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Antenna Theory and Design
Antenna Theory and Design
Associate Professor:
WANG Junjun 王珺珺
School of Electronic and Information Engineering,
Beihang University
F1025, New Main Building
[email protected]
13426405497
Chapter 7
Antenna Measurements
Introduction
• Accurate measurements are necessary to establish the actual performance of
antennas: their gain, pattern, polarization, bandwidth, efficiency, etc. Antennas
having strict specifications are needed in many applications as in mobile and
personal communications, satellite communications, remote sensing, and radar.
• In many cases, antenna properties can be calculated theoretically very accurately.
However, for complex antennas this might not be possible-too many idealizations or
simplifications have to be made. Often the modeling of the usage environment is
difficult, e.g., if the antenna is close to the human head or is installed on an airplane.
Even if the properties of ideal antennas can be calculated, performance of real world
antennas has to be checked by measurements, because due to fabrication tolerances,
and in some cases due to fabrication errors, they may not work as well as predicted.
The measurement results give valuable information for troubleshooting.
Chapter 7 Antenna Measurements
▫ Basic Concepts
▫ Radiation Pattern
▫ Gain Measurements
▫ Typical Sources of Error In Antenna Measurements
7.1 Basic Concepts
• The
most
common
antenna
measurement is to measure its
radiation properties like directional
pattern, gain or phase pattern in the
far field. Typical configuration of the
measurement of radiation properties is
shown in Fig.7-1.
• The basic procedure is to place a
transmitting or receiving source
antenna at different locations with
respect to the Antenna Under
Test(AUT) and thus get a number of
samples of the pattern.
Figure 7-1 Typical configuration for the
measurement of the radiation properties
of an antenna
7.1 Basic Concepts
7.1.1 Reciprocity in Antenna Measurements
• It was indicated already above that the AUT can act as either a receiving
antenna or a transmitting antenna. This is of course due to the reciprocity
principle.
• Two important consequences of the principle from the antenna measurement
point of view were given:
▫ The transmitting and receiving patterns are the same.
▫ Power flow is the same either way.
• Thus it is clear that all radiation parameters of the AUT can be measured in
either transmission or reception mode. This is especially useful in cases,
where, for example, the AUT is an integral part of a larger device acting as
either a receiver or a transmitter thus defining the direction of the signal.
7.1 Basic Concepts
7.1.1 Reciprocity in Antenna Measurements
• However, in practical antenna measurements one has to be careful in applying
the reciprocity principle：
①The emfs in the terminals of the interchanged antennas are of the same
frequency.
②The media are linear, passive and isotropic.
③The power flow is equal for matched impedances only.
• In standard careful measurements the 1st condition should be met without
problems. The 2nd condition should be considered always, when the AUT
includes any active or ferrite components as an integral part. The 3rd condition
is perhaps the most difficult and may lead into measurement results, which
seem to violate the reciprocity principle.
7.1.1 Reciprocity in Antenna Measurements
Fig.7-2 shows a typical configuration with the instrumentation for the measurement of the radiation
properties of an AUT.
Fig.7-2
Where
VR
e 1 1
e  2 2

 tFS  (1   AUT ) 
2 1 1
VT 1  T  SAe
1   AUT  R e2 2
V𝑔 = voltage detected by the receiver from transmission line 2, V
V𝑇 = voltage supplied by the transmitter into transmission line 1,V
𝛾1 = complex propagation factor of transmission line 1 between the transmitter and source antenna,𝑚−1
𝛾2 = complex propagation factor of transmission line 2 between the AUT and receiver antenna, 𝑚−1
𝑙1 = length of transmission line 1,m
𝑙2 = length of transmission line 2,m
𝜌𝑇 = voltage reflection coefficient of the transmitter output
𝜌𝑠𝐴 = voltage reflection coefficient of the source antenna
𝜌𝐴𝑈𝑇 = voltage reflection coefficient of the AUT
𝜌𝑅 = voltage reflection coefficient of the receiver input
𝑡𝐹𝑆 = voltage transmission coefficient between the antenna termimals
2
(7-1)
7.1.1 Reciprocity in Antenna Measurements
VR
e 1 1
e  2 2

