014
and also serves as member of the board of
directors. He is an Edmund S. Gillespie
Fellow of the Antenna Measurements
Techniques Association (AMTA). Rodriguez
is a member of the Applied Computational
Electromagnetic Society (ACES), where heserves on the board of directors. Rodriguez
is a member of several standard committees
including IEEE STD 149, IEEE STD 1148
and RTCA DO-213. Rodriguez is also a full
member of the Sigma Xi Scientific Researchand Eta Kappa Nu Honor Societies.

Cable & Antenna Analyzers from 1 MHz to 18 GHz

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(cable & antenna analyzers) perform lab-quality measurements of the S11 parameter invarious presentation formats up to 18 GHz.
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http://www.coppermountaintech.com/products/Ohm-50/category/10/9Basic Rules for Anechoic
Chamber Design, Part Two:
Compact Ranges and Near Field
Measurements
Vince Rodriguez

MI Technologies, Suwanee, Ga.

The task of adequately specifying performance for an indoor anechoic chamber without
driving unnecessary costs or specifying contradictory requirements calls for insight that is
not always available to the author of the specification. Although there are some articles
and books1-3 that address anechoic chamber design, a concise compendium of reference
information and rules of thumb on the subject would be useful. This second part of the series
intends to do that, concentrating on the sizing of compact ranges and chambers for near fieldsystems. As was done in part one, simple approximations are used for absorber performanceto generate a series of equations that help specify performance and size of facilities.

Ps Fig. 1 Simulated results of a parabol-

ic reflector, showing plane wave behavior
on the right.

art one of this series identified limitations in using far
field chambers, mainly related to the electrical size of antennas that can be tested. As
was shown, an 18” dish used bya popular satellite TV service willbe almost impossible to test in a
far field chamber. The satellite service operates at 18.55 GHz, thedish antenna is 28.29 wavelengths(l) in size, so the far field is at approximately 1600 l or 25.9 m (84.8ft). Clearly for such an electricallylarge antenna, far field illuminationindoors is not economically feasible. For this antenna, a compact
range or near field measurement ismore suitable.
10

COMPACT RANGES
Although barely mentioned in
the IEEE standard test procedure
for antennas,4 the compact range(CR) has become an important tool
for measuring electrically large antennas. The CR uses a parabolic reflector to create a plane wave illumination at the location of the antenna under test (AUT). This plane
wave simulates the field distribution that the antenna experiencesin the far field. Figure 1 shows aparabolic reflector illuminated by asource located at the focal point of
the parabola. The plane wave behavior can be seen a short distancefrom the reflector. The reflector sys-TABLE 1
COMMERCIALLY AVAILABLE COMPACT RANGE REFLECTORSQZ Size(Length andDiameter)(cm)

Overall
Reflector Size
(Including
Serrations)(cm)

Length ofSerrations(cm)

Frequency ofOperation(GHz)

Focal Length fl
(cm)

61

216 × 18838

4 to 200

182

122

432 × 335

76

2 to 200

366

182

488 × 41676

2 to 200366

244

864 × 670

152

1 to 200

732

366

975 × 833

152

1 to 200732

est frequency of operation. Table 1
provides a typical list of reflectors,
showing their overall size and frequency ranges. Note that as frequency increases, the reflector becomes more efficient. While somereflectors can operate well into themillimeter wave range, extra careshould be taken during manufacturing and surface finishing, as surface
imperfections will affect the performance.
Reflector size is the determining
factor when sizing the width andtem is the controlling factor whensizing the range. The reflector mustbe large enough to provide a plane
wave that illuminates the entire antenna being tested, and the reflector should be properly terminated.
The purpose of the termination is to
reduce the effects of the terminatedparaboloid on the illumination. The
two most common ways of terminating a reflector are serrations androlled edges.6 In the case of serrated edge reflectors, serrations canbe between 3 l and 5 l at the low120

90

60
30

150
180

–60 –40 –20 0

–150

–30

–120

–60

–90

60

150

30

180

–60 –40 –20 0
–30

–150
–60

–120

5
fl
(1)
3
where fl is the focal length of thereflector. Referring to the satellite
TV antenna, requiring a far field
distance of 25 m to test, one might
expect long chambers and large
distances for CR testing. However,Table 1 with Equation 1 indicates
the test distance for a 61 cm QZ is
3 m. This is sufficient to test the satellite TV antenna.
As a rule, the length of a CR
chamber is given by the following
equation:
51
L = R clr + f l + QZ + ( 2 + t ) λ
(2)
3
2
r=

where Rclr is the reflector clearance.
This includes the mechanical structure to support the reflector, whichranges from 60 cm to 2 m, depending on the overall reflector size. In
general, the wall behind the reflector has a small absorber, usuallyl/2 in thickness, and only covers
the perimeter of the wall. The parameter t is the thickness of the end
wall absorber. For a CR, this is themost critical wall and should havethe lowest reflectivity; it is recommended the value of t be no lessthan 3 to 4.
The width of the chamber is calculated using:

W = CR w + ( 4 + 2t ) λ90120height of the chamber. The lengthof the chamber will be affected by
the focal length of the reflector.
The distance from the vertex of thereflector to the quiet zone (QZ) is
given by the following rule:–90s Fig. 2 A typical compact range layout showing the reflector pattern, side (a) andtop (b) views. At 2 GHz, the energy incident on the side walls, floor and ceiling is morethan 40 dB down.
11(3)

where CRw is the overall width of
the reflector. There is an additional
2 l from the tips of the serrations
to the absorber tips on each side of
the reflector, although in some cases the spacing can be as small as
one wavelength on each side. The
final item determining the width of
the range is the thickness of the absorber.
While for far field ranges the absorber on the ceiling, floor and sidewalls should be thick enough toprovide good bistatic reflectivity at
oblique angles, in the CR the sidewall absorber does not need to be
as thick. Figure 2 shows a typical

2.5

2.0

2.0

1.5

1.5

Y (m)

Y (m)

2.5

1.0
0.5

1.0
0.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
X (m)

(b)

2.5

2.5

2.0

2.0

1.5

1.5

Y (m)

Y (m)

(a)

1.0
0.5

1.0
0.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
(c)

5
4
3
2
1
0
–1
–2
–3
–4
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 –5
X (m)

60
30

–60 –40 –20

5
4
3
2
1
0 –90
–1
–2
–3
–4
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 –5

(d)

X (m)

90

0

–30
–60

X (m)

s Fig. 3 Wave propagation from the source horn vs. time – 6.6 ns (a) 10.4 ns (b) 11.3 ns (c) 15.1 ns (d) – compared to the far
field pattern.

CR chamber. The radiation patternof the CR reflector has been superimposed over the chamber drawing. The reflector in the figure provides a 3.66 m × 1.82 m elliptical
QZ. The depth of the QZ is 3.66 m.
The important aspect of the CR isthat it has a very directive pattern,with directivities in excess of 25
dBi. As Figure 2 shows, the energy
incident on the absorber on theside walls is already 40 dB below
the direct path. A 1l thick absorber
will provide 10 dB of absorption atover 60 degrees of incidence (seeFigure 4 in part one, publishedin January 2016). Combining thereflectivity with the difference inmagnitude between the direct ray
and the reflected ray, results in areflected energy level of approximately -50 dB. The reflector is being used in the near field while theradiation pattern of the reflector isa far field concept. However, this isan acceptable approximation, as itprovides a method for estimating
the level of energy that radiatesfrom the reflector in the directionof the walls. As Figure 3 shows,
the reflector will send some energy
towards the side walls, estimatedfrom the far field pattern of the reflector.
The height of the chamber has asimilar equation for calculating thesize:
H = CR h + ( 2 + K + 2t ) λ(4)where CRh is the overall height of
the reflector. The spacing between
the tips of the reflector and thetips of the ceiling absorber is 2 l.
The parameter K provides a factor
for the spacing between the floor
and the reflector. For the floor absorber, we want a larger separation
between the edge of the reflector
and the tips of the floor absorber.
This reduces the angle of incidence
at the specular point between thereflector feed and the reflector tominimize the impact of the floor reflection on the reflector illumination(see Figure 4). Equation 4 includes
K wavelengths of space between
the tips of the floor absorber andthe serration tips. K should be large
enough to provide sufficient space
for the feed positioner supporting
the feed antenna that illuminates
the reflector. As was the case withthe side walls, the absorber on thefloor and the ceiling can be 1lthick. Special consideration mustbe given to the floor absorber between the feed and the reflector,
which may be 2l thick. In general,
the absorber electrical thickness at
the lowest frequency can be t # 1.2
and t $ 0.75 for the side wall andceiling treatments, respectively.
NEAR FIELD RANGES
Different techniques are used
for performing near field measurements; they align with the type ofantenna being measured. With all
12

