ELEC6103 Satellite Communications

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Page 1 of 12 ELEC6103 Satellite Communications Laboratory: ‐‐ Radiation patterns measurements of different antennas in satellite communications systems 1. Objectives On completion of the laboratory, the students will be familiar with: the system configuration of the Satimo antenna measurement equipment, SG24 or StarLab; measurements of the antenna radiation patterns; and radiation patterns of different types of monopole antennas. 2. Equipment Required Satimo antenna measurement equipment (SG24) a reflector antenna (labeled Ant 1) a horn antenna (labeled Ant 2) a monopole antenna (labeled Ant 3) 3. Introduction 3.1 Radiation Pattern An antenna radiation pattern is defined as “a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates.” [1]. Usually, the radiation pattern is determined in the far field region and represented as a function of the directional coordinates A convenient set of coordinates is shown in Figure 1. Fig.1 Coordinate system for antenna analysis.

Transcript of ELEC6103 Satellite Communications

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ELEC6103 Satellite Communications Laboratory: ‐‐ Radiation patterns measurements of different antennas in satellite communications systems 

1. Objectives

On completion of the laboratory, the students will be familiar with: the system configuration of the Satimo antenna measurement equipment,

SG24 or StarLab; measurements of the antenna radiation patterns; and radiation patterns of different types of monopole antennas.

2. Equipment Required

Satimo antenna measurement equipment (SG24) a reflector antenna (labeled Ant 1) a horn antenna (labeled Ant 2) a monopole antenna (labeled Ant 3)

3. Introduction

3.1 Radiation Pattern

An antenna radiation pattern is defined as “a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates.” [1]. Usually, the radiation pattern is determined in the far field region and represented as a function of the directional coordinates A convenient set of coordinates is shown in Figure 1.

Fig.1 Coordinate system for antenna analysis.

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An antenna radiation pattern may consist of several lobes. The major power is concentrated in the main lobe. The 3-dB beamwidth is the angle between two points which is 3 dB lower than the main lobe, and it shows half of the maximum power intensity.

Different antennas have different antenna radiation patterns. The distance between the transmitting antenna and receiving antenna affects the received power of the antennas, but the antenna radiation pattern will be remained the same. The E-plane radiation pattern is defined as “the plane containing the electric field vector and the direction of maximum radiation”, so for dipole and monopole antennas, an H-plane antenna radiation pattern is plotted when both the transmitting and receiving antennas are placed vertically. The H-plane radiation pattern is defined as “the plane containing the magnetic field vector and the direction of maximum radiation”, so an E-plane antenna radiation pattern is plotted when both antennas are placed horizontally. Moreover, the polarization of the transmitting and receiving antennas affect the plotting of the radiation pattern.

The main source of errors in antenna measurements is reflections from nearby obstacles including the ground. To reduce the reflections and hence the errors, the measurement is done inside an echoic chamber.

3.2 SG24

The SG24 system, as shown in Fig. 1, is a compact measurement system for both of the passive antenna and also the active performance testing of wireless devices. It has 23 dual polarized (vertical and horizontal) probes covering the testing frequency from 400 MHz to 6 GHz and can be used to measure the complete radiation information with 180 degree mechanical rotation [2].

Fig. 1 SG24.

The SG24 uses a switching Unit to commute between passive and active antenna measurements. For passive antenna measurements, it uses a Vector Network Analyzer

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(VNA) as the RF source/receiver. The control Unit drives the two positioning motors and the electronic scanning of the probe array. For active antenna measurements, the test is performed through a Multi-Protocol Radio-Communication Tester, R&S CMU200. The Amplification Units are added on both transmit (Tx) and receive (Rx) chains. The macro system architecture of SG24 is shown in Fig. 2.

Fig. 2 Macro system architecture of SG24.

The SG24 can perform the following measurements: Gain, directivity, radiation pattern in any polarization, linear or circular, 3D radiation pattern, beam width, cross polar discrimination, side lobe levels, back to front ratio, as well as antenna efficiency. A combination of automated mechanical and electronically scanned probe array is available to provide unlimited scan resolution in both elevation and azimuth. The spacing between two probes is 15º. This spacing is suitable for small antenna testing. For larger antennas, an additional mechanical rotation is combined to the probe array. In this case, the positioning mast rotates with ±7.5° in elevation, in order to adjust the position of the equipment under test (EUT) on offset locations. This “fills in the gaps” and provides the possibility of unlimited sampling as illustrated in Fig. 3. The coordinate system definition of SG24 is shown in Fig. 4.

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Fig. 3 Principal of oversampling function.

Fig. 4 SG24 coordinate system.