 tFS  (1   AUT ) 
2 1 1
VT 1  T  SAe
1   AUT  R e2 2
2
• Above it has been assumed that the voltage transmission coefficients of the
transmitter output, source antenna input and receiver input are included in the
calibration procedures into 𝑉𝑇 ,𝑉𝑅 and 𝑡𝐹𝑆 respectively. Therefore, only the voltage
transmission coefficient 1-𝜌𝐴𝑈𝑇 of the AUT output is taken into account. The first and
last term on the right side of(1)are the voltage transmission coefficients of the
transmission lines including multiple reflections. Now, according to the reciprocity
principle we can exchange the input and the output(𝑉𝑅 and 𝑉𝑇 ) if we fulfill the
conditions above.
7.1.1 Reciprocity in Antenna Measurements
• However, this is not possible normally, because we have to exchange at least the
transmitter (generator) and receiver and sometimes also parts of the cabling. In this
case the exchange does not change the result significantly, if the numerators of the
first and last term on the right side of (1) do not change much, which means that
▫ The reflection coefficients 𝜌𝑇 and 𝜌𝑅 and the cables (length, attenuation) are
identical, which is seldom the case in practice,
▫ The reflections are small , i.e. 𝜌𝑇 𝜌𝑆𝐴 , 𝜌𝑅 𝜌𝐴𝑌𝑇 , 𝜌𝑇 𝜌𝐴𝑈𝑇 and 𝜌𝑅 𝜌𝑆𝐴 ≈ 0
. This depends on the measurement system and can usually be approximated to
be true only for a measurement where a calibrated vector network analyzer is
used.
• Problems occur especially when the matching of the antennas or the measurement
devices is poor. In this case the exchange of the generator and receiver will change
the result and it would appear that the reciprocity does not hold.
7.1 Basic Concepts
7.1.2 Near-Field and Far-Field
• The measuring practices and constrains depend largely on the distance of this surface from
the AUT. It has been defined already that one can find several regions of radiated field in
the vicinity of the antenna. There are the reactive near-field region, the radiative near-field
or Fresnel region, and the far-field or Fraunhofer region(Fig 7-3).
Figure 7-3 Radiation pattern in near- field and far-field regions.
7.1 Basic Concepts
7.1.2 Near-Field and Far-Field
• As we are almost always interested in the radiation properties in the far field,
it is obvious that the measurement usually also takes place in the far field.
There are several advantages of the far-field measurement:
▫ The measured field pattern is valid for any distance in the far-field region;
only simple transformation of the field strength according to l/r is required.
▫ If a power pattern is required, only power (amplitude) measurement is
needed.
▫ The result is not very sensitive to the changes in the location of the phase
center of the antennas and thus the rotation of the AUT does not cause
significant measurement errors.
▫ Coupling and multiple reflections between the antennas are not significant.
7.1.2 Near-Field and Far-Field
• The main disadvantage of the far-field measurements is the required
large distance between the antennas leading to large antenna ranges. The
distance can be too large for a measurement in an anechoic chamber or it
can result in atmospheric attenuation. In these cases one needs to
consider measurements in the radiating near-field region, where so called
near-field measurements take place.
• The reactive or evanescent near-field region is seldom used for antenna
measurements, because it normally is located too close to the AUT and
because mutual impedance caused by reactive coupling between the
antennas makes the measurement complicated.
7.1.2 Near-Field and Far-Field
• The border of the regions around the AUT are defined based on
dominances of certain properties of the field. The outer boundary of the
reactive near-field region is typically defined to be at the distance
beyond which the amplitude of the far-field component is larger than
those of reactive near-field components. For small elementary dipoles, it
is at the distance determined by the radius of the radian sphere, that is,

(7-2)
r 
rnf
where
2
𝑟𝑟𝑛𝑓 = distance to small AUT at the outer boundary of the reactive near
field, m
𝜆 = wavelength, m
7.1.2 Near-Field and Far-Field
• The inner boundary of the far-field region is most often defined as the
distance, where the curvature of the spherical wave front at the edge of the
AUT causes a phase difference of 𝜋/8 radians 22.5° compared to the
center of the antenna, which is equal to a curvature of 𝜆/16 . This lead to
the well-known Rayleigh distance giving the border between the Fresnel
and Fraunhofer regions:
2D2
(7-3)
rff 
where