approaches, the field (amplitude
and phase) radiated from the AUT is
measured on a surface, and the far
field behavior is derived mathematically from this measurement. Threedifferent near field techniques —planar (PNF), cylindrical (CNF) andspherical (SNF) — represent the surface where the data is measured.7-9
The most basic near field measurement approach is planar scanning,where the field radiated from theantenna is scanned on a single
plane. This is a good techniquefor high gain antennas, as there isa very small amount of energy radiating to the back of the antenna.
Cylindrical scanning is where thefield is measured on the surface of acylinder excluding the top and bottom surfaces. This is ideal for longantennas that are omnidirectionalor have a wide beam on one of theprincipal planes but a narrow beamin the perpendicular plane. Spherical scanning is a more general measurement approach. Here the field
is measured on a sphere that contains the entire antenna. In general,
the test distance for PNF measurements is between 3l and 10l. For
SNF, the probe can be further away.
The same equations developed
for far field chambers can be used
for SNF with the exception of thetest distance. In general, the equation is given by:
L = d pp + ( n + 6 + 2t e ) λ

(5)

–60

be treated with absorber, such as
those behind the AUT. The test distance, as stated above, is between
3l and 10l. The dominant factor
for sizing a PNF range is the scanner, where the scan size is given by:
L x = n + 2k tan ( θs ) λ
(9)

(

–90

–30

us is the maximum angle for accurate far field and nl is the electrical size of the antenna being tested(see Figure 5). The variable k is
the test distance in wavelengths;
hence, 3 < k < 10. The physicalscanner will usually be slightly larger than the scan plane. Typically, 2lis the separation to the absorber
tips.
The width of the range becomes:

–120
–5
– 10
– 15
– 20
– 25

0
–150

30
–180
60
90

150

120

s Fig. 4 Absorber on the floor between the feed positioner and the reflector is critical to reduce the reflected energy from illuminating the reflector.

where dpp is the depth of the probe(measuring antenna) and its positioner. The variable n is the diameter in wavelengths of the minimum
sphere that contains the AUT. The
absorber on the two end walls will
have a thickness of tel, where te is
the thickness, in wavelengths, ofthe end wall absorber. As is customary, 2l is added between theminimum sphere and the absorbertips. Finally, 4l is estimated to be
the distance between the probe
and the sphere containing the antenna.
The width of the SNF chamber
is given by:
W = ( n + 4 + 2t s ) λ(6)

In this case, ts is the thickness,in wavelengths, of the side wallabsorber. This is a rough approximation. For both Equations 5 and6, a minimum of 1 meter should
be added to prevent the positioning equipment from hitting theprobe as it rotates the antenna being measured. The chamber also
should provide room for people
to work inside to set up the measurement. This is more critical forhigher frequencies (above 2 GHz),
where the 4λ separation may not
be enough for the positioner toclear the probe.
The angle of incidence onto theside absorber is:⎛ 4n + 16 ⎞
θ = arctan ⎜(7)⎝ 2n + 16 ⎟⎠Taking the limit as n → `, u <63.4 degrees. Using the absorberapproximations presented in partone of this series, we can estimatethat ts < 2te. To do this, we checkthe reflectivity of the end wall absorber at normal incidence and select the thickness of the absorberthat will provide similar reflectivity
for the 63.4 degree incident angle.
The ceiling and the floor will havethe same absorber as the sidewalls.
The chamber height can be estimated using the following equation:
H = h p + ( n + 4 + ts ) λ(8)where the variable hp accounts forthe height of the positioning equipment. In a typical roll-over azimuth
positioner used in SNF measurements, hp should include the heightof the floor slide, the azimuth positioner and the offset slide. The
positioning equipment in the far
field chamber equations or the CR
equations (except for the feed positioning) is not an issue because
other dimensions are so dominant
in these ranges (i.e., the far field test
distance or the reflector size).
PNF systems use a planar scanner to measure highly directive
antennas (i.e., gain > 20 dB). The
high gain of the AUT benefits thedesign of the range, as some regions of the range do not need to
13

)

(

)

W = n + 2k tan ( θs ) + 4 + 2t s λ +
Δ scn
(10)
which can be written as:
W=L x +( 4 + 2t s ) λ + Δ scn(11)where Dscn is the additional spacerequired for the scanner structure,and ts is the thickness of the absorber.
The length of the range is givenby the following equation:
L = Sclr + A d + ( 4 + k + t ) λ(12)Where Sclr is the scanner depth,
which should include the spacing
to the absorber, if any (the scanner can be placed very close to
the tips), and the probe length.
Ad is the depth of the AUT andthe support structure for aligning that antenna with the scanner.
The 4l in Equation 12 is the space
between the back of the AUT andthe range wall. For very high gain
antennas, this wall does not needabsorber treatment. If absorber isdesired, the thickness of absorber
for this wall can be as small as l/4.
The thickness of the absorber on
the wall behind the scanner takes
Lx
s

k

s

n

s Fig. 5 Geometry of a planar nearfield measurement.

()

W ≈ n + 2k tan ( θs ) + 4 + 2t s λ(13)Using the approximation that( n + 2k tan (θs ) + 4 + 2ts ) λ > Δscn (14)ititfollows
the angle
angleofofincidence
incidence
follows thatthat theonto
the sidesidewalls
wallsis:is:onto the⎛⎞kθ=arctan ⎜⎟⎝ kn + k tan ( θs ) + 4 ⎠(15)

Notice that the angle of incidence
is only dependent on the size of the
AUT, the maximum angle for accurate far field and the test distancein wavelengths. Figure 6 shows thateven at 10λ for a test distance, thelargest angle of incidence is close to
20 degrees. From the absorber approximations presented in part one,the reflectivity of a given piece of absorber of certain electrical thickness
does not deteriorate much withinthat range of angles of incidence. If
the AUT is a simple passive antenna,
the high gain can be a benefit. Since
the antenna will not radiate much energy to the side walls, a smaller absorber (t < 1) may be used. However,if the AUT is a complex antenna withbeam steering, then the side walls
should have more thickness (t $ 2).
The height of the chambershould be calculated in the same
way as the width. In some cases,
the scan distance is different between vertical and horizontal; it is
not rare for the chamber to havea non-square cross section. The
equation for the height is:
H = L y + y o + ( 2 + ts ) λ(16)

Where yo is the minimum heightof the probe, i.e., the location of
the probe at the bottom of the vertical motion. This includes the rails
on which the scanner moves in thehorizontal axis and should also belarge enough to include the floor
absorber; at a minimum, yo > tsλ.
The above rules for SNF andPNF ranges can be combined toarrive at a range size for a CNF system.
CONCLUSIONPart two of this series provided
an overview of the rules and physics that guide the selection and sizing of indoor anechoic chambers
for compact ranges and for nearfield scanning measurements. All
the equations are approximations.
The length, in most cases, is a minimum; more space may be required
for the loading and unloading of
the AUT, changing feeds and range
antennas and connecting additional
equipment. Both parts of this series provide a general overview andequations for sizing anechoic rooms
for the most common antenna measurement methods currently being
used. n20.0
Angle of Incidence on
Side Wall Absorber (°)advantage of the directivity of theprobe used to scan the plane.
Thus, t $ 2.
The remaining value to be defined is the absorber on the sidewalls. This is dependent on the angle us and the factor k. The width isapproximated as:

k=3
k=5
k=7
k = 10

17.5
15.0
12.5
10.0
7.5
5.0
2.5
30

40
50
60
70
Maximum Angle for
Accurate Far Field (s°)s Fig. 6 Angle of incidence on the

80side wall absorber vs. maximum angle
for accurate far field patterns, plottedfor several test distances. The antenna
aperture is 20λ.

