Chapter 17 : Antenna Measurement

• Introduction
• Antenna Ranges
• Radiation Patterns
• Gain Measurements
• Directivity Measurements
• Impedance Measurements
• Polarization Measurements
• Scale Model Measurements

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 Introduction
• Practical antennas : complex structural
configuration & excitation method
• Need experimental results to validate
theoretical data.
• Ideal condition : test antenna in receiving
mode and illumination of test antenna by
plane wave, i.e., uniform amplitude and phase.
• Actual system : “large enough” separation ->
far-field region.
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 Phase Error

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 Measurement Drawbacks
• Pattern Measurements : far-field distance too long ->
difficult to suppress “unwanted” reflections from ground
and surrounding objects.
• Impractical to move antenna to measuring site.
• Take time to measure characteristics, e.g., antenna array.
• For outside measuring systems, uncontrolled
environment and no all-weather capability
• For enclosed systems, cannot accommodate large
antenna systems, e.g., ship, aircraft, antenna arrays, etc.
• Measurement techniques are expensive.

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 Antenna Ranges
• By antenna facilities : Outdoor & Indoor ranges.
• Receiving mode & far-field required -> Ideal
incident field : uniform plane wave -> large
space required.
• Antenna Ranges:
– Reflection : suppress reflection effects
– Free-space : suppress contributions from surrounding
environments; Elevated ranges, Slant ranges,
anechoic chambers, compact ranges, near-field
ranges.
– Near-field ranges use Near-field/Far-field method to
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convert measured near-field data to far-field.
 Reflection Range
• “constructive interference” is desirable -> “quiet
zone”.

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 Elevated Ranges
• Reduce contributions from surrounding environments by 1.
select proper directivity & side lobe of source antenna. 2.
clear between line-of-sight. 3. redirect or absorb reflected
energy that cannot removed. 4. utilize some special signal-
processing techniques.

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 Slant Ranges
• Pattern maximum of free-space radiation toward center of the
test antenna
• First null toward the ground to suppress reflection.
• More compact than
elevated ranges.

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 Anechoic chambers
• Controlled environment, all-weather capability, security &
minimize EM interference -> Indoor anechoic chambers.
• After high-quality absorbing
materials are available.
• Two basic types to minimize
specular reflections.
• Rectangular : maximize quiet
zone and simulate free-space.
Require absorbers.
• Tapered : phase difference
between direct and reflected
waves is small.
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 Compact Ranges
• To obtain “ideal” plane wave illumination.
• Compact Antenna Test Range (CATR) can generate nearly
planar wavefronts in a very short distance.

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 Radiation Patterns
• Spherical coordinate system -> 3-dimensional pattern.
• Impractical to obtain 3-d pattern.
• Alternatively, a number of 2-d patterns (pattern cuts) are
measured and used to construct 3-d.
• To obtain pattern cuts:
– Fix φ and vary θ. ->
Elevation patterns
– Fixθ and vary φ. −>
Azimuthal patterns.

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 Instrumentation
Classification of instrumentations:
1. Source antenna &
Transmitting system
2. Receiving system
3. Positioning system
4. Recording system
5. Data-processing
system

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 Instrumentation (2)
• Source antenna : typically log-periodic antenna,
parabolas with broadband feeds, horn antenna;
polarization must be controllable.
• Source : frequency control, frequency stability, spectral
purity, power level, and modulation.
• Receiving system : bolometer detector, amplifier,
recorder; or a heterodyne system.
• Recording system : linear plot or polar plot; record
relative field or power patterns -> relative pattern.

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 Instrumentation (3)
• Positioning system : must be capable to rotate in various
planes to generate pattern cuts.

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 Instrumentation (4)
• Typical systems for measuring antenna & RCS pattern.

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 Amplitude Pattern
• Amplitude pattern = vector sum of two orthogonally polarized
field components. Can be measured using same system as
radiation pattern measurement.
• In situ measurement : preserve environmental performance
characteristics.

