We performed a fundamental study on a multi-beam phased-array antenna for satellite broadcasting which can simultaneously receive signals from a number of satellites in different orbital positions. This paper describes the basic design of a multi-beam antenna that can receive signals from satellites in range of ±20 degrees. We also performed a reception experiments with an experimental model. The model was able to form two beams separated by 40 degrees.
- 1. INTRODUCTION
- Satellite broadcasting services are currently provided in Japan using four orbital positions, one operates in the BSS band (11.7 - 12.2 GHz) and the other operate in the FSS band (12.2 -12.75 GHz). A number of satellites located in different orbital positions will provide in the future. A commonly used dish antenna can receive signals from only one satellite at a time. A multihorn-feed reflector antenna can receive signals simultaneously from several satellites in different orbital positions, but the number of satellites may be limited by the arrangement of the feed horns. A multi-beam phased-array antenna can receive signals simultaneously from a great number of satellites in different orbital positions.
This paper describes the fundamental design of a multi-beam receiving antenna and measured results of an experimental model.
- 2. ANTENNA DESIGN
- The multi-beam receiving antenna receive signals simultaneously from many satellites in different orbital positions (Fig.1). Figure 2 shows a block diagram of the antenna. Multiple beams are formed using multiple beam forming circuits, one for each satellite direction. To receive both linearly polarized waves with an arbitrary angle and circularly polarized waves, the waves are received as two orthogonal components.
Figure 1 Multi-beam receiving antenna
Antenna elements receive two orthogonal components
Figure 2 Block diagram of multi-beam receiving antenna
We designed this antenna to meet the following requirements.
- Receive signals from satellites in range of ±20 degrees (corresponding 110°E - 150°E)
- Suppress interference from adjacent satellites
- Provide a gain of 32 dBi, which corresponds to the gain of a 45 cm dish antenna
- 2.1 Range of beam position
- Figure 3 shows the orbital positions of the four satellites providing satellite broadcasting in Japan. These four satellites are positioned within 40 degrees in azimuth and 0.4 degrees in elevation. We set the range of the beam position to ± 20 degrees only in the direction of the azimuth axis, because the difference in elevation angles is negligible compared with the beam-width of the antenna. To form a beam along the azimuth axis, we used the antenna configuration shown in Fig. 4.
Figure 3 Orbital positions of satellites providing satellite broadcasting
Figure 4 Subarray configuration
- 2.2 Antenna size
- The radiation pattern along the azimuth axis, which coincides with the orbital arc, depends on the width of the antenna. To suppress interference from adjacent satellites, the antenna width should be designed taking into consideration the side-lobe level in their directions. To make nulls in the azimuth pattern that coincide with the directions of the adjacent satellites (± 4.4 and ±6.6 degrees), the antenna width needs to be 65 cm (Fig. 5). However, to reduce the number of subarrays, we set the width to 37 cm. In this case, the first side-lobes occur in the directions between ± 4.4 and ±6.6 degrees; they have the same levels in the directions of the adjacent satellites. To achieve a gain of 32 dBi with a width of 37 cm, the antenna needs to be 70 cm long (assuming 50% aperture efficiency).
- 2.2 Antenna size
- The radiation pattern along the azimuth axis, which coincides with the orbital arc, depends on the width of the antenna. To suppress interference from adjacent satellites, the antenna width should be designed taking into consideration the side-lobe level in their directions. To make nulls in the azimuth pattern that coincide with the directions of the adjacent satellites (4.4 and 6.6 degrees), the antenna width needs to be 65 cm (Fig. 5). However, to reduce the number of subarrays, we set the width to 37 cm. In this case, the first side-lobes occur in the directions between 4.4 and 6.6 degrees; they have the same levels in the directions of the adjacent satellites. To achieve a gain of 32 dBi with a width of 37 cm, the antenna needs to be 70 cm long (assuming 50% aperture efficiency).
- 2.3 Number of subarrays
- Figure 6 shows the calculated results for both the main-lobe level and the grating-lobe level when the beam is tilted to 20 degrees. To avoid the occurrence of a grating-lobe, the number of subarrays must be at least 22 (Fig. 6, A). If we allow the occurrence of the grating-lobe in order to reduce the number of subarrays, the subarrays can be reduced to 15 while still keeping the condition that the difference between the main-lobe and grating-lobe level is large (Fig. 6, B).
Figure 5 Calculated azimuth pattern
Figure 6 Main-lobe level and grating-lobe level
- 2.4 Grating-lobe
- When the beam is tilted to 20 degrees, the grating-lobe occurs in the direction of -39 degrees, and its relative level is -7 dB. The distance between the azimuth axis and the orbital arc is 1.2 degrees in the direction of the grating-lobe (Fig. 3). Therefore, the relative level of the grating-lobe at the orbital arc is reduced to -12 dB.
- 3. MEASURED RESULTS ON EXPERIMENTAL MODEL
- 3.1 Experimental model
- Figure 7 shows the configuration of a two-beam experimental model. Delay lines are used as phase shifters to simplify the beam forming circuits. Figure 8 shows an external view of the experimental model. The antenna consists of 960 microstrip patch elements (0.76 mm thick PTFE substrate, r= 2.17). The size of the antenna aperture is 372 672 mm.
- 3.2 Radiation pattern
- Figure 9 shows the measured radiation pattern for a beam formed at 0 degrees in the azimuth axis. The maximum side-lobe level is -17.5 dB in the direction of adjacent satellites (-4.4 degrees). Figure 10 shows the measured radiation pattern when the beam is tilted to 20 degrees. The grating-lobe occurs in the direction of -39 degrees, and its relative level is -7.3 dB.
Figure 7 Configuration of experimental model
Figure 8 External view of experimental model
Figure 9 Azimuth pattern ( Beam position : 0 deg. )
- 3.3 Reception experiments
- We performed a reception experiments in an anechoic chamber. The two beams were tilted to ± 20 degrees. Figure 11 shows their measured radiation patterns. In the direction of each main-lobe, isolation levels of over 20 dB were obtained. We transmitted two different TV programs on the same frequency from two points 40 degrees apart. The experimental model was able to receive both programs simultaneously.
Figure 10 Azimuth pattern ( Beam position : 20 deg. )
Figure 11 Two-beam pattern ( Beam position : +20 deg., -20 deg. )
- 4. CONCLUSION
- We performed a fundamental study on a multi-beam receiving antenna for a satellite broadcasting. We presented that 15 subarrays are suitable for the antenna considering interference from adjacent satellites while trying to minimize the number of subarrays. The experimental model was able to form two beams separated by 40 degrees.
Even though multi-beam receiving antennas have superior performance capabilities, the large number of discrete components, such as phase shifters and down-converters, is a major obstacle to their realization. To reduce the component count and cost, we have been studying an active integrated subarray antenna and a simplified feed system. In the future, we expect that beam forming circuits will be integrated into satellite TV tuners to handle both satellite and frequency selection.