14References

1.		L. Hemming, “Electromagnetic Anechoic Chambers: A Fundamental
Design and Specification Guide,”IEEE Press/Wiley Interscience: Piscataway, N.J., 2002.
2.		G. Sanchez and P. Connor, “How
Much is a dB Worth?,” 23rd Annual
Symposium of the Antenna Measurement Techniques Association (AMTA),
Denver, Colo., October 2001.
3.		 J. Hansen and V. Rodriguez,“Evaluate
Antenna Measurement Methods,”
Microwaves and RF, October 2010,
pp. 62267.
4.		ANSI/IEEE STD 149-1979 149-1979- IEEE Standard Test Procedures for
Antennas, 1979, reaffirmed 2008.
5.		J.R.J. Gau, D. Burnside and M. Gilreath “Chebyshev Multilevel Absorber Design Concept,” IEEE Transactions On Antennas Propagat., Vol. 45,
No. 8, pp. 128621293, 1997.
6.		T.H. Lee and W. Burnside, “Performance Trade-Off Between Serrated
Edge and Blended Rolled Edge
Compact Range Reflectors,” IEEE
Transactions on Antennas and Propagation, Vol. 44, No. 1, January 1996,
pp. 87296.
7.		D. Hess, “Near-Field Measurement
Experience at Scientific Atlanta,”
White Paper, www.mitechnologies.
com/papers/91/Near-Field%20Measurement%20Experience%20at%20
Scientific-Atlanta.pdf.
8.		Yaghjian, “An Overview of Near-Field
Antenna Measurements,” IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 1, January 1986,
pp. 30245.
9.		J. E. Hansen ed., “Spherical NearField Antenna Measurements,” IEEE
Peter Peregrinus Ltd.: London, UK,1988.
Vince Rodriguez is a senior applications
engineer with MI Technologies in Suwanee,
Ga., where he brings his expertise innumerical modeling, RF absorber andanechoic range design to the design ofantenna, radar cross section and radome
testing facilities. His full biography
appeared in part one of this series, publishedin the January 2016 issue.

Extending the Quiet Zone
Using an RF Lens on a
Conical Tapered Chamber
to 18 GHz
V. Rodriguez

ETS-Lindgren L.P., Cedar Park, Texas

S. Matitsine

Matsing Pte. Ltd. and Temasek Laboratories, National University of Singapore

T.T. Chia

DSO National Laboratories and Temasek Laboratories, National University of Singapore

P. Lagoiski, L. Matytsine and M. Matytsine
Matsing Pte. Ltd.

P.K. Tan

Temasek Laboratories, National University of SingaporeA

tapered chamber is traditionally constructed using a square based pyramidal shaped taper that transitions
to an octagon and then finally
into a cylindrical launch section. This approach is related
to the manufacturability of different absorber cuts. In this article, we introduce a chamberwhere the conical shape of thelaunch continues throughout
the entire length of the tapered
chamber. The results of frees Fig. 1 A typical tapered anechoic
space VSWR measurements
chamber.
at different frequencies over a
1.5 m diameter quiet zone (QZ)
are presented. The conical taper appears to have a better
illumination wave front than
the traditional approach andbetter QZ VSWR levels even at
frequencies above 2 GHz.
As with all antenna chambers, when the frequency increases, the usable or far field
illuminated QZ is reduced. At
s Fig. 2 Shaping from square to oca 12 m distance from the feed
tagonal cross-section at the feed.

15

to the turntable, the quiet zone at 8 GHz
is reduced to 45 cm. A solution to extend
the quiet zone at high frequencies employsa large dielectric lens installed in front of
the turntable to improve the phase distribution of the field. A lightweight, broadband
lens with a diameter of 2 m was developed
and weighs just 35 kg with a focal lengthof 10 m. With the lens installed, the usablefar field QZ is increased, allowing electrically larger antennas to be measured in thechamber. The use of the lens can also be
applied to traditional square cross-section
tapered chambers.
BACKGROUND
Tapered anechoic chambers have beenaround for almost 50 years,1,2 introduced
to address issues present in rectangularchambers at frequencies below 500 MHz.3,4
At lower frequencies, high gain antennas
used in an antenna measurement range become physically large and can be difficult tohandle inside an anechoic chamber, so lessdirective antennas are used. These radiate
more energy to the side walls, ceiling andfloor of the chamber forcing it to grow inEHP-24PCL
Absorber
EHP-12PCL
Absorber
Scanner

B
EHP-24WW Absorber

CRV-60PCL
Absorber

EHP-24PCL
Absorber
AUT
Positioner
D

A

C

EHP-36PCL
AbsorberCustom EHP-18WGCL
Absorber on Taper

(a)

(b)

s Fig. 3 The conical tapered anechoic range plan and elevation (a), and a picture of the taper section (b).
size in order to accommodate thicker absorbers needed to reduce reflections. Tapered anechoic chambers were introduced to solve thislow frequency problem. Instead oftrying to eliminate specular reflections in the quiet zone, the speculararea is brought closer to the measuring antenna and the specular reflections are used to create a QZ illumination.2,5 Traditionally, tapered
anechoic chambers were built having a square based pyramid as thetaper (see Figures 1 and 2). To better accommodate different feed
antennas, the square section may
be gradually transformed to a cylindrical cross section taper. Thesechanges in cross section require a
lot of special cuts of absorber tos Fig. 4 The tapered section as seenfrom the feed location.

s Fig. 5 The scanning antenna at theQZ viewed from a point right behind thesource antenna at the apex of the taper.

make the transition from the conical section to the square section assmooth as possible and to create
the illumination in the QZ.2-5
The design presented in thisarticle introduces a conical taper(see Figure 3). The entire tapered
structure maintains a constantangle and a circular cross section.
The tapered section is about 10 m
in total length. Results for the freespace VSWR6 are presented andcompared with similar chambers
employing a traditional design. To
improve performance at high frequencies a dielectric lens is used tocreate a plane wave behavior.7
MEASURED RESULTS
The chamber QZ is scanned using free space VSWR tests6 at a series of frequencies from 200 MHz to
18 GHz. The chamber is lined with60” (152 cm) curvilinear absorbers
on the back (i.e., receive) wall and a
combination of 24” (61 cm) pyramidal absorbers and 36” (91.44 cm)
on the sidewalls, floor and ceiling.
The tapered section has a specially
cut wedge material that lines thetapered section from the feed location to the QZ area. The wedges
range from 18” (45.72 cm) at the
QZ end to 8” (20.32 cm). Figure
4 shows a picture of the conicaltreatment. The tapered section isbuilt inside an RF shielded room to
avoid outside interference during
measurements.
The source antenna is an ETSLindgren model 3164-06 dual linearly polarized open boundary
quad-ridge horn,8 rated from 300
MHz to 6 GHz. In this application,
the antenna is used from 200 MHz
16

with attenuators at the feed to reduce the effects of the high VSWR.
The QZ is scanned with an ETSLindgren model 3106B dual ridge
horn. The scanning antenna andsource antenna are shown in Figure 5.
Figure 6 shows reflectivity levelsof the QZ versus direction for horizontal and vertical polarizations.
Results are shown for 200, 400,
800 and 1,000 MHz. All of these results are measured with the source
antenna at a fixed position in theapex of the taper. The antenna iscommonly moved as frequencychanges to maintain the phasecenter close to the reflections andmaintain a QZ illumination free ofripples.3,4
For frequencies above 2 GHz,an ETS-Lindgren 3164-05 dual
linearly polarized open boundary
quad-ridge horn, rated from 2 to
18 GHz, is used. For scanning theQZ, a series of standard gain horns
are used with gains ranging from10 to 20 dBi. Additionally, since asmaller horn is used as the source,
it is positioned inside an extensionof the conical taper. Figure 4 shows
one of the two halves that make up
this high frequency extension.
Figure 7 shows the results ofthe scans at high frequencies. As
discussed by Rodriguez and Hansen,5 tapered chambers are better
suited for low frequencies and caremust be taken to properly position
the source antenna. However, it ispossible to use them at these highfrequencies once the chamber ischaracterized.

