ANTENNA MEASUREMENTS
Measurement of Radiation Pattern
Gain
Polarization
VSWR
Test Ranges
Antenna Measurement
 Antenna measurements are part of the analysis of
antenna parameters.
 Antenna measurements are useful for the
application oriented specific designs of the
antennas.
 Accurate measurements are necessary
establish the actual performance of antennas.
to
Antenna Measurements are
required for the following purpose
1. To verify design.
2. To control quality.
3. To analyze different parameters.
4. To find analytical and statistical errors.
5. To indicate actual performance of antennas.
6. To adjust critical components and dimensions.
7. To calibrate and store data for different types of
antennas.
Drawbacks in Measurements of
Antenna Parameters
1. Measurement are time consuming.
2. Measuring equipment is expensive.
3. Open site measurement are not accurate.
4. It is difficult to bring large antennas to the measuring
site.
5. It is sometimes difficult to provide far-field distance.
6. For large antennas, at higher frequencies the pattern
measurement becomes very difficult because
distance to the far field region becomes too large for
even outside ranges.
Methods to overcome
Drawbacks in Measurements
1. Determining of far-field patterns from near field
measurement.
2. Making scale model measurements.
3. Using automated measuring equipment.
4. Using computerized techniques.
Accurate Measurements
 The following methods are used for a accuracy,
1. Near field techniques.
2. Polarization techniques.
3. Computer controlled test systems.
4. Compact ranges.
Radiation Pattern Measurement
 We need two antennas for measurement of radiation
pattern one will transmit and other will receive.
 One is the origin other at some distance away from
the first antenna.
 There are 2 procedure for Radiation Pattern
Measurement.
 First procedure is suitable for low frequency
antennas and Second procedure is suitable for high
frequency antennas.
Radiation Pattern Measuring setup
Secondary
Antenna
Primary
Antenna
RF Cable
Transmitter
Antenna Rotator Mechanism
Receiver
Indicator
First procedure is suitable for
low frequency antennas
 Primary antenna is not moving, secondary is
moved along a circular path at a constant radius.
 Secondary is directional antenna, usually the
primary antenna is transmitting.
 The field strength readings and direction of the
secondary antenna with respect primary antenna
are recorded along the circle at different points.
 The plot of radiation pattern is made either polar
form or rectangular form.
Second procedure is suitable for
high frequency antennas
 Both the antennas are kept in fixed positions
having a suitable spacing between them. Now,
 Primary antenna is rotated and secondary antenna
is transmitting.
 So that the field strength reading and direction of
the primary antenna with respect to secondary
antenna can be made.
 The readings are taken at number of points, by
stopping the rotation of primary antenna for
recording the readings or continuous reading can
also be taken if pattern recorder is available.
Experimental setup for Antenna Test
Primary
Antenna
Direct Rays
Secondary
Antenna
d – Maximum linear
direction of either antenna
Tower
Tower
Ground
Measurement of Antenna Gain
 Radiation Intensity is defined as the power
radiated from an antenna per unit solid angle.
 Basically there are two standard methods used for
the measurement of gain of an antenna such as,
1. Gain comparison method or direct comparison
method.
2. Absolute gain method.
1. Measurement of gain by Direct
Comparison Method
 This method is commonly used at HF.
 The measurement of gain is carried out by
comparing the signal strengths transmitted or
received with the test antenna and a Standard
Gain Antenna (SGA).
• A standard gain antenna is that antenna whose gain
is accurately known so that it can be used in
measurement of other antenna.
• HORN antenna at microwave frequencies (300MHz
and 300GHz) is mostly used as SGA.
 The secondary antenna may be an arbitrary
transmitting antenna and it is not necessary to
know its gain.
 In place of primary antenna, there will be two
antennas, one is test antenna and another one is
standard antenna at a considerable distance.
 The distance between primary and secondary
antennas must satisfy the condition
and
reflections between them should be minimized.
 The attenuator pad is inserted to the input of
receiver, in order that AUT & SGA work into a
matched load.
Measurement of gain by Direct Comparison Method
Procedure
 At first, standard antenna is connected to the receiver
with the help of switch “ S ”.
 The input to the transmitting antenna (secondary
antenna) is adjusted to a convenient level and
corresponding reading at the receiver (primary
antenna circuit) is recorded.
 The attenuator dial setting and the power bridge
reading are also recorded. Say it is W1 and P1
respectively.
 Now connect the test antenna whose gain is to be
measured in place of standard gain antenna.
 The attenuator dial is adjusted such that receiver
indicates the same previous reading as was with SGA.
 Let the attenuator dial setting reading W2 and power
bridge reading P2.
Case I.
When P1 = P2
 If P1 = P2, then no correction need to be applied
and the gain of the test antenna measurement with
respect to standard gain antenna is given by
Power gain (GP) = W2 / W1
Case II. When P1 ‡ P2
 The power level is changed during the
measurements and P1 ‡ P2, then actual power gain
of test antenna can be obtained by multiplying Gp
with ratio (P1 / P2) = P.
Power gain (G) = GP * (P1 / P2)
2. Absolute gain Method
 There are two methods with which the calibration of
standard gain antenna can be done.
i.
Two Identical Antennas
ii.
Three Arbitrary Antennas
i). Two Identical Antennas
 The input to the transmitting antenna is adjusted to
an appropriate level and corresponding receiver
reading level is recorded.