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4. Measurements of Antenna Radiation Patterns

4.1 Measuring Ant 1 (reflector antenna)

1. Set up the equipment as shown in Fig. 2. Check the status of the VNA and the active switching unit.

2. Mount Ant 1 (with its aperture facing the z-direction) on the master cable of the system with proper interface. Adjust the height of the antenna to make sure that the centre of the antenna is in alignment with the laser cross point which is the centre of SG24.

3. Launch the measurement software, Satimo Passive Measurement (SPM).

4. Set the start frequency and the stop frequency for the antenna according to the operating frequency of the antenna.

5. Set the frequency step to be 10 MHz

6. Keep the default oversample settings (the deg/step is 15º).

7. Click “Start A Measurement” to measure the antenna radiation pattern.

8. Launch the postprocessing software Satimo Satenv.

9. In Satenv, cerate a new project and save it.

10. In SPM, export the measurement raw data from SPM to Satenv by click the commond “to Satenv” in SPM.

11. In Satenv, post-process the raw data from near-field (NF) to far-field (FF).

12. In Satenv, execute the macro “3 cuts” and get the 2D radiation patterns at 3 most important cuts (one horizontal cut and two vertical cuts) in the x-y, x-z and y-z planes at the center frequency of the operating frequency band.

13. Record the measured signal strength in Appendix A1.

14. Record the peak gain, the 3-dB beamwidths in the three principle planes and the -10 dB operating bandwidth of the antenna.

15. Export the 3D plot and record in Appendix A.

4.2 Measuring Ant 2 (horn antenna)

Repeat the procedures in Section 4.1 with Ant 2, except that

i) in steps 13, export the three 2D radiation patterns and show them in Appendix B, instead of recording the signal strength; and

ii) from these plots, record the peak gain, and the 3-dB beamwidths in the three principle planes.

4.3 Measuring Ant 3 (monopole antenna)

Repeat the procedures in Section 4.1 with Ant 3, except that:

i) in step 2, mount the antenna in a horizontal position on the master cable of the system with proper interface; and

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ii) in steps 13 and 14, export the three polar plots and show them in Appendix C.

4.4 Measuring Ant 3 (monopole antenna)

Repeat the procedures in Section 4.1 with Ant 3, except that:

iii) in step 2, mount the antenna in a vertical position on the master cable of the system with proper interface; and

iv) in steps 13 and 14, export the three polar plots and show them in Appendix D.

5. References

[1] C.A. Balanis, “Antenna Theory: Analysis and Design,” 3rd Edition, Wiley, 2005.

[2] “Data sheet of SG24”, available on: http://www.satimo.com/sites/www.satimo.com/files/Product%20sheet_SG24_2010.pdf .

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Appendix A

A1 Polar Chart for Ant 1

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Eh and Ev: Electric fields in horizontal and vertical cuts, respectively

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A2: 3D plot for Ant 1

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Appendix B

B1 Polar Chart for Ant 2

Eh () at =900 (x-y plane) Ev () at =00 (y-z plane) Ev () at =900 (x-z plane)

B2 3D plot for Ant 2

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Appendix C

C1 Polar Chart for Ant 3

Eh () at =900 (x-y plane) Ev () at =00 (y-z plane) Ev () at =900 (x-z plane)

C2 3D plot for Ant 3

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Appendix D

D1 Polar Chart for Ant 4

Eh () at =900 (x-y plane) Ev () at =00 (y-z plane) Ev () at =900 (x-z plane)

D2 3D plot for Ant 4

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6. Discussions

1. What are the -10-dB operating bandwidths of the antennas?

2. What are the measured beamwidths and gains of Ant 1 and Ant 2?

3. Comment the uses of these 2 types of antennas based on the results of 2) & 3)

4. Notice the sidelobes of Ant 1 and explain why it should not be used for transmission.

5. Discuss why no tracking mechanism is needed for Ant 1 (the reflector antenna) to receive signals from a geostationary satellite.

6. Use equations given in lecture notes to calculate the beamwidth and gain of Ant 1, compare it with the measured result and comment on it.

7. Also calculate the beamwidth and gain of a 32-m reflector antenna. If the antenna is used at an Earth Station to receive signals from a geostationary satellite, explain whether tracking mechanism is needed to be installed in E/S?

7. If the geostationary satellite is maintained within a box ±0.05° by the TT&C system, what is the largest size of reflector antenna to use without requiring to use the tracking mechanism at the E/S? What is the gain of the antenna?

8. When a satellite is launched but not in the correct orientation, the dish reflectors may not in the correct orientation to communicate with ground station. Based on the measured results, explain how monopole antennas can be used in such situation.