Antennas Measurement

Hon Tat Hui Antenna Measurements

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Antenna Measurement1 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)


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Block diagram of a typical antenna-measurement system

Realized by a network analyser


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A compact antenna range


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2 Pattern Measurement

By means of the reciprocity theorem, the radiation patterns of an antenna can be measured by its receiving patterns.

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. Thesepatterns are called principal plane patterns.

( ) plane in the offunction a as ,90 xyEE ==( ) plane in the offunction a as 0, xzEE ==( ) plane in the offunction a as ,90 xyEE ==( ) plane in the offunction a as 0, xzEE ==

Principal Plane Patterns


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5Antenna position and the coordinate system for pattern measurement


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Principal plane radiation patterns

Antenna under testAntenna under test


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3 Gain MeasurementComparison MethodThe gain of an antenna can be measured by the comparison method using a standard gain antenna whose gain and reflection coefficient are know accurately. The power received by the standard gain antenna and the test antenna are measured, respectively, under the same conditions.


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GT = gain of the test antennaGS = gain of the standard gain antennaPT = power received by the test antennaPS = power received by the standard gain antennaT = reflection coefficient of the test antenna S = reflection coefficient of the standard gain antenna

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

( )( ) STS sTT GPPG 22

11

=

( ) ( )

+= 2

2

101011

log10log10dBdBS

T

S

TST P

PGG


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Partial Gain Method for Elliptically Polarized AntennasFor an EP polarized (including CP ploarized) 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 one orientation, the vertical orientation GTV. Then rotate the antenna about its axis through 90 and measure its gain GTHagain. The total gain of the antenna GT is given by:

( ) ( ) (dBic) log10dB 10 THTVT GGG +=dBic is a unit to indicate that the gain is relative to an isotropicand perfectly CP antenna.


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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 Ps = 3.16 mW (5 dBm) and with the test antenna PT = 31.6 mW (15 dBm). The standard antenna has a VSWRS = 1.1 and the test antennas VSWRT = 1.3. Find the gain of the test antenna assuming both antennas are LP and polarization matched to each other.

Solution

( ) ( )

dB 06.28 05.0113.01log10

16.36.31log10dB 18

11

log10log10dBdB

2

2

1010

2

2

1010

=

+=

+=

S

T

S

TST P

PGG

13.01VSWR1VSWR ,05.0

11.111.1

1VSWR1VSWR =+

==+=+

=T

TT

S

SS


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4 Polarization Measurement4.1 Polarization Pattern Method

This method can be used to measure the AR and the tilt angle of the polarization ellipse but not the sense of polarization.

The test antenna is connected as the source antenna while a LP antenna such as a dipole antenna is used to receive the power atdifferent rotation angles. The square root of the received power plotted against the rotation angle indicate the AR and title .


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Polarization pattern method

O

AB

AR = OA/OB


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A LP antenna is used as the source antenna and made to rotate continuously in the vertical plane while the test antennas 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 testantenna 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 polarization.


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Radiation pattern obtained by the rotating source method

AR at


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5 Input Characteristic Measurement5.1 Input Impedance Measurement

The input characteristics of an antenna such as the input impedance ZA 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.


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S parameter measurement using a network analyser


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

less)(dimension 0

0

ZZZZ

A

A

+=

less)(dimension 11

VSWR

+=

5.2 Reflection Coefficient MeasurementThe reflection coefficient of an antenna can be obtained from its input impedance measurement.

5.4 S Parameter MeasurementThe S parameters of two antennas treated as a two-port network can be measured by a network analyser after proper calibration.


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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 York, 1998, Chapter 9.

Antennas Measurement

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