𝑟𝑓𝑓 = distance to AUT at the inner boundary of the far-field region, m
𝜆 = wavelength, m
D = largest dimension of the physical aperture of the antenna, m
7.1.2 Near-Field and Far-Field
Phase error at the edges of a test antenna in the far-field when illuminated by a spherical wave.
7.1.2 Near-Field and Far-Field
• In some cases the allowed error due to the curvature is smaller and thus large
far-field distances like 2𝑟𝑓𝑓 are sometimes applied. For small antennas, the
curvature criterion may not be adequate to define the far-field condition.
′
Actually, when 𝐷 < 𝜆/4 , 𝑟𝑓𝑓 is smaller than 𝑟𝑓𝑓
! Thus in these cases
additional criteria for the proper distance for far-field measurement must be
used. If we assume that the near fields add in random phase to the far field and
set the peak-to-peak ripple caused by this to ∆𝐿(𝑑𝐵) , we get an additional
condition for the measurement distance:
r 
'
ff
where
rrnf
10L/40  1
′
𝑟𝑓𝑓
= distance to AUT with certain level of the reactive near fields, m
∆𝐿 = peak-to-peak error caused by the reactive near fields , dB
(7-4)
7.1.2 Near-Field and Far-Field
• Another far-field condition is based on the requirement that the change in
the measurement distance r due to rotation of the AUT should not have a
large effect on the result obtained. Setting the peak-to-peak limit of this
effect to ∆𝐿(𝑑𝐵) , we get the condition
2 Dm
(7-5)
''
r 
ff
10L /40  1
′
𝑟𝑓𝑓
= distance to AUT with certain effect of the rotation, m
𝐷𝑚 = largest mechanical dimension of the antenna, m
∆𝐿 = peak-to-peak error caused by the rotation of AUT, dB
In (7-5) we have estimated that the peak-to-peak change of the distance
between the phase center of the AUT and the source antenna is 𝐷𝑚 /2.
where
7.1.2 Near-Field and Far-Field
• Example Near-and Far-Field Distances for a Normal-Mode Helix on a
Handheld Cellular Phone. A normal-mode helical antenna is located on top of a
handset at 900MHz. The length of the antenna is 25 mm and the length of the
handset chas sis 110 mm. Find the distances where different near-and far-field
criteria are met, when required peak-to-peak measurement uncertainty due to
proximity effects is 0.5 dB.

