Estimation for degradation of radiation pattern due to excitation coefficient error for onboard array-fed reflector antenna

The next generation satellite broadcasting system is planning to use a 21-GHz-band. One of the difficulties which we have to overcome in using the 21-GHz band for satellite broadcasting is rain attenuation. Therefore, we have proposed to increase the satellite equivalent isotropic radiated power (e.i.r.p.) for limited areas, where there is heavy rainfall, by using an onboard phased array antenna. In this paper, we estimate the degradation of radiation pattern caused by the excitation coefficient error of a phased-array-fed single reflector antenna for 21-GHz satellite broadcasting. Introduction The 21.4 to 22.0-GHz band, that is allocated for broadcasting satellite services in Europe and Asia (ITU Regions 1 and 3), will be available in April 2007 [1]. 21-GHz satellite broadcasting is expected to develop as a large-capacity transmission medium for advanced broadcasts featuring multi-channel and ultra-high-definition TV using large display screens. However, one of the difficulties we have to overcome in using the 21-GHz band for satellite broadcasting is rain attenuation, which is much larger than that in the 12-GHz band [2]. The traditional design of the transmission scheme and the onboard antenna design for the satellite system may not be efficient for digital broadcasting services in the 21-GHz band, since a large RF power is needed to compensate for such attenuation. We have proposed two promising approaches for solving this problem [1]: one is to disperse the continuous data loss into a long data sequence by using long-block-length interleaving [3]; the other is to increase the satellite equivalent isotropic radiated power (e.i.r.p.) for limited areas with heavy rainfall by using an onboard phased array antenna [4]. The feasibility of a variable radiation pattern in which the gain is increased by 10 dB for a limited area of 100 km was confirmed by a numerical estimation in reference [4]. For practical application, however, it is necessary to estimate for the gain degradation of the radiation pattern caused by the error of the excitation coefficient in the phased array antenna. In case of two dimension antenna array, the main beam loss caused by random error of the excitation coefficient was investigated[5]. But in case of a phased array-fed reflector, the degradation of radiation pattern has not investigated. In this paper we estimate the degradation of radiation pattern caused by the excitation coefficient error of the phased-array-fed reflector antenna. Concept of Rain Attenuation Compensation by a Onboard Phased Array Antenna Figure 1 illustrates the concept of the compensation of rain attenuation by a phased-array-fed reflector antenna. Heavy rainfall, which may interrupt broadcasting services, almost occurs within limited areas. Therefore, it is available to concentrate the radiation power on limited areas in order to compensate for rain attenuation. Fig.1 Concept of Compensation for Rain Attenuation by Phased Array Antenna 東 京 大 阪 boosted beams Tokyo Osaka nationwide beam E. I.R .P E. I.R .P 21st International Communications Satellite Systems Conference and Exhibit AIAA 2003-2225 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. American Institute of Aeronautics and Astronautics 2 In Fig.1, the radiation power from antenna is controlled to be a nominal level of e.i.r.p. across the entire service area as a “nationwide beam” while being concentrating “boosted beams” of a greater e.i.r.p. only to areas with heavy rainfall, to compensate the rain attenuation. Figure 2 shows an example of the radiation pattern for forming the boosted beam on the nationwide beam. The gain variation in the nationwide beam caused by forming the boosted beam occurs as shown in Fig. 2. Because the minimum gain of the nationwide beam is one of the important parameters to determine the radiation power of the broadcasting satellite system, the minimum gain should be designed so that the difference between the average gain and the minimum gain is as small as possible. Performance of a Phased-array-fed Single Reflector Antenna Antenna Parameters and Design Conditions Figure 3 shows the basic configuration of a phased-array-fed single reflector antenna. Table 1 shows the assumed design conditions. The shape of the nationwide beam is assumed to be ellipsoidal. The diameter of the boosted beam is assumed to be 100 km, which corresponds to an antenna aperture diameter of approximately 10 m. The primary feed array consists of 217 conical horns arranged in a triangular pattern with a distance of 1.5 wavelengths. The complex excitation coefficients of the primary feed array are calculated by the least squares method [4]. Table 1. Assumed design conditions Orbital position 110 °E Frequency 21.7 GHz Polarization Linear Diameter of the rainfall area 100 km Number of boosted beams 1 Intensified gain for the boosted beam 10 dB Radiation Pattern with a 10-dB Boosted Beam Figure 4 shows the contour of the radiation pattern with a 10-dB boosted beam that is formed at the center of the nationwide beam. Figure 5 shows a cross-sectional view on gain along Cut A in Fig. 4. We set 685 observation points to evaluate the radiation pattern inside the dashed ellipsoidal line. A gain of more than 40 dBi for the nationwide beam and a gain of 51.1 dBi for the boosted beam were achieved without an excitation coefficient error. Gain of boosted beam Minimum gain of nationwide beam Side lobe Nationwide Beam Boosted Beam Fig.2 Illustration of Radiation Pattern 36 ° 53.13 ° 10 m 6 m Focal point Off-focal-point distance Fig.3 Antenna configuration