200 MHz Horizontal 200 MHz Vertical–30 dB limit
Measured Reflectivity Data
270
225

315

–35
–40
–45
–50
–55
–60

180

0

45

135
90

(a)

400 MHz Horizontal 400 MHz Vertical–36 dB limit
Measured Reflectivity Data
270
315

225

0

Measured Reflectivity Data
270
225

315

–35
–40
–45
–50
–55
–60

180

0

45

135
90

(c)

1,000 MHz Horizontal 1,000 MHz Vertical–42 dB limit
Measured Reflectivity Data
270
315

225

–35
–40
–45
–50
–55
–60

180

45

135
(d)

0

90

s Fig. 6 Reflectivity results for the conical tapered chamber at 200 (a), 400 (b),800 MHz (c) and 1 GHz (d).

–40
–50
–60
–70
–80

800 MHz Horizontal 800 MHz Vertical–39 dB limit

–40
–50
–60
–70
–80

90

(b)

–40
–50
–60
–70
–80

45

135

–40
–50
–60
–70
–80

–35
–40
–45
–50
–55
–60

180

either side of the source antenna.
COMPARISON WITH
These variations are not seen in the
TRADITIONAL CHAMBERS
conical tapered chamber.
Comparison with traditional
The chamber is configured withchambers is difficult. There are notwo ranges, a far field tapered range
two identical chambers that have theand a NF-FF planar and spherical
exact same absorber treatment withrange. Figure 3 shows the plan ofthe exception of the taper geomthe chamber with the two ranges.
etry. A qualitative comparison, howThe antenna under test uses theever, suggests some advantages.
same positioner for both ranges,
With traditional chambers, antenand the QZ is the same as well. For
nas with gains of 16 dBi and above
the spherical range the probe is loare required to achieve adequate
cated between the QZ and the plaillumination in the QZ. It appears
nar scanner on the opposite wall.
that one of the features of the coniThe planar scanner can be used forcal taper is that lower gain antennas
can be used. At 10
4,000 MHz Horizontal
2,000 MHz Horizontal
GHz, the source
4,000 MHz Vertical
2,000 MHz Verticalantenna has a–42 dB limit–42 dB limit
directivity of 12
Measured Reflectivity Data
Measured Reflectivity Data
270
270
dBi,8 whereas theconical quad ridge
315
225
225
315
horn used in many
traditional tapered
anechoic
cham- 1800 180
0bers has a directivity of 14 dBi. The
openboundary
45
45
135
135
ridge horn is suc(b)
(a)
90
90
cessfully used in
the conical cham10,000 MHz Horizontal
18,000 MHz Horizontal
10,000 MHz Vertical
18,000 MHz Verticalber design; how–42 dB limit–42 dB limit
ever, when used in
Measured Reflectivity Data
Measured Reflectivity Dataa traditional cham270
270
ber, a smooth am225
225
315
315
plitude taper is notachieved (see Figure 8).
0
0 180
In Figure 9, a 180
comparison of thereflectivity of the
135
135
45
45
conicaltaperedchamber and a tra- (c)
(d)
90
90
ditionally implemented chamber s Fig. 7 Reflectivity levels in the QZ versus angle at 2 (a), 4(b), 10 (c), and 18 GHz (d).
at 400 MHz showsa slight difference
Ref –34 dBm
Atten 0 dB
in back wall re- PeakLog
flectivity(180°),
but this is related 5 dB/
to differences inabsorbertreatThe different curves are fromment between themoving the antenna back and forthin the feed section trying to “tune”
chambers. For thethe best position
traditional cham- W1 V2
V3FS
5 dB/div
ber, one can seeAAa large variationin reflectivity forthe horizontal poCenter 10 GHz
VBH 1 kHz
Span 0 Hz
Res BW 1 kHz
Sweep 20.3 s (3000 pts)
larization as the
direction changes
from 15° to 60° on s Fig. 8 Data for a transverse scan of a traditional chamberusing the same horn used in the conical design.

17400 MHz Horizontal
400 MHz Vertical–30 dB limit
400 MHz Horizontal Traditional
400 MHz Vertical Traditional

Antenna Stand for
Planar Near Field and Spherical Near Field
Far Field and Spherical Near Field Positioner

315

–40
–40

–50
–50

–60
–60

–70
–70

–80
–80

180

2000 mm8000 mm

Measured Reflectivity Data
270
225

Planar Near Field Scanner

5305 mm0

Lens

16097 mm
Half Cones With Wedge Absorbers

135s Fig. 10 Placement of lens in tapered chamber.

45
90s Fig. 9 Comparison with a traditional
chamber.

testing high gain arrays. These arrays can be positioned at the QZ or
closer to the scanner by mounting
them on a tripod, depending on
the frequency of operation or thesize of the scanner.
INCREASING THE QZ SIZEUSING AN RF LENS
While the tapered chamber isused to overcome some of thelimitations of the standard rectangular chamber for antenna testingat lower frequencies, the size of its
quiet zone decreases significantly
as frequency increases. For example, the tapered chamber installedat the National University of Singapore (NUS) has a quiet zone of 1.4
m at 500 MHz but only 45 cm at
8 GHz. To increase the quiet zone
at the higher frequencies, a custom RF lens is integrated inside thechamber. We are not aware of any
other method to increase the quiet
zone without physical alterations to
the original chamber.
The design of the RF lens isbased on the principle of opticalyrefraction to transform a sphericalwave from a point source to a planar wave. By precisely controllingthe dielectric constant of the lens,
the focal length of the lens can becustomized based on the lens aperture.
A plano-convex RF lens is integrated into the tapered chamber
at NUS (see Figure 10). Its focal
length (ƒ) of 10 m is equal to the
distance between the source antenna and the end of the chamber’s
tapered section. The diameterof the lens is chosen to be 2 m inorder to cover a large area of thechamber’s aperture, while allowing
easy mobility of the lens inside thechamber.
The lens has a comparatively
high ratio of the size of the planar
wave front to the lens diameter (afactor of about 0.7D, where D isthe diameter of the lens). Hence, a
2 m diameter lens can produce a
1.4 m plane wave front. The profile
P(x,y) of the lens is designed using
the following equations from Kraus
and Marhefka:9yr
xx == r −− y ,,tantantan θθmtan θθ
m
where
where
y=
y=ρρ sin
sin θθ,,andandP(x, y)rmhs Fig. 11 Lens geometry.

Source Antenna Mast

x

(

72.0
68.0
64.0
60.0
56.0
52.0
48.0
44.0
40.0
36.0

(a)
Z E-Field Phase (deg)
200.0
160.0
120.0
80.0
40.0
0
–40.0
–80.0
–120.0–160.0–200.0

(2)
(2)

)

18

Z E-Field Magnitude (mV/m)

(1)
(1)

ff εε −− 11ρρ== ε cos θ − 1'ε cos θ − 1'
with
with
r ⎛ 1
1 ⎞
hh == r ⎜⎛ 1 −− 1 ⎟⎞⎜εε −− 11 ⎝⎝ sin
sin θθm tantan θθm ⎠⎟⎠m

The variables are defined in Figure 11.
Given its size (2 m), the RF lenscannot be manufactured with traditional dielectric materials due tothe difficulty of controlling permittivity throughout the lens to a highdegree of accuracy. Furthermore, itwould be extremely heavy (∼1,000kg), making installation difficult andrequiring a special support structure, potentially causing undesirable diffractions.
To overcome these issues, a new
low-loss, lightweight metamaterial

m

(3)
(3)

(4)
(4)

(b)

s Fig. 12 Predicted field distribution at
8 GHz, both in magnitude (a) and phase(b).

NUMERICAL ANALYSISThe performance of the lens is
first evaluated using FEKO EM simulation software. A half-wavelength
dipole is placed at the focal lengthof the 2 m lens. The focal length
corresponds to the distance (10 m)
between the feed and aperture of
the tapered chamber. The field isobserved at a vertical plane at 2 m

10
No Lens
With Lens

8

MAGNITUDE (dB)

6
4
2
0
–2
–4
–6
–8
–10
–1

–0.5

0

0.5

1

TRANSVERSE DISTANCE (m)80
No Lens
With Lens

60

PHASE (deg)

40
20
0
–20
–40
–60
–80
–1

–0.5

0

0.5

1

TRANSVERSE DISTANCE (m)s Fig. 13 Computed field distribution
at 2 GHz.

MEASURED PERFORMANCE
The lens is installed at the aperture of the tapered chamber asshown in Figure 15 using a special frame made from low reflection material to easily place andhold it. For the field measurementof the quiet zone, a simple linear
scanner is set up as shown in Figure 16. A broadband dual-ridged
horn is used as the probe antenna.
The field is measured along an axis
transverse to the lens axis at about
2 m separation. The lens is thenremoved and the measurement
repeated.
The results at 2 and 8 GHz are
shown in Figures 17 and 18, respectively. The magnitudes andphases are “normalized” in thesame manner as the numerical re19

10
No Lens
With Lens

8
6
MAGNITUDE (dB)(corresponding to the quiet zone
region) on the other side of thelens. For simplicity, the lens and thedipole are simulated in free-space
without the tapered chamber since
the primary aim of the simulation isto ensure that for the given lengthof the taper, the lens provides thebest possible illumination. Including the chamber with its absorbers in the simulation model would
drastically increase the problem
size and complexity beyond the capability of the numerical package
at these high frequencies.
Figure 12 shows the predicted
fields (for a quadrant) at 8 GHz.
The circles in the plots represent
the outline of the 2 m lens. Cuts ofthe fields along the lens diameterare shown in Figures 13 and 14
for 2 and 8 GHz, respectively. The
fields of the dipole in the absenceof the lens are superimposed in
the figures for reference. For easeof comparison, the magnitudes
are normalized to their respective
mean values – the phase without
the lens is normalized to its peakvalue and the phase with the lens isnormalized to its mean value. Fromthese figures, it is observed that thefield with the lens deviates slightly
from the dipole field due mainly todiffraction from the lens. However,the lens significantly reduces thelarge phase variation of the dipole
field, producing a reasonably goodplane wave in the vicinity of thequiet zone of the tapered chamber.

4
2
0
–2
–4
–6
–8
–10
–1

–0.5

0

0.5

1

TRANSVERSE DISTANCE (m)80

No Lens
With Lens

60
40
PHASE (deg)manufactured by Matsing Pte Ltd.
is used. The material allows thecontrol of the dielectric permittivity to a high degree of accuracy. It
has extremely low-loss (ε’ < 10-4).
Its low density (40 kg/m3) meansthat the 2 m lens weighs only 35
kg, making it portable and easily
installed. The material is also isotropic and broadband, making thelens suitable for both vertical andhorizontal polarizations over a wide
range of frequencies.

20
0
–20
–40
–60
–80
–1

–0.5

0

0.5

1

TRANSVERSE DISTANCE (m)s Fig. 14 Computed field distribution
at 8 GHz.

s Fig. 15 View of the lens from thesource antenna.

s Fig. 16 The QZ scanned with the lensin place at the end of the taper section.

10

10
No Lens
W/Lens8

6

4

4

MAGNITUDE (dB)MAGNITUDE (dB)

6

2
0
–2
–4

2
0
–2

–6

–8

–8

–10

–10

0

0.3

0.6

0.9

1.2

1.5

TRANSVERSE DISTANCE (m)No Lens
W/Lens

60

0

80

80

40

20

20

PHASE (deg)

40

–20

–20
–40

–60

–60
0.3

0.6

0.9

1.2

No Lens
With Lens

0

–40

–80
0

0.3
0.6
0.9
1.2
1.5
TRANSVERSE DISTANCE (m)

60

0

1.5

–80
0

TRANSVERSE DISTANCE (m)

0.3
0.6
0.9
1.2
1.5
TRANSVERSE DISTANCE (m)s Fig. 17 Measured field distribution at 2s Fig. 18 Measured field distribution at 8

sults. Note that the transverse distance in these figures, unlike that of
Figures 13 and 14 is relative to thestart of the measurement position
at 0 m. The plots show that the lenshas indeed improved the phase significantly without adversely affecting the amplitude. The size of thequiet zone for ±10° phase variation
with and without the lens is summarized in Table 1, demonstrating thatthe lens has significantly improved
the phase performance of the tapered chamber. Measurements arealso done from 500 MHz to 1 GHz
to confirm that the lens does not affect the original quiet zone of thechamber at low frequency.

CONCLUSION
This article introduces a new approach to manufacturing tapered
anechoic chambers that provides
good QZ reflectivity results overwide frequency ranges. Additionally, it appears to allow the useof lower directivity antennas thanthe ones used in traditional chambers. A lower directivity antenna
provides smaller amplitude tapers
across the QZ, reducing errors
during gain measurements. With
the addition of an RF lens, thephase of the chamber’s quiet zone
at higher frequencies (2 to 10 GHz)is significantly improved. The lensprovides a quick and easy way toGHz.

enhance the performance of thetapered chamber. Its light-weight
construction enables easy user
installation. The NUS tapered
chamber with an RF lens is now
capable of far field measurementof relatively large antennas from
0.3 MHz to 10 GHz. ■References–4–6

PHASE (deg)No Lens
With Lens

8

GHz.

TABLE I
SIZE OF QUIET ZONE (IN CM) FOR ±10° PHASE VARIATIONWITH AND WITHOUT LENSf (GHz)

2

4

6

8

10

Without lens

95

65

55

45

40

With lens

140

140

140

140

140

20

1.		W.H. Emerson and H.B. Sefton,“An Improved Design for Indoor
Ranges,” Proceedings of the IEEE,
Vol. 53, No. 8, August 1965, pp.
1079-1081.
2.		H. King, F. Shimabukuro and J.
Wong, “Characteristics of a Tapered Anechoic Chamber,” IEEE
Transactions on Antennas andPropagation, Vol. 15, No. 3, May
1967, pp. 488-490.
3.		L. H. Hemming, Electromagnetic
Anechoic Chambers: A Fundamental Design and Specification Guide,
Wiley-Interscience, John Wiley andSons and IEEE Press, Piscataway,
N.J., 2002.
4.		
V. Rodriguez, “Using Tapered
Chambers to Test Antennas,” Evaluation Engineering, Vol. 43, No. 5,
May 2004, pp. 62-68.
5.		V. Rodriguez and J. Hansen, “Evaluate Antenna Measurement Methods,” Microwaves and RF, October
2010, pp. 61-67.
6.		R.E. Hiatt, E.F. Knott and T.B.A.
Senior, “A Study of VHF Absorbers and Anechoic Rooms,” Technical Report 5391-1-F, University ofMichigan, February 1963.
7.		
V. Rodriguez, S. Matitsine, T.T.
Chia, P. Lagoiski, L. Matytsine, M.
Matytsine and P.K. Tan, “A Cone
Shaped Tapered Chamber for Antenna Measurements Both in Near
Field and Far Field in the 200 MHz
to 18 GHz Frequency Range andExtension of the Quiet Zone using
an RF Lens,” Journal of the Applied
Computational
Electromagnetic
Society, Vol 28, No. 12, December
2013, pp. 1162-1170.
8.		ETS-Lindgren horn, www.ets-lindgren.com/pdf/3164-05.pdf.
9.		J.D. Kraus, R.J. Marhefka, Antennas for All Applications, 3rd edition,
McGraw-Hill, 2001.
Vicente Rodriguez attended Ole Miss,in Oxford Miss., where he received his
B.S.E.E in 1994, his M.S. in 1996 andPh.D. in 1999. Dr. Rodriguez joined ETSLindgren as an RF and Electromagnetics
engineer in 2000. In 2004 Dr. Rodriguez
became senior principal antenna design
engineer, placing him in charge of thedevelopment of new antennas. In 2006

