During the past few decades reflector antenna designs have evolved through several configurations to increase performance and/or reduce structural complexity. Electrical parameters that are of prime interest are aperture efficiency, sLL, and, more recently, XPOL. All topics herein discussed apply to the various types of reflectors previously addressed. However, the offset configuration is likely to retain, in the near future, the largest percentage of the reflector antenna market.
We first focus our attention on XPOL. Reflector antennas presenting low XPOL (e. g., XPOL < —35 dB) are necessary for frequency-reuse applications, in which an overlap of orthogonally polarized channels is permitted. Many efforts are being conducted to develop these kinds of antennas for mass production (7,35). Dual offset reflectors can be designed for low-cost construction, provided that specific manufacturing constraints are carefully taken into account (7), an effort only possible due to increased interest from industry. single offset reflector systems illuminated by a matched feed (23) or a feed with a lens (35) can also be designed to satisfy stringent requirements on XPOL, yielding very compact designs. In the latter case, the lens is designed to replace the subreflector, and in both cases bandwidth performance is not as straightforwardly obtained as with the dual reflector configuration. Research continues to be conducted within the area, yielding innovative solutions that provide satisfactory XPOL performance while attending to practical manufacturing specifications. Cost-effective solutions normally require that attendance to a particular specification, such as low XPOL, be achieved with minimal capital outlay, which implies using the maximum amount of infrastructure and technology already implemented. As mentioned previously, there is a tendency to
employ existing molds for the main reflector, a concept referred to as reflector upgrading (7).
Within this context, existing single offset reflector molds are normally used to construct the main reflector of a dual configuration. However, many such molds are for just fully offset geometries, which, in general, produce a dual reflector configuration that is Gregorian with the feed axis zf intersecting the main reflector (Fig. 11). The same problem may also occur in certain Gregorian configurations even when the main reflector is not just fully offset. The final design should provide suitable clearance between the bottom of the main reflector and the feed axis in order to access the feed antenna with a straight section of waveguide, thus reducing the complexity and cost of the manufacturing process. This setup is achieved by rotating the parent ellipsoid (i. e., the conical surface from which the subreflector is generated) until the desired clearance is obtained. The rotation is performed in such a way that the feed remains pointed toward the intersection of the new subreflector and the ray coming from the center of the main reflector, thus avoiding the introduction of spillover and phase errors. The amount of rotation /3R that yields a desired angle y’ between the main reflector and feed axes can be determined from (7)
1 + e
1 — e cos(180° — в — eR — frC)
1 + e
1 — e cos(180° — в — eR — ^C)
sin(180° — в — eR — ^c ) — sin(e + eR + Y!) (39)
Given the initial configuration and the desired angle y’, Eq. (39) can be solved to determine /3R. In general, values for y’ smaller than the one used in the original configuration bring the feed axis away from the main reflector. However, the nonconventional design obtained after the rotation of the parent ellipsoid may lead to a XPOL degradation due to the fact that the minimum-XPOL conditions, Eqs. (27) to (29), are no longer satisfied. A simple solution to this problem is to alter the value of the subreflector eccentricity while keeping all orientation angles constant. In general, eccentricity values greater than the one employed before the rotation will reduce the system XPOL (7), yielding a low-cross-polarization dual offset Gregorian antenna that has adequate clearance between the feed axis and the bottom of the main reflector. In addition, the resulting configuration has the ability to operate with either a linearly polarized or a circularly polarized feed over a wide bandwidth without the need to be repositioned (no substantial beam squint).
Compact designs for reflector systems have been investigated for years. It is desirable, for example, to upgrade a main reflector with a subreflector that is as small as possible. In addition, with the proliferation of satellite TV at Ku band employing single offset reflector systems for reception, there is now interest in minimizing the size of the reflector while maintaining required gain performance. This is only possible by increasing aperture efficiency through the reduction of diffraction effects and feed blockage. High-performance feeds are also necessary, especially if they are located close to the reflector, as in a very compact design requiring a complete nearfield analysis. Finally, microelectronics technology is integrating both low — and high-frequency hardware into the feed system. It is common nowadays to find feeds that already include low-noise amplifiers, downconverters, and other electrical devices in a single unit.