tracking can be achieved without presenting to the satellites an antenna response much greater than isotropic. There will be a small additional interference-reducing effect resulting from the resolution of the 0.5- x 1.O-arc- minute collector arrays by the spaced antennas. This resolution effect, incidentally, enables the array to operate in the daytime in the presence of the quiet sun, which is, of course, much more highly resolved than a satellite. Since the line of satellites is essentially constant in declination, at -5.5° as viewed from the VLA site, there will be a zone of declinations centered on -5.5° in which the array cannot operate without receiving interference above a tolerable level. In this zone interference could be avoided only by reducing the range of hour angle tracking by something more than the 60° longitude range of the satellites, and satisfactory mapping of celestial objects would not be possible. At the present time the low level response of the antennas is not well enough known to define the zone with accuracy. The antennas operate in the Cassegrain mode with the feeds placed on a circle of 0.97-m radius about the vertex (Weinreb et al. 1977, Gustincic and Napier 1977). As a source of radiation moves away from the beam center, the antenna response falls sharply, but then becomes dependent on spillover of the feed response at the edge of the subreflector. The subreflector subtends an angle of 18° at the feed, and the gain of the feeds falls from 26 dB near the center of the subreflector to 13 dB at the edge. Thus, at 9° off axis the antenna gain is close to 13 dB, ' and it fallsto the isotropic level at the angle at which the feed response becomes isotropic. This angle is given for each feed in Table 2. The very large angle for 18 to 21 cm results from the horn being placed 1.5 m behind the correcting lens, which provides a compact and economical design, but allows some spillover at the edge of the lens. However, it should be possible, by using much more expensive feeds, to bring the side lobe performance at 18 to 21 cm much closer to that of the other feeds. Because the tolerable interference level falls as the declination of observation approaches that of the satellites, observations with satellites in the 13 dB side lobe at the subreflector edge would be very close to the (a) Because of the offset feed arrangement, the spillover side lobe is not precisely centered on the main beam and the 9° figure is the mean distance from the beam axis.
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