Dr. Rodriguez became antenna productmanager, placing him in charge ofdevelopment, marketing and maintenanceof the antenna product line. Dr.
Rodriguez is the author of more than 50
publications and holds patents for hybrid
absorber and dual ridge horn antennas.
Serguei Matitsine graduated withhonors from the Moscow Institute ofPhysics and Technology in 1979 andreceived his Ph.D. in 1982. From 19821984 he held the position of seniorresearcher at the Institute of RadioEngineering and Electronics of theRussian Academy of Sciences. From1984 until 1995 he has held several
positions including senior researcher,head of the electromagnetic laboratoryand deputy director at the Institute ofTheoretical and Applied Electromagneticsof Russian Academy of Sciences. In
1995 Dr. Matitsine joined the researchand development group at Singapore
Technologies Aerospace as technicaldirector and later moved to the position ofchief engineer. Since 2001 Dr. Matitsine
has also been working at Temasek
Laboratories of the National University ofSingapore as an adjunct senior principal
research scientist. He is also the chairman

and technical director of Matsing Pte.
Ltd. His research interests include
electromagnetic materials, metamaterials,
smart materials, multi-beam antennas,
antenna measurement techniques, andmost recently, lightweight, large-size RF
lenses. He has more than 60 publicationsin these areas, as well as four patents.
Tse-Tong Chia received his B.Eng.
degree with first class honors in1986 from the National University ofSingapore, and his M.S. and Ph.D. in1991 and 1994, respectively, from Ohio
State University. He has been with the
DSO National Laboratories in Singapore
since 1986, where he is currently adistinguished member of the technicalstaff. Chia was a laboratory head from1995 until 2010 when he stepped down tofocus on research. He is currently also a
principal research scientist in the Temasek
Laboratories at the National Universityof Singapore. His research interests
include computational methods forelectromagnetic scattering and installed
antenna performance, as well as the use oflenses for antenna applications.
Pavel Lagoiski received his B.S. E.E.
from the National University of Singapore
in 2010. Since then he has held theposition of engineer at Matsing Pte. Ltd.
His research interests include antenna
measurement system and RF lenses.
Leo Matytsine received his B.S. fromthe University of Southern California in2009 and his MBA from the AustralianGlobal School of Management in 2013.
He has been with Matsing Pte. Ltd. since2009 and currently holds the position of
director. His interests include RF convex
lenses and antenna measurement systems.
Michael Matytsine received his B.A.
from Chapman University in 2006 andhis MBA from La Verne University in2010. He has been with Matsing Pte.
Ltd. since 2006 and currently holds the
position of director. His interests include
Luneburg and convex RF lenses.
Peng-Khiang Tan received his degree
in electronic and computer engineeringfrom Ngee Ann Polytechnic in 1999 andhis Bachelor of Technology in Electronics
Engineering (second Class Honors) fromthe National University of Singapore in2008. He currently works as a laboratory
technologist within the antenna group at
the Temasek Laboratories of the National
University of Singapore.

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Characterizing and Tuning
Antennas Using an AutomatedMeasurement System anda VNA
APP NOTE

Copper Mountain Technologies
Application
Indianapolis,
Ind.Profile

As the number of devices featuring wireless connectivity
DiamondEngineering
grows, ensuring
their performance specifications while staying

Characterizing and Tuning Antennas Using an
Automated Measurement System and a VNA

APP NOTE

Characterizing and Tuning Antennas Using an
Automated Measurement System and a VNA

DiamondCA
within Springs,
regulatory requirements
becomes even more important.
Antenna pattern measurement is a critical step in the design
process of antennas and wireless devices. Compact antenna
measurement systems combined with a high performance
VNA are necessary to characterizeparameters suchProfileas pattern,
APPLICATIONPROFILE
Application
gain, VSWR, and efficiency. These results are used to validate
As thenumber of ofdevices featuring wireless connectivityAsthenumbersimulated designs and identify possible performance issuesdevices featuring wireensuring
theirgrows,
performance
specifications
whileperstaying
lessconnectivity
their
before final testing. By using
angrows,
in-housemeasurement
system, ensuring
withinrequirements becomes even more important.
multiple design revisions formancecan betestedregulatory
andpre-certified
specifications while staying withpatternmeasurement is a critical step in the design
without the high cost of using Antennaan accredited
or certified
in
regulatory
requirements
becomeseven
processofantennas
andlikewireless devices.
Compact antenna
measurement facility for each test. Other considerations
more
important.
patternmeasureportability and low cost areimportantto engineers
inAntennavariousmeasurement
systemscombined witha high performance
Application:
Characterizing the Antenna
environments like defensement
or education,
respectively.
criticaltostep
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designprocess
VNAis
areanecessary
characterizesuchasanpattern,
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withtoa configurationconsisting of: Diamond
gain, VSWR, andefficiency.
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andwireless
Compact
Engineering
DAMS 5000
positioner with an FSM sphericalsimulateddesigns
andidentify
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combined reference antenna, a Copper
mount, RF
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Challenges

12

21)

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CHALLENGES

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exported.
measured, the antenna can be manually positioned
Measurement Studionoise byreducingtheIFbandwidthbuttheto a point of lowest signal to confirm that the signal
Scalar or Vector* Calibration
analyzer
speed
isgreatly
reduced,asingleis still above the noise floor and the trace noise is(* Requires VNA)
sweepacceptable
can takeformany
seconds tocomplete.
testing requirements.
After
the setup is
GPIB / Ethernet / USBUSB
verified,the DAMS
antennaisPC-based
VNA’s
offer
the measurement
benefit ofsoftware
greater
VNA, Rec. Only orController
used to make range,a measurement.
The software
indexes
initial dynamic
higherspeeds,andSource/Receiver
the antenna to every physical point and executes adirect datatransfer.
transmission sweep on the VNA. Optionally, VSWR
Copper Mountain Technologies
22
data can be automatically collected at every point.
Precision RF Cables
Once complete, the entire data set can be viewed,
DAMS Antenna
processed and exported.

Left:
is istuned
at 2420
MHzMHzLeft:thetheAUT
AUT
tuned
at 2420
Right: the VNA and DAMS software
Right: the
VNA andsoftwareoperatein tandem
on aDAMS
PC