7.1.2 Near-Field and Far-Field
• Solution
Now the maximum dimension of the antenna should be estimated to be the combined length
of the phone chassis and the antenna element, so 𝐷 = 𝐷𝑚 = 135𝑚𝑚. This is due to the fact
that for a device smaller than half wavelength typically the whole metallic body is occupied
by the currents inducing the radiation.
(a) The outer boundary of the reactive near field is at a distance given by Eq. 7-2 :
𝑟𝑟𝑛𝑓 = 0.333/2𝜋 𝑚 = 53𝑚𝑚
(b) The Fraunhofer region starts at a distance given by Eq.7-3
𝑟𝑓𝑓 = 2 ∙ 0.1352 /0.333 m=110mm
(c) Now∆𝐿 = 0.5dB and we get from Eq.7-4 the minimum distance, where the effect of the
near fields is small enough:
′
𝑟𝑓𝑓
= 0.333/2𝜋 ∙ 34.2 𝑚 = 1.8𝑚
(d) Eq 7-5 gives the minimum distance, where the effect of the rotation of the AUT is small
enough:
′′
𝑟𝑓𝑓
= (2 ∙ 0.135) ∙ 8.20 𝑚 = 2.2𝑚
7.1.3 Coordinate System
• The
IEEE
standard
spherical
coordinate system is shown in Fig.74.
• Moving the source antenna along
lines of constant 𝜙 or constant 𝜃
results in conical cuts or 𝜙 cuts,
when 𝜃 is constant, and great-circle
cuts or 𝜃cuts, when 𝜙 is constant.
• In measuring the polarization of the
antenna a standard practice is to
establish a local coordinate system for
each measurement direction (𝜃, 𝜙 ).
Figure 7-4 Standard coordinate system for
antenna measurements showing conical, great
circle and principal plane cuts
Chapter 7 Antenna Measurements
▫ Basic Concepts
▫ Radiation Pattern
▫ Gain Measurements
▫ Typical Sources of Error In Antenna Measurements
7-2 Radiation Pattern
7-2-1 Instrumentation
• The instrumentation required to accomplish a measuring task depends
largely on the functional requirements of the design. An antenna-range
instrumentation must be designed to operate over a wide range of
frequencies, and it usually can be classified into five categories :
1. source antenna and transmitting system
2. receiving system
3. positioning system
4. recording system
5. data-processing system
7-2-1 instrumentation
• A block diagram of a system that possesses these capabilities is shown in
Fig 7-5
Figure 7-5 Instrumentation for typical antenna-range measuring system
7-2-1 instrumentation
• The source antennas are usually log-periodic antennas for frequencies below 1 GHz,
families of parabolas with broadband feeds for frequencies above 400 MHz, and even
large horn antennas. The system must be capable of controlling the polarization.
Continuous rotation of the polarization can be accomplished by mounting a linearly
polarized source antenna on a polarization positioner. Antennas with circular
polarization can also be designed, such as crossed log-periodic arrays, which are often
used in measurements. The transmitting RF source must be selected so that it has
frequency control, frequency stability, spectral purity, power level, and modulation.
• The receiving system could be as simple as a bolometer detector, followed possibly by
an amplifier, and a recorder. More elaborate and expensive receiving systems that
provide greater sensitivity, precision, and dynamic range can be designed. One such
system is a heterodyne receiving system, which uses double conversion and phase
locking, which can be used for amplitude measurements. A dual-channel heterodyne
system design is also available, and it can be used for phase measurements.
7-2-1 instrumentation
• To achieve the desired
plane cuts, the mounting
structures of the system
must have the capability to
rotate in various planes.
This can be accomplished
by
utilizing
rotational
mounts (pedestals), two of
which are shown in Figure
7-6.
Figure 7-6 Azimuth-over-elevation and elevation-over-azimuth rotational mounts.
7-2-1 instrumentation
• There are primarily two types of recorders; one that provides a linear
(rectangular) plot and the other a polar plot. The polar plots are most
popular because they provide a better visualization of the radiation
distribution in space.
• The recording instrumentation is usually calibrated to record relative field
or power patterns. Power pattern calibrations are in decibels with dynamic
ranges of 0–60 dB. For most applications, a 40-dB dynamic range is
usually adequate and it provides sufficient resolution to examine the
pattern structure of the main lobe and the minor lobes.
7-2-1 instrumentation
• In an indoor antenna range, the
recording equipment is usually
placed in a room that adjoins the
anechoic chamber. To provide an
interference free environment, the
chamber
is
closed
during
measurements. To monitor the
procedures, windows or closed
circuit TVs are utilized. In
addition, the recording equipment
is connected, through synchronous
servo-amplifier systems, to the
rotational mounts (pedestals) using
the raditional system shown in
Figure 7-7(a).
Figure 7-7a Block diagrams of typical
instrumentations for measuring rectangular and
polar antenna and RCS patterns. Traditional
system
7-2-1 instrumentation
• The system can record rectangular or polar plots.
Position references are recorded simultaneously
with measurements, and they are used for angular
positional identification. As the rotational mount
moves, the pattern is graphed simultaneously by
the recorder on a moving chart. One of the axes of
the chart is used to record the amplitude of the
pattern while the other identifies the relative
position of the radiator.
• A modern configuration to measure antenna and
RCS patterns, using a network analyzer and being
computer automated, is shown in Figure 7-7(b).
Figure 7-7(b) Block diagrams of typical
instrumentations
for
measuring
rectangular and polar antenna and RCS
patterns. Computer automated system
7-2-1 instrumentation
Figure 7-7c Indoor range with automatic control.
7-2-1 instrumentation
Figure 7-7d Anechoic chamber room
7-2-1 instrumentation
RCS and far-field antenna measurement
system
is
based
on
intelligent
instruments, and personal computers
(IPC), the automatic monitoring system,
with data acquisition, analysis and
processing capabilities. Testing System to
control computers at the core command
and control functions with the
corresponding vector network analyzer,
the receiving table. Polarization turntable
and other peripheral equipment, and farfield antenna measurement and automatic
completion of the measurement tasks
RCS.
RCS & far-field Antenna Measurement System
7-2-1 instrumentation
7-2 Radiation Pattern
7-2-2 Amplitude Pattern
• The total amplitude pattern of an antenna
is described by the vector sum of the two
orthogonally polarized radiated field
components.
• In many applications, antenna pattern
measurements must be made in situ to
preserve the environmental performance
characteristics.
A
typical
system
arrangement that can be used to
accomplish this is shown in Figure 7-8.
Figure 7-8 System arrangement for in situ antenna pattern
measurements.
7-2 Radiation Pattern
7-2-3 Phase Measurements
Two basic system techniques that can be used
to measure phase patterns at short and long
distances from the antenna are shown
respectively, in Figures 7-9(a) and 7-9(b). For
the design of Figure (a), a reference signal is
coupled from the transmission line, and it is
used to compare, in an appropriate network,
the phase of the received signal. For large
distances, this method does not permit a direct
comparison between the reference and the
received signal. In these cases, the
arrangement of Figure (b) can be used in
which the signal from the source antenna is
received simultaneously by a fixed antenna
and the antenna under test.
Figure 7-9 Near-field and far-field phase pattern
measuring systems.
Chapter 7 Antenna Measurements
▫ Basic Concepts
▫ Radiation Pattern
▫ Gain Measurements
▫ Typical Sources of Error In Antenna Measurements
7-3 Gain Measurements
7-3-1 Absolute-Gain Measurements
• There are a number of techniques that can be employed to make absolute-g
ain measurements.
A. Two-Antenna Method
 Pr 
4 R 