 The attenuator dial setting and the power bridge
reading are
recorded. Say it is Wt and Pt1
respectively.
Absolute gain of Identical Antenna Measuring setup
r
Receiving Antenna
(AUT)
Transmitting Antenna
Precision Calibrated
Variable Attenuator
Signal
Source
Attenuator
Padd.
Receiver Detector
Detector
Power Bridge
Indicator
Modulator
 Now the transmitter is disconnected from the
antenna and is connected to the receiver through
pads.
 The attenuation dial is adjusted until the receiver
reads the same previous levels.
 The attenuator dial setting and power bridge reading
are noted, say Wr and Pt2.
If Pt1 = Pt2
 Then no correction would be needed.
 If drifting of power is involved, then correction
would be needed. The second method is needed
only when two identical antennas are not available.
ii). Gain Measurement by 3 Antenna Method
This method consists of
 Three unknown antennas.
 Using antenna 1 as transmitter and antenna 2 as
receiver, the received power W1 is measured. Let
the transmitter power be P1.
Gain Measurement by 3 Antenna Method
 Replacing antenna 2 by antenna 3, the received
power W2 is measured for the same transmitted
power (P2 = P1).
 When antenna 2 is used as transmitter and
antenna 3 is used as receiver, receiver power W3 is
measured. Let the transmitter power be P3.
Measurement of Polarization
Polarization is a property of waves that can oscillate
with more than one orientation.
Types of Polarization
There are three types of polarization
linear
circular
elliptic
The basic setup for Polarization Measurements
Output of Measurement when the Test
Antenna is Vertically Linearly Polarized
Left: Rectangular Plot & Right. Polar Plot
Output measurement of Linearly Polarized
Test Antenna (Horizontal Polarization)
Output of Measurement when the Test
Antenna is Circularly Polarized
Output of Measurement when the Test
Antenna is Elliptically Polarized
Measurement of VSWR
 VSWR stands for Voltage Standing Wave Ratio,
and is also referred to as Standing Wave Ratio
(SWR).
 VSWR is a function of the reflection coefficient,
which describes the power reflected from the
antenna.
 If the reflection coefficient is given by Г, then the
VSWR is defined by the following formula
 VSWR is the ratio of the peak amplitude of a
standing wave to the minimum amplitude of a
standing wave.
 The VSWR is always a real and positive number
for antennas.
 The smaller the VSWR is, the better the antenna is
matched to the transmission line and the more
power is delivered to the antenna.
 The minimum VSWR is 1.0. In this case, no power
is reflected from the antenna.
TEST RANGES
Free Space Ranges
 The most popular free space ranges are
1.Anechoic Chambers, ("an-echoic" meaning
non-reflective, non-echoing or echo-free).
2. Elevated Ranges.
3. Compact Ranges.
1. Anechoic Chambers
 An anechoic chamber is a room designed to
completely absorb either sound or electromagnetic
waves. They are also insulated from exterior
sources of noise.
 Anechoic chambers are indoor antenna ranges.
 Indoor ranges are desirable because the test
conditions can be much more tightly controlled
than that of outdoor ranges.
 Anechoic Chambers are most often used for
frequencies above 300 MHz.
 Anechoic Chamber is shown in the following picture,
along with some test equipment.
2. Elevated Ranges
 Elevated Ranges are outdoor ranges.
 Placed higher than the surrounding area.
 In this setup, the source and antenna under test are
mounted above the ground.
 These antennas can be on mountains, towers,
buildings.
 For elevated ranges, once a source and test
antenna location are determined, the test operators
then determine where the significant reflections will
occur, and attempt to minimize the reflections
from these surfaces.
 The basic diagram of an elevated range is shown in
Figure.
3. Compact Ranges
 The source antenna must be placed in the far field
of the test antenna.
 The reason is that the wave received by the test
antenna should be a plane wave for maximum
accuracy.
 The compact range operates over the 400 MHz to
100 GHz band.
 The parabolic antenna changes spherical waves
into plane waves towards AUT. This involves
small distances, and hence it is called compact
range.
Differences between indoor and
outdoor ranges
S.
No.
Indoor Ranges
Outdoor Ranges
1.
Space is limited
Space is unlimited
2.
Only small antennas can be
tested
Antennas of any size can be
tested
3.
Ground and other reflections
are controlled
Ground and other reflections
cannot be controlled fully
4.
They have controlled
environment
They have uncontrolled
environment
5.
They have all weather
capability
They do not have all weather
capability
The selection range for antenna measurement
depends on the following factor:
1. Frequency,
2. Cost,
3. Accuracy required,
4. Size of the antenna and so on.
Measurement of Antenna Efficiency
 Antenna radiation efficiency is the ratio of power
radiated to the power supplied to the antenna.
Measurement of Directivity
 Directivity is defined in terms of radiation intensity.
Problems
1.A parabolic reflector antenna is designed for
operation at 3000MHz. Its largest aperture
dimensions is 20 ft. for measurement of radiation
pattern, what should be minimum distance between
primary and secondary antenna (one feet = 0.3048
meter)
Given
f = 3000 MHz,
d = 20 ft
20 * 0.3048 = 6.0960 meters
To Find
r
Solution
λ =c/f
= ( 3*10Λ8 ) / 3000MHz
= 3 / 30
λ = 1 / 10
r ≥ ( 2 * (6.0960) Λ 2 ) * 10
r ≥ 743.22 meters
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YOU