operate in tandem on a PC

environment reflections,or S21), the VNA will show the reAPPLICATION:
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PC-Based
VNA
the VNA can be switched to S21
sponse of the entire link. At thisCHARACTERIZING THE
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may
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ANTENNA
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this
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response
issues. Since
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antennas
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Diamond Engineering DAMS 5000
block thelittle tracetonoise.
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fullunwanted
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erenceCopperMounoverposi33,000 data
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tioned
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point
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Technologies
measurements using the Planar 804/1 VNA scripts and DAMS
VNA canbe switched toPlanar
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operate in tandem
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up as unusually
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end
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noise
floor
and
the
trace
noise
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measurement
andVNA
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and Tuning
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VNA
In this application
the VNA’s high measurement speed of 100respikes indicating thatthe signal
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The
sult
a refined
antenna design withis
acceptablefor
testingrequireware.
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begin,
the
VNA
is
set
to
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may
revealcertain
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accurate,publishable
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ments.
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key
in
this
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frequency
range,
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changes havebeen madeto theallantenna and/or the test
response
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Planar 804/1 VNA
is still fully
dynamicantenna
range, Time Domain,and sweep speed.
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measurement
RFsetup,cable
loss(including
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Traditional
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saved many
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outside of the direct path will show up as unusually largesion,reflection, VSWR, dynamic
can5000
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antenna is connected then posiIn this application the VNA’s
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spikes indicating that the signal is being reflected. The
sourceand receiver,limiting
range, Time Domain, and sweep
point.
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real-timetuningcomplete,and fast data collection.
absorbercan now
placedto block the unwanted
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frequency
or stepped CW
modesAllof operation.
speed.
of theseThearetestcrucial for
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1been
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isoften
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limited of
setup,
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measurement
can
be
made
quicklytoverifythe measurement andcessedand exported.
the antennas are connected andSummary
and record these changes. The end result is a refined antenna
to
just
the
measurementofthe
antennawithout
parameters
antennas
andrelatedsystems.
the VNA is set to transmission (S12Complete antenna measurement and characterization candesign with accurate, publishable test data.
as phase
VSWR.
The Planar804/1 VNA contains
all of
POST-MEASUREMENT
Antennameasurements
containbe quickly achieved using sucha compact
antennaand
measurement
system poweredby a PC-based
VNA andthecapability
DAMS studio.
ANALYSISAND
TUNING
the tools
andfor the measurement,a requiredlarge amountof data: adesign,
singleTraditional pattern measurement
systemsof
often
a separateFast, broadband vector basedtuningtheuseantenna.
polarization full spherical measureAnalyzing the andmeasurement
source and receiver, limiting measurements to either single
ment
10dynamic
degreeresolution
data
may
revealcertainproblems
measurementswithlownoise using
and highrange
arefrequency or stepped CW modes of operation. The test
and
51 frequencies
containsoversuch
as low
gain,
pattern
distoressential
forthe andwireless
technology
of today andtomorrow.
equipment
for these
systems
is often dedicatedlimitedto justmeasurement responseofCombining
the antenna issues.
without
parameters
33,000 measurement
data points.system
The withability totion,orthefrequency
a compact
antennasuch asphase
and VSWR.
The Planar
804/1
contains
all ofautomatethesemeasurements
Since
the
Planar
804/1
VNA
isVNA
still
the flexibilityof
an automation
ready,
PC-based
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the provides
Planar a804/1
VNA
scripts
fully
calibrated
it
can
be
used
to
Mountainlow cost
solutionand tuning of the antenna.Copper
Fast, broadband
vectorTechnologies
based
and
DAMS
5000
positioner
saved
tune
and
troubleshoottheantennameasurements with low noisehigh dynamic
range
are
thatandgreatly
reducesdevelopment
time and increases product
many
hours
comparedtomanuin essential
real time.
live
antennatunfor the For
wireless
technology
of todayand tomorrow.
performance.
Combininga compact antenna
measurement
system with
The yellow trace shows S22 of the AUTing,after
tuningal testing. In this application thetheanalyzeris switchedto S22the flexibility of an automation ready, PC-based VNA from
VNA’s high measurement speedmode, and the user can make small
Copper Mountain Technologies provides a low cost solutionof 100 μs enabled real-time tuning
modifications
totheantennawhilethat greatly reduces development time and increases productand fast data collection.
seeingtheresults
immediately
disperformance.
Theyellow
yellow
trace
shows
the
AUT
The
trace
shows
S22 ofS22
the ofAUT
after
tuningplayed. To identify multi-path andafter tuning
22

21

COPPER MOUNTAIN TECHNOLOGIES23
US: +1.317.222.5400 | Asia: +65.63.23.6546

COPPER MOUNTAIN TECHNOLOGIESSUMMARY
Complete antenna measurement and characterization can bequickly achieved using a compact
antenna measurement system powered by a PC-based VNA and theDAMS studio. Traditional patternmeasurement systems often use aseparate source and receiver, limiting measurements to either single
frequency or stepped CW modesof operation. The test equipment
for these systems is often dedicated and limited to just the measurement of the antenna without parameters such as phase and VSWR.
The Planar 804/1 VNA contains allof the tools and capability required
for the measurement, design, andtuning of the antenna. Fast, broadband vector based measurementswith low noise and high dynamic

range are essential for the wirelesstechnology of today and tomorrow.
Combining a compact antenna
measurement system with the flexibility of an automation ready, PCbased VNA from Copper Mountain
Technologies provides a low cost
solution that greatly reduces development time and increases product performance.

2- and 4-Port USB VNAs from 100 kHz to 20 GHz

Copper Mountain Technologies’ Cobalt Series
USB VNAs’ industry-leading dynamic range,
152* dB typ. (10 Hz IFBW), and sweep speed,
10* µs min. typ., make them ideal for fast
production and diverse research applications.  
2- and 4-Port models with options for direct
receiver access and frequency extension.
* model specific.
http://www.coppermountaintech.com/products/Ohm-50/category/13/24

Practical Antenna
Connection for Accurate
Testing
Clayton Karmel, Pdicta Corp, San Diego, CA and Ben Maxson,
Copper Mountain,
Indianapolis, Ind.

C

onnecting RF test and measurement
equipment to an Antenna Under
Test (AUT) usually involves tradeoffs
among measurement accuracy, electricalconsiderations, cost, and mechanical ruggedness. In this article, we describe some
aspects of this connection with practical

SMA to RF switch, SMA to U.FL (both sexes), SMB, SMA bullet and SMA
to U.FL short cable, and a Vector Network Analyzer.
25

advice for such test and measurement scenarios.

Tools of the TradeTESTING VIA AN RF TESTCONNECTOR: U.FL VS COAXIAL
SWITCHWhen provided, an antenna test port willusually be at a 50 ohm point. If the design
includes a coax switch for inspection
and final test purposes, anantenna test sample canbe produced or reworked
with the switch rotated
180°. This reverses the in
and out terminals so connecting a test cable interrupts the feedline and the cable faces theantenna instead of the device.
A test port and test cable of the appropriate type reduces the need for strain relief
as the cable can be connected just before
measuring. A right-angle connector at thedevice end may further reduce stress andallow easier routing through the device
housing. Note that most ultra-miniatureconnectors have a mating life on the orderof a few dozen cycles, but failure will become self-evident in the measurements.

An RF Switch allows production testingof the WiFi module. Turn it around totune the antenna. But it adds cost to
every unit.

testing either the WiFi module or
the antenna.

U.FL connector sits on a stub with thesame length as the trace to the WiFi
module. In production, omit the U.FL
connector.

The example PCB on the left has
a U.FL connector on a very shortstub. One can isolate the WiFi device by omitting the 100pf cap andinstalling the U.FL to obtain coaxial
access to the antenna. Alternatively, and as shown at right, an RF
switch can be installed allowing for

CUSTOM CAL STANDARDS
Lumped element tuning for PCB
antennas requires that your VNA
calibration is in the proximity of thenext series or shunt element to be
added. You have options:• Use a co-axial calibration standard, usually to an SMA interface, and then extend the reference plane across the lengthof your board connection. Thisextension may compensate foran SMA-U.FL adaptor, or maybe
a short cable. Add reference
plane time until the ‘Open’ isat the right edge of the Smith
chart.
• Create a home-brew set of Calibration standards. Take threesparsely populated boards;
add 50 Ohm shunt, 100pf shunt(short) and one left open.
• 
If the reference plane isn’t extended by either custom calibration standards or by manuallyextending the reference plane,
then matching efforts will be extremely confused because theSmith chart will be rotated.
This test board omitted the
WiFi IC, installing a U.FL connector, instead. We made four of theseboards, one each configured for
Load, Open, Short and Thru to theun-matched Antenna.
Now we can calibrate to theentry point of the antenna withthe short, open and 50 ohm load.
In production, there are no extra
components or space required.
Alternatively, if we had usedcoaxial calibration standards (N or
SMA), we would subsequently “extend” the reference plane by an adjustment in the VNA, usually called
Port Extension. How far? Use theOpen and Short configurations andextend the reference plane until
these align with the right and leftedges of the Smith Chart.
If your instrument and cablingmass is substantial compared tothe counterpoise (ground-plane) of
the board, add a DC-DC connector prior to calibration. When you
start to match, move the reference
plane (Port Extension) backwards,
bringing the reference plane backwith you as you add tuning elements.
TUNING A LOOPThe keyfob design below was
based on a radio chip with a differential output. The round shapeof the board and differential drive
suggested a loop antenna which isless susceptible to de-tuning thana monopole, dipole or folded F.
Dielectric detuning happens when
the keyfob is held by different
hands or in different orientations orsits on different surfaces.
On the right is the production
board with differential RF feed at
the top of the image. On the left

