Horn Antennas as Reflector Feeds
Horn antennas have served in numerous applications for much of the last century, but most notably for their role as feeds for reflector antennas. The simplest horn antennas (such as that shown in Fig. 2.1) have radiation patterns much like those of open-ended waveguides, with relatively high sidelobes in the Я-plane and relatively low sidelobes in the Я-plane. This behavior results from the aperture field distribution, which corresponds to the fundamental waveguide mode. For rectangular horns, the fundamental mode is typically TE10, which has a uniform field in the Я-plane and a smoothly tapered field in the Я-plane. One of the important considerations for the design of a reflector antenna system is spillover loss, which can be mitigated by reducing the sidelobes in the feed’s radiation pattern.
Figure 2.1 A simple pyramidal (rectangular) horn antenna, highlighting the principal planes and the typical corresponding aperture field distributions.
Reflector antennas are commonly used in satellite communication links, where the weight of the system is a critically important constraint. As a result, many satellite antennas reuse frequencies (and thus antennas) through dual-polarized communication links.
Antennas for dual-polarized systems must exhibit minimal cross- polarized radiation, as cross-pol from one polarization directly interferes with the alternative polarization.
Moreover, circular symmetry is often desirable for dual-polarized systems in order that the two communication channels will have similar characteristics. Simple circular (conical) feed horns exhibit high cross-pol as a result of the field line configurations of their associated mode distributions. To reduce cross-pol, hybrid-mode horns have been commonly employed for several decades, since the introduction of the corrugated horn , followed by the thorough description of hybrid-mode horns by Minnett and Thomas . Ideal hybrid modes, which are linearly polarized modes characterized by perfectly straight field lines, can be supported by a waveguide or horn having an anisotropic boundary on its walls, such that it meets the balanced hybrid condition:
where ZTE and ZTM are the TE and TM boundary (surface) impedances, respectively, and n0 is the free-space wave impedance. In the lossless case, the surface impedances become surface reactances, and the balanced hybrid condition may be expressed as: