Antenna Measurement 1 Antenna Ranges antenna range

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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)

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Block diagram of a typical antenna-measurement system

Realized by a network analyser

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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 I1 at its terminal. Its radiation pattern is measured by a standard horn antenna #2, which

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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 Voc2. 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 I2 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

Voc1. Then by the reciprocity theorem,

2 1 1

oc oc

V V I

V V

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Thus the transmitting radiation pattern (E2) is equal to the received field pattern (E1) 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 E1 and

E2 through the effective lengths, Le1 and Le2, of the antennas as: 1 1 1, 2 2 2 oc e oc e VE L VE 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

 

,   0 E as a function of in the

xz plane

3. E

 90 ,

E as a function of in the

xy plane

4. E

 

,   0 E as a function of in the

xz plane

2.2 Principal Plane Patterns

1. E

 90 ,

E as a function of in the

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:

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Principal plane radiation patterns

Antenna under test

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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.

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GT = gain of the test antenna

GS = gain of the standard gain antenna

PT = power received by the test antenna

PS = 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 log10 10 log10 1 22 1 T T T S S S P G G P          

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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

GTV. Then rotate the antenna about its axis through 90º and measure its gain in the horizontal orientation GTH. The total gain of the antenna GT is given by:

 

GT dB  10 log10

GTVGTH

(dBic)

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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 antenna’s VSWRT = 1.3. Find the gain of the test antenna assuming both antennas are linearly polarized (LP).

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 

 

10 10 22 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

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4 Polarization Measurement

4.1 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 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

indicate the AR and title

.

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Polarization pattern method (1)

Receiving dipole antenna (receiving)

Test antenna (transmitting)

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Polarization pattern method (2)  O A B AR = OA/OB     2 2 2 2 cos sin rA    B   r

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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 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

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

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5 Input Characteristic Measurement

5.1 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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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

   1 VSWR (dimensionless) 1

  

5.2 Reflection Coefficient Measurement

The reflection coefficient

(or S11) of an antenna can be obtained from its input impedance measurement.

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5.4 S Parameter Measurement

The S parameters (S11, S12, S21, S22) of two antennas treated as a two-port network can be measured by a network analyser after a 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

Figure

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References