Reflector Antennas
Reflector antennas are used as high-gain, narrow-beam antennas in fixed radio links, satellite communication, radars, and radio astronomy.
A parabolic reflector antenna is the most common of reflector antennas. Figure 9.26(a) shows a parabolic antenna fed from the primary focus. The equation of the surface is
1 + cos where F is the focal length. The rays coming from the focal point are converted parallel by the reflector or vice versa. A more physical interpretation is that the fields radiated by the feed antenna induce surface currents, which in turn produce the aperture fields. The feed antenna is often a horn antenna. The phase center of the feed antenna should coincide with the focal point to obtain maximum gain.
The Cassegrainian antenna shown in Figure 9.26(b) is fed from the secondary focus behind the reflector. The subreflector is a hyperbolic reflector. This configuration has several advantages compared to the primary focus-fed antenna: Transmitters and receivers can be placed behind the reflector; transmission lines between the feed and the radio equipment are short; positioning of the feed antenna is less critical; the phase and amplitude distribution of the aperture field can be adjusted with a shaped subreflector; and the feed pattern over the edge ofthe subreflector is directed toward a cold sky in satellite reception, which leads to a lower antenna noise temperature. A more complicated structure and often a larger blockage of the aperture are the disadvantages of the Cassegrainian antenna.
Figure 9.26 Parabolic reflector antennas: (a) a primary focus fed antenna; and (b) a Cassegrainian antenna.
The theoretical directional patterns of circular apertures are only rough approximations of real patterns. Aperture field distribution depends on the radiation pattern of the feed antenna and on the ratio of the focal length and diameter of the reflector, F/D. A constant aperture field gives the highest gain but also the highest sidelobe level. A high edge illumination leads also to a high feed radiation over the edge of the reflector. Tapering the aperture illumination toward the edge leads to a lower aperture efficiency and gain but improves the pattern by lowering the sidelobes. Typically, the field at the edge is 10 dB to 12 dB lower than that at the center, and the aperture efficiency is about 0.6.
Many factors have an effect on the pattern of a reflector antenna: amplitude and phase pattern of the feed; positioning of the feed; aperture blockage due to the struts, feed, and subreflector; multiple reflections between the feed and reflector; and errors in the shape ofthe reflector and subreflector. Small random errors in the surface of the reflector reduce the gain and move energy from the main beam to the sidelobes. If the rms value of the surface errors is e, the aperture efficiency is
where tjq is the aperture efficiency for an ideal surface. The surface error e should not exceed A/16.
The struts and the feed or the subreflector block a part ofthe aperture, which reduces rjap and changes the level of sidelobes. They also scatter and diffract energy over a large solid angle. Also the edge ofthe reflector diffracts, which often produces a back lobe opposite to the main lobe.
The aperture blockage can be avoided by using offset geometry. Figure 9.27 shows offset-fed reflector antennas. The single-offset antenna has a simpler structure but the offset geometry inherently produces cross polarization. The dual-offset antenna has two distinct advantages compared to the single-offset antenna: The cross-polar field can be compensated and the aperture field distribution can be adjusted by shaping the reflectors. Dualoffset reflectors may have excellent sidelobe properties.
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