10log
 G0t dB   G0r dB  20log10 

10 

P
  
 t
(𝐺0𝑡 )𝑑𝐵 = gain of the transmitting antenna (dB)
(𝐺0𝑟 )𝑑𝐵 = gain of the receiving antenna (dB)
𝑃𝑟 = received power (W)
𝑃𝑡 = transmitted power (W)
R= antenna separation (m)
𝜆 = operating wavelength (m)
• If the transmitting and receiving antennas are identical(𝐺0𝑡 = 𝐺0𝑟 ), (7-6) reduces to
 Pr  
1
 4 R 
G

G

20log

10log
 0t dB  0 r dB 

10 
10 

2
P
  
 t 
(7-6)
Where
By measuring R, λ, and the ratio of Pr/Pt , the gain of the antenna can be found.
(7-7)
7-3-1 Absolute-Gain Measurements
B. Three-Antenna Method
• If the two antennas in the measuring system are not identical, three antennas (a, b, c) must be
employed and three measurements must be made (using all combinations of the three) to determine
the gain of each of the three. Three equations (one for each combination) can be written, and each
takes the form of (7-6). Thus
 Prb 
4 R 
(a-b Combination)

10log
 Ga dB   Gb dB  20log10 

10 

  
 Pta 
(7-8a)
(a-c Combination)
(b-c Combination)
 Prc 
4 R 

10log

10 

P
  
 ta 
 Ga dB   Gc dB  20log10 
 Gb dB   Gc dB
 Prc 
 4 R 
 20log10 

10log

10 

P
  
 tb 
(7-8b)
(7-8c)
From these three equations, the gains (Ga)dB, (Gb)dB, and (Gc)dB can be determined provided R, λ,
and the ratios of Prb/Pta, Prc/Pta, and Prc/Ptb are measured.
7-3-1 Absolute-Gain Measurements
C. Extrapolation Method
• The extrapolation method is an absolute-gain method, which can be used with the
three-antenna method, and it was developed to rigorously account for possible errors
due to proximity, multipath, and nonidentical antennas.
7-3-1 Absolute-Gain Measurements
D. Ground-Reflection Range Method
• A method that can be used to measure
the gain of moderately broad-beam
antennas, usually for frequencies
below 1 GHz, has been reported. The
method takes into account the
specular reflections from the ground
(using the system geometry of Fig 710), and it can be used with some
restrictions and modifications with
the two or three-antenna methods.
Figure 7-10 Geometrical arrangement for reflection range.
7-3-1 Absolute-Gain Measurements
D. Ground-Reflection Range Method
• To make measurements using this technique, the system geometry of Figure 7-10 is utilized.
Usually it is desirable that the height of the receiving antenna ℎ𝑟 be much smaller than the
range 𝑅0 (ℎ𝑟 ≤ 𝑅0 ) . Also the height of the transmitting antenna is adjusted so that the field
of the receiving antenna occurs at the first maximum nearest to the ground. Doing this, each
of the gain equations of the two- or three-antenna methods take the form of
 Pr
 4 RD 
G

G

20log

10log
 a dB  b dB
10 
10 

  
 Pt


rRD 

20log
D
D



10 
A B
R

R 

(7-9)
The only quantity that needs to be determined is the factor r which is a function of the
radiation patterns of the antennas, the frequency of operation, and the electrical and
geometrical properties of the antenna range.
7-3 Gain Measurements
7-3-2 Gain-Transfer (Gain-Comparison) Measurements
• The method most commonly used to measure the gain of an antenna is the gaintransfer method. This technique utilizes a gain standard (with a known gain) to
determine absolute gains. Initially relative gain measurements are performed, which
when compared with the known gain of the standard antenna, yield absolute values.
The method can be used with free-space and reflection ranges, and for in situ
measurements.
• The procedure requires two sets of measurements. In one set, using the test antenna
as the receiving antenna, the received power (𝑃𝑇 )into a matched load is recorded. In
the other set, the test antenna is replaced by the standard gain antenna and the
received power (𝑃𝑠 ) into a matched load is recorded.
7-3 Gain Measurements
7-3-2 Gain-Transfer (Gain-Comparison) Measurements
• Writing two equations of the form of (7-6) or (7-9), for free-space or reflection
ranges, it can be shown that they reduce to
P 
( 7-10)
 GT dB   GS dB  10log10  T 
 PS 
where (𝐺𝑇 )𝑑𝐵 and (𝐺𝑠 )𝑑𝐵 are the gains (in dB) of the test and standard gain
antennas.
7-3-2 Gain-Transfer (Gain-Comparison) Measurements
• If the test antenna is circularly or elliptically polarized, gain measurements using the
gain-transfer method can be accomplished by at least two different methods. One
way would be to design a standard gain antenna that possesses circular or elliptical
polarization.
• The other approach would be to measure the gain with two orthogonal linearly
polarized standard gain antennas. The total gain of the circularly or elliptically
polarized test antenna can be written as
(7-11)
G
 10log G  G
 T dB
10

TV
TH

𝐺𝑇𝑉 and 𝐺𝑇𝐻 are, respectively, the partial power gains with respect to vertical-linear
and horizontal-linear polarizations.
Chapter 7 Antenna Measurements
▫ Basic Concepts
▫ Radiation Pattern
▫ Gain Measurements
▫ Typical Sources of Error In Antenna Measurements
7-4 Typical Sources of Error In Antenna Measurements
• Any measured quantity has a margin of error. Thus, the complete value for the gain of
an antenna might be 15𝑑𝐵𝑖 ± 0.5𝑑𝐵 indicating a half decibel uncertainty. To reduce
the measurement uncertainty to an acceptable level, the critical sources of error have
to be recognized.
• A pure plane wave (uniform phase and amplitude) is an ideal test field for the
measurement of far-field pattern. However, there are inevitably deviations from the
plane wave.
• The test field imperfections are virtually indistinguishable from antenna errors and
therefore, it is worth to spend a lot of expense and labor to reduce them.
• Coupling to the reactive near field, alignment errors, interfering signals, effects of the
atmosphere, leaking and radiating cables, instrumentation errors, etc., are further
sources of error in antenna measurements.
7-4 Typical Sources of Error In Antenna Measurements
7-4-1 Phase Error and Amplitude Taper Due to Finite Measurement Distance
• Let us assume that the AUT is a planar antenna, which is receiving a wave coming from
the direction of the main beam axis. If the measurement distance is too small, the fields
received by different parts of the AUT will not be in phase and there will be a quadratic
2𝐷2
𝜆
phase error(Fig). At the far-field limit
, the phase difference between the aperture
edge and center is 22.5° . Doubling the measurement distance halves this phase error.
Figure 7-11 Phase error and amplitude taper across the aperture of an AUT
7-4-1 Phase Error and Amplitude Taper Due to Finite Measurement
Distance
• Due to the phase error, the measured gain is smaller and the side lobes
are higher than in the ideal plane wave case. Also the nulls of the
radiation pattern become filled. For measuring antennas having
2𝐷2
𝜆
moderate side-lobe levels(down to about -30dB), the distance
is
usually adequate. At this distance, the measured gain is about 0.06dB
smaller than the real far-field gain. If the measurement distance gets
shorter, the measurement errors increase rapidly and the near-in side
lobes merge with the main beam and either appear as shoulders or
disappear altogether.
7-4 Typical Sources of Error In Antenna Measurements
7-4-2 Reflections
• Reflections from surroundings produce field variations(amplitude and
phase ripples)in the test zone as the direct wave and reflected waves
interfere.
• Even small reflected waves may cause large measurement errors,
because the fields of the waves are added, not the powers.
• Reflections are especially harmful in the measurement of low side
lobes. A small reflection coupled to the AUT through the main lobe
may completely mask the direct wave coupled through the side lobe.
Different ways to reduce the reflections or their influence are studied.
7-4 Typical Sources of Error In Antenna Measurements
7-4-3 Other Sources of Error
In addition to these common measurement range imperfections, there are
some other possible sources of error:
Coupling to the reactive near field may be significant at low frequencies.
Antenna measurements are three-dimensional vector field measurements.
Therefore, many kind of alignment errors are possible.
Man-made interfering signals may couple to the sensitive receiver
especially on outdoor ranges.
At large measurement distances, the effects of the atmosphere may be
considerable.
……..
Questions:
What are the near- and far-field distance for a pyramidal standard qaln
horn antenna at 10GHz?