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 Phase Measurements
• In general, far-field component given by
− jkr
e
Eu = uˆ E (θ , φ )e jψ (θ ,φ )
r
• E, ψ : amplitude, phase
• Phase is “periodic”.
• Need reference.

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 Gain Measurements
• Two basic methods: absolute-gain and gain-transfer.
• Absolute-gain used to calibrate antennas that can be used
as standards for gain measurements and requires no a
priori knowledge of the gains.
• Gain-transfer (or gain-comparison) used in conjunction
with standard gain antennas to determine absolute gain
of the antenna under test.
• Typical antennas used for gain standards:
– Resonant λ/2 dipole (gain around 2.1 dB) : broad pattern,
affected by surrounding environments.
– Pyramidal horn antenna (gain 12-25 dB) : very directive, less
affected by environments.

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 Absolute Gain : 2-antenna
 4πR   Pr 
• Gain equation: (G0t ) dB + (G0 r ) dB = 20 log10   + 10 log10  
 λ   Pt 
G0t,G0r : transmitting,
receiving gains.
Pt,Pr : transmitting,
receiving powers.
R : antenna separation
λ : wavelength
• For identical antennas:
(G0t )dB = (G0r )dB =
1  4πR   Pr 
20log10  +10log10 
2  λ   Pt 
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 Absolute Gain : 3-antenna
• 2-antenna method is not applicable for
“nonidentical” antennas.
• Need 3 antennas with 3 following gain equations:
 4πR   Prb 
(Ga ) dB + (Gb ) dB = 20 log10   + 10 log10  
 λ   Pta 
 4πR   Prc 
(Ga ) dB + (Gc ) dB = 20 log10   + 10 log10  
 λ   Pta 
 4πR   Prc 
(Gb ) dB + (Gc ) dB = 20 log10   + 10 log10  
 λ   Ptb 
Then solve for Ga,Gb,Gc .
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•Homework #4 [pdf] Solution [pdf]

Gain Transfer
• Most commonly-used.
• Use gain standards to determine absolute gain.
• Use two set, first AUT as receiving antenna,
second AUT replaced by standard gain antenna.
 4πR   PT 
Test: (GT ) dB + (G0 ) dB = 20 log10   + 10 log10  
 λ   P0 
 4πR   PS 
Standard: (GS ) dB + (G0 ) dB = 20 log10   + 10 log10  
 λ   P0 
 PT 
Thus, (GT ) dB = (GS ) dB + 10 log10  
 PS 

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 Directivity Measurement
• Simplest method:
1. Measure two principal E- and H-plane patterns.
2. Determine half-power beamwidths of each pattern.
3. Compute directivity using
4π (180 / π ) 2 22.181(180 / π ) 2
D0 = (2-27) OR D0 = (2-30b)
Θ1d Θ 2 d Θ12d + Θ 22 d
• Alternative method:
4πU max  π  2π  M  N 
D0 = ; Prad = B0   ∑ ∑ F (θ i , φ j ) sin θ i 
Prad  N  M  j =1  i =1 
• If there are both θ and φ components:
4πU {θ ,φ }
D0 = Dθ + Dφ ; D{θ ,φ } =
( Prad )θ + ( Prad )φ
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 Polarization Measurement
• Polarization-pattern method.
• Need additional measurement to determine sense of
rotation:
– Use two antennas:
CW & CCW.
– Use dual-polarized probe.

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 Impedance Measurement
• Two types of impedances : Self and Mutual
• If antenna radiating into unbounded medium, i.e. no
coupling, Self-impedance = driving-point impedance.
• If there’s coupling between AUT and other sources or
obstacles, driving-point impedance is a function of both
self and mutual impedances.
• In practice, driving-point impedance = input impedance
• Typically, use vector network analyzer (VNA), slotted
lines.

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 Scale Model Measurement
• In many applications (e.g., aircraft, ship, etc.), antennas
and structures are too “large” in weight/size to move.
• Furthermore, moving changes environments.
• Use Geometrical scale modeling to
– physically accommodate, within small ranges, measurements
that can be related to large structures,
– experimental control,
– minimize costs.
• Need “exact” replicas,
physically & electrically.

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