Antenna Measurement 1 Antenna Ranges antenna range

Transcription

1 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) 1

2 Realized by a network analyser Block diagram of a typical antenna-measurement system 2

3 A compact antenna range 3

4 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 4

5 5 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, Voc2 Voc1 I1 Voc2 Voc1 I I I 1 2 2

6 6 The open-circuit voltages are related to the far fields E 1 and E 2 through the effective lengths, L e1 and L e2, of the antennas as: V E L, V E L Hence, oc1 1 e1 oc2 2 e2 I I L EL EL E E ce 1 1 e1 2 e2 1 e1, I2 I2 Le 2 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.

7 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 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: 1. E 90, E as a function of in the xy plane 7

8 Antenna position and the coordinate system for pattern measurement 8

9 Antenna under test Antenna under test Principal plane radiation patterns 9

10 3 Gain Measurement 3.1 Comparison Method 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. 10

11 We have the following relation from which the gain of the test antenna can be determined. G T P P T S 1 1 s T 2 2 G S 2 PT 1 T db S db 10log10 10log10 2 P S 1S G G T 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 T = reflection coefficient of the test antenna S = reflection coefficient of the standard gain antenna 11

12 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 T is given by: G G G db 10log (dbic) T 10 TV TH where dbic is a unit to indicate that the gain is relative to an isotropic and perfectly CP antenna.

13 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 T = 31.6 mw (15 dbm). The standard antenna has a VSWR 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). 13

14 Solution VSWR S S 0.05 VSWR VSWRT 1 T 0.13 VSWR 1 S T 2 PT 1 T db S db 10log10 10log10 2 P S 1S G G T db 10log 10 10log db 14

15 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. 15

16 Test antenna (transmitting) Receiving dipole antenna (receiving) Polarization pattern method (1) 16

17 AR = OA/OB r A B sin 2 cos B O r A Polarization pattern method (2) 17

18 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. 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.

19 3 db AR beamwidth Radiation pattern obtained with a rotating linear source AR at Rotating Source Method 19 Test antenna (receiving) Fast-rotating dipole antenna (transmitting)

20 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. 20

21 S parameter measurement using a network analyser 21

22 5.2 Reflection Coefficient Measurement S Z Z A 0 or 11 (dimensionless) ZA Z0 5.3 VSWR Measurement The VSWR of an antenna can be obtained from its reflection coefficient measurement. 1 VSWR (dimensionless) 1 The reflection coefficient (or S 11 ) of an antenna can be obtained from its input impedance measurement. 22

23 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. 23

24 References: [1] IEEE Standard Test Procedures for Antennas, IEEE Std , published by IEEE Inc., 1979, distributed by Wiley-Interscience. [2] G. E. Evans, Antenna Measurement Techniques, Artech House, Boston, MA, [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. 24