Application-Specific RF test board26

is a test board with the same antenna and same ground-plane.
Notice tuning elements both at theentry of the differential drive andat the mid-way point. L5 enables
the loop to be extended, in casesimulations (or back-of-the-handguesses) are off when judging loop
size/characteristics.
Notice Y2. It’s a 1:1 balun, configured so that a single-ended RF
interface can drive the loop differentially. Look up the radio IC’s
component output match and tune
the antenna impedance to thecomplex conjugate of that match.
Remember to extend the reference
plane to the lumped elements first.
TESTING WITHOUT A COAXIAL
CONNECTOR
Sometimes there just isn’t anopportunity to have a U.Fl or Coaxial switch. Maybe it was overlooked

in design, or maybe the productrequirements were just too tight.
Here are some coaxial cable options if you need to forge a reliabletransmission line connection.
First, you will most likely want to
identify a position on your board
where the impedance is, or is expected to be, 50 ohms. This isn’t always required, but simplifies things.
Remember that you still face two
calibration/reference plane options– calibrating with normal standards
at the ‘connector’ end of your coaxial cable, then extending the reference plane, or building multiple
boards and implementing a custom ‘calibration set’.
The latter option can be a problem in this case, however, because
it requires that your coaxial cablehave the same electrical lengthon each of 3-4 boards, or thatyou have to regularly solder yourOpen/Short/Load values in and outof place.
50 Ohm matches are increasingly typical at the output of mixed
signal RF SOC IC’s, but if not, review your options at your duplexer,
diplexer, balun or within your RF
matching circuit to the antenna.
The ideal test point has close by access to the antenna’s ground plane,
preferably one well reinforced withvias.
Whether your coaxial cable attached with “horizontal” or “vertical” launch, the goal is to maintainthe TEM modes that exist between
the center conductor and shield of
the test cable the feedline microstrip
and its ground plane through thetransition between them.
A 50 ohm microstrip feed line
may be as little as 0.03 to 0.1 mm(.015” to .025”) wide if over anembedded ground plane of a thincircuit board assembly. Protecting such a thin trace from being
pulled up by the test cable requires
a mechanically sound connection
between the cable sheath or shield
and the ground plane, preferably inan area where the ground plane isreinforced by vias.
HORIZONTAL LAUNCH
For a horizontal launch, to the extent possible the test cable should
access the broken microstrip feedline trace directly from the groundplane and in the direction of thefeedline. Keep the feed line andcenter conductor and their separate
ground return currents close by, in
proximity, parallel to each other,and minimally different in length.
Avoid 90° connections and bends
until both currents are safely within
the test cable. Eisenhart’s “edgelaunch design” can be approximated even if the test point is not on
the edge of the circuit board.

Small board example RF Test Circuit

Small board example, loaded as Calibration ‘Load’

VERTICAL LAUNCH
For a vertical launch, it may bepractical to break the feedline traceand reflow solder a U.FL or similar surface mount connector to thefeedline at the break and the groundplane on both sides. This gives theruggedness advantage of the connector, albeit at the presence ofsome reflection. If this isn’t practi-

Small board example, loaded as Calibration ‘Short’27Small board example, loaded as Calibration ‘Open’

Small board example, Measuring the Antenna raw match with extended reference
plane.

Test circuit for Loop antenna.

feedline, the approach below may
work. A clearance
hole is drilled forthe test cable dielectric, which istrimmed to theboard
thickness,and the emerging
center conductor
soldered closely tothe feed trace at the
break. The shieldshould be tightly
flared so as to besoldered closely to
the ground plane
beneath.
USING A BALUN
A further refineRF test and production board with Loop Antenna.
ment might help
exclude the test
cal, the device doesn’t need to recablefromthemeasurement
if themain functional, and ground plane
DUT has insufficient ground planeis accessible on the other side of theand common mode currents on
circuit board assembly beneath the28

the cable result. This can be doneby forming a 1-turn loop aroundthe minimum bending radius fora semi-flexible or hand-formable
test cable, or with a ferrite beador two placed over the test cable
where it departs from the groundplane. While ferrite beads are notcharacterized above 500 MHz andall of them lose performance as
frequency increases, they are inexpensive and unlikely to impair
the test results. Type 61 ferrites areusually recommended above 300
MHz but Type 43 is available in agreater range of dimensions andmay work.
A bead can be selected for fit
and evaluated by placing a sample
at the shorted end of a coax stuband extending the R54 port to thatpoint. As a useful example, theKemet B-20L-44 RG-178 stub demonstrated |Z| in excess of 58 ohms
out to 3 GHz, which includes theGSM, UMTS, and LTE to Band 7.
It was in excess of 30 ohms out to5GHz. Any common mode current
from the DUT can be arbitrarily reduced by stacking multiple beads.
CABLE LENGTH AND TYPE
Regarding the choice off cable,
here are some points to consider:• 
Test Cable: The test cableshould be long enough to place
the test instrument out of thenear field of the AUT and preferably a few wavelengths away.
This reduces reflections off thetest instrument itself so the antenna measurement will better
approach the limits of the test
environment.
• 
Test Cable Type: The thinnerand more flexible the cable, theless strain relief required for ruggedness under test; on the other hand, the thinner the cable,
the greater the attenuation loss
which limits the measurement
accuracy. To follow is a table ofcable types typically available incommercial cable assemblies.
If the required test cable length
introduces significant loss, it may
be possible to include the coaxial
adaptors and test cable in the instrument calibration using a tightly
constructed short, open, and a 50
Ohm chip resistor on a brassboard

of similar construction to the DUT.

of multiple band requirements andsize and space constraints; accordingly, the typical SWR may be far
more significant than reflection or
attenuation from a less than ideal

CONCLUSION
The typical integral antenna in
the 2.4GHz ISM bands and up is acompromise to begin with because

test cable interface. Every device
will be different, but keeping theTEM modes in mind in making thetest connection won’t hurt and will
make for the most accurate test.

Test Cable

Braid

Avoid 90°
Connection
if possible

Solder to
Ground
Plane

OK to
Bend

Eisenhart
Edge Launch

Clearance
Hole

Dielectric

Sharp Flarein Braid

29

Break
Feedline

Matching
Network

Chip orOther AntCompact 2-Port USB VNAs up to 8.5 GHz Frequency

Copper Mountain Technologies’ popular Compact Series delivers full-featured lab-grade
performance in a compact package suitable
for many research and production applications. These instruments are a great value
solution for magnitude and phase measurements from 9 kHz to 8.5 GHz andan option for 75 Ω measurements to 3 GHz.
http://www.coppermountaintech.com/products/Ohm-50/category/11/30

EXTEND YOUR REACH
Diamond Engineering has integrated the
CobaltFx Frequency Extension System into
their antenna measurement system.

C4209
Available options are 2- and4-port VNAs with a maximum
frequency of 9 GHz or 20 GHz

COST-EFFECTIVE
MM-WAVE SYSTEM

CobaltFx Frequency Extension System is an unprecedented
approach for millimeter-wave S-parameter measurements, suited forantenna range measurements, automotive radar & sensor testing,material characterization, and many other mm-Wave applications.
The system is anchored by a Cobalt Series USB VNA

Three dedicated waveguidebands, 50-75 GHz, 60-90
GHz and 75-110 GHz
Visit the websites below for
more information on the
CobaltFx System:
cpmt.link/cobaltfx or
cpmt.link/farrancobaltfx

Includes frequency extension modules and all necessary
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Read the white paper from Frost & Sullivan about the high-performance
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