**Antenna Measurement**

**1 Antenna Ranges**

**An antenna range is a facility where antenna radiation **
characteristics are measured. An antenna range includes
the following typical components:

1. A substantial space for hosting the test antenna and the source antenna

2. A source antenna

3. An antenna positioner

4. A transmitter and receiver system (e.g. a Network Analyser)

**Block diagram of a typical antenna-measurement system**

Realized by a network analyser

**2 Pattern Measurement**

**2.1 Reciprocity for Antenna Radiation Patterns**

(a) Test antenna in transmission (b) Test antenna in reception

As shown above, in (a) a test antenna #1 is fixed in space
*and excited by a current I*_{1} at its terminal. Its radiation
pattern is measured by a standard horn antenna #2, which

moves around a spherical surface at the far-field region of
the test antenna and receives an open-circuit voltage at its
*terminal indicated by V _{oc2}*. Note that the horn antenna is
always pointing towards the test antenna as it moves. In
(b), the horn antenna #2 is fixed in space and excited by a

*current I*

_{2}at its terminal. It radiates towards the test antenna #1, which is now in the receiving mode and made to rotate (but with no translational motions) around all possible angles of

###

and###

. The received open-circuit voltage at the terminals of the test antenna is indicated by*V _{oc1}*. Then by the reciprocity theorem,

2 1 1

*oc* *oc*

*V* *V* *I*

*V* *V*

*Thus the transmitting radiation pattern (E*_{2}) is equal to the
*received field pattern (E*_{1}*) through just a constant (c) which *
can be removed after normalization of the far fields. This is
**known as the Reciprocity for Antenna Radiation Patterns**
and the radiation pattern of an antenna can be measured by
using it to receive the far field of another fixed antenna.

*The open-circuit voltages are related to the far fields E*_{1} and

*E*_{2} *through the effective lengths, L _{e1}*

*and L*, of the antennas as: 1 1 1, 2 2 2

_{e2}*oc*

*e*

*oc*

*e*

*V*

*E L*

*V*

*E L*Hence, 1 1 1 2 2 1 1 2 1 1 2 2 2 ,

_{e}

_{e}*e*

*e*

*L*

*I*

*I*

*E L*

*E L*

*E*

*E*

*cE*

*I*

*I L*

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2. *E*_{}

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, 0*E*

_{}as a function of in the

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*xz*plane

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3. *E*_{}

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90 ,###

*E*

_{}as a function of in the

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*xy*plane

###

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4. *E*_{}

###

, 0*E*

_{}as a function of in the

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*xz*plane

**2.2 Principal Plane Patterns**

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###

1. *E*_{}

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90 ,###

*E*

_{}as a function of in the

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*xy*plane

Usually radiation patterns are measured over certain planes cut through the antenna. There are four typical planes at which the far-field characteristics (magnitude and phase) are measured. These patterns are called principal plane patterns:

**Principal plane radiation patterns**

Antenna under test

**3 Gain Measurement**

The gain of an antenna can be measured by the comparison
**method using a standard gain antenna whose gain and **

**reflection coefficient are known accurately. The power **

received by the standard gain antenna and the test antenna are measured, respectively, under the same conditions.

*G _{T }*= gain of the test antenna

*G _{S }*= gain of the standard gain antenna

*P _{T }*= power received by the test antenna

*P _{S }*= power received by the standard gain antenna

= reflection coefficient of the test antenna

We have the following relation from which the gain of the test antenna can be determined.

##

##

##

##

2 2 1 1*T*

*s*

*T*

*S*

*S*

*T*

*P*

*G*

*G*

*P*

###

dB###

dB 10 log_{10}10 log

_{10}1 2

_{2}1

*T*

*T*

*T*

*S*

*S*

_{S}*P*

*G*

*G*

*P*

_{ }

_{}

_{}

_{}

_{}

_{}

_{}

**3.2 Partial Gain Method for Elliptically Polarized**
**Antennas**

For an EP polarized (including CP polarized) antenna, its gain is measured by measuring its partial gains at two orthogonal orientations, for example the horizontal and the vertical orientations. That is, first measure (use the comparison method) its gain in the vertical orientation

*G _{TV}*. Then rotate the antenna about its axis through 90º

*and measure its gain in the horizontal orientation G*.

