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Shoji TANAKA,
Digital Satellite Broadcasting Systems
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As a single transmission medium for future ultra high-definition TV broadcasting or multi-channel HDTV broadcasting, research is underway on a 21-GHz band broadcasting system. During heavy rainfall, satellite broadcasting services using frequencies in the 21-GHz band suffer from approximately three times the amount of dB attenuation for received signals than those employing the 12-GHz band currently used for satellite broadcasting, which results in the problem of longer broadcasting interruptions. To deal with such problems, studies have been done on constructing an on-board phase array antenna that can efficiently compensate for rain attenuation by selectively increasing transmitting power intensity in the areas of rainfall.
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| Figure 1: Rain attenuation compensation technology image by phased array antenna technology |
The inserted Figure 1 is a concept image of rain attenuation compensation technology, which concentrates its transmitting power on the areas of rainfall by adjusting its radiation pattern through control of the amplitude and phase of the signals being fed to the phased array antenna. This system first radiates beams over the country at an approximately uniform transmitting power (hereafter called the nationwide beam); it then concentrates its transmitting power on areas receiving precipitation in accordance with the precipitation intensity (hereafter called the intensified beam). This intensified beam can travel with the movement of precipitation, but if this intensified transmitting power is not needed, broadcasting services can be provided exclusively with the nationwide beam radiation.
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| Figure 2: Phased array antenna |
The on-board phased array antenna system, as Figure 2 shows, consists of a direct radiating array antenna that is composed of a number of aligned radiating elements, and an array-fed reflector antenna that combines multiple radiating elements (a primary feed-horn array) and a reflector. Since this array-fed reflector antenna requires a fewer number of array elements and a smaller feeder circuit in comparison with that of a direct radiating array antenna, current examinations are focusing on this type of phased array antenna system.
Figure 3 shows an example of a radiation pattern computation when transmission power is increased, using a reflector with an aperture 10 m in diameter and covering an area 100 km in diameter, centered on Osaka. It forms a nationwide beam in the shape of the national land area of Japan, and provides an intensified beam that is approximately 10 dB greater at maximum than the nationwide beam. Currently, a prototype antenna is being used to verify the effectiveness of this design approach (Figure 4).
Progress is expected to continue on producing further refinements in detailed radiation pattern engineering with variations in the intensified beam forming location, the number of beams, and their intensity.
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| Figure 3: Radiation pattern computation example |
Figure
4: Prototype antenna |
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2003/03