_{TH}*The total gain of the antenna G*is given by:

_{T}###

*G*dB 10 log

_{T}_{10}

###

*G*

_{TV}*G*

_{TH}###

(dBic)**Example 1**

A standard gain antenna has a gain of 63 (18 dB). It is used
to measure the gain of a test antenna. The received power
*with the standard gain antenna P _{s}* = 3.16 mW (5 dBm) and

*with the test antenna P*= 31.6 mW (15 dBm). The standard antenna has a VSWR

_{T}_{S}= 1.1 and the test antenna’s VSWR

_{T}= 1.3. Find the gain of the test antenna assuming both antennas are linearly polarized (LP).

###

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_{10}

_{10}

_{2}2 2 10 10

_{2}1 dB dB 10 log 10 log 1 31.6 1 0.13 18 dB 10 log 10 log 3.16 1 0.05 28.06 dB

*T*

*T*

*T*

*S*

*S*

_{S}*P*

*G*

*G*

*P*

_{ }

_{}

_{}

_{}

_{}

_{}

_{}

_{}

_{}

_{}

_{}

_{}

_{} VSWR 1 1.1 1 0.05 VSWR 1 1.1 1 VSWR 1 0.13 VSWR 1

*S*

*S*

*S*

*T*

*T*

*T*

**Solution**

**4 Polarization Measurement**

**4.1 Polarization Pattern Method**

**This method can be used to measure the AR and the tilt **
angle

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**of the polarization ellipse but not the sense of**polarization.

The test antenna is connected as the source antenna while a linearly polarized antenna such as a dipole antenna is used to receive the power at different rotation angles. The square root of the received power plotted against the rotation angle

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indicate the AR and title###

.**Polarization pattern method (1)**

Receiving dipole antenna (receiving)

Test antenna (transmitting)

**Polarization pattern method (2)**
O
A
B
AR = OA/OB
2 2 2 2
cos sin
*r* *A* *B*
**r**

A LP antenna is used as the source antenna and made to rotate continuously in the vertical plane while the test antenna’s radiation patterns are being measured. The result is a radiation pattern with rapid fluctuation in field strength. The difference between adjacent maximum and minimum points of the fluctuation gives the AR at that particular direction. The rotation speed of source antenna must be much greater than the rotation speed of the test antenna in the azimuth or vertical plane.

**4.2 Rotating Source Method**

**This method can be used to measure the AR at different **

**directions but not the tilt angle or the sense of **

**AR at **
Radiation pattern
obtained with a
rotating linear
source
**3 dB AR beamwidth**
Test antenna

**5 Input Characteristic Measurement**

**5.1 Input Impedance Measurement**

The input characteristics of an
antenna such as the input
impedance *Z _{A}* can be

**measured by a network**

**analyser. The advantage of a **

network analyser is its ability to measure both the magnitude and the phase of the power received.

**5.3 VSWR Measurement**

The VSWR of an antenna can be obtained from its reflection coefficient measurement.

0
11
0
or *A* (dimensionless)
*A*
*Z* *Z*
*S*
*Z* *Z*

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1 VSWR (dimensionless) 1###

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**5.2 Reflection Coefficient Measurement**

The reflection coefficient

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*(or S*

_{11}) of an antenna can be obtained from its input impedance measurement.

**5.4 S Parameter Measurement**

*The S parameters (S*_{11}*, S*_{12}*, S*_{21}*, S*_{22}) of two antennas
treated as a two-port network can be measured by a
network analyser after a proper calibration.

**References:**

[1] *IEEE Standard Test Procedures for Antennas, IEEE Std 149-1979, published *

by IEEE Inc., 1979, distributed by Wiley-Interscience.

[2] *G. E. Evans, Antenna Measurement Techniques, Artech House, Boston, MA, *

1990.

[3] *John D. Kraus, Antennas, McGraw-Hill, New York, 1988, Chapter 18.*

[4] *C. A. Balanis, Antenna Theory, Analysis and Design, John Wiley & Sons, Inc., *

New Jersey, 2005, Chapter 17.

[5] *W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, Wiley, New *