A transversely corrugated surface as used in corrugated horn antennas represents a soft boundary. A hard boundary is made by using longitudinal corrugations filled with dielectric material. The concept of soft and hard surfaces is treated in detail, considering different geometries. It is shown that both the hard and soft boundaries have the advantage of a polarization-independent reflection coefficient for geometrical optics ray fields, so that a circularly polarized wave is circularly polarized in the same sense after reflection. The hard boundary can be used to obtain strong radiation fields along a surface for any polarization, whereas the soft boundary makes the fields radiated along the surface zero. >

Artificially soft and hard surfaces in electromagnetics

Horn antennas with soft and hard boundaries are analyzed. A soft boundary which exists in classical hybrid-mode horns gives zero field intensity at the wall. A hard boundary corresponds to a uniform field distribution over the horn aperture. Soft and hard horn antennas are compared with respect to directivity, sidelobes, and beamwidth. The dependency between the edge taper directivity, and sidelobes is also calculated based on the solution to the spherical hybrid modes in a conical horn with arbitrary wall impedances. This makes it possible to study how to compromise between directivity and sidelobes. Also discussed is how the different wall impedances may be realized, and some preliminary experimental work on hard horns is presented. >

Soft and hard horn antennas

A mechanically simple dielectric loaded hybrid mode horn is presented. It can be designed to satisfy either hard or soft boundary conditions, or any boundary between hard and soft under balanced hybrid condition (low cross-polarization) for any aperture size. Design curves are developed based on a plane wave model. Radiation performance is computed based on a single mode circular cylindrical wave model (CCWM) as well as by method of moments (MoM). Based on CCWM aperture efficiency of about 94% has been computed at the design frequency for a 3.38/spl lambda/ aperture with hard boundary condition and a dielectric constant of 4.0. The same horn with a dielectric constant of 2.5 can provide higher than 89% aperture efficiency and under -30 dB cross-polarization over a 15% frequency range. Predicted peak sidelobes ranging from -19 to -26.5 dB and corresponding aperture efficiency between 92% and 78% at the design frequency under balanced hybrid condition have been obtained for various horn designs. Computations based on MoM show that cross-polarization patterns agree well with what was predicted based on an ideal single hybrid-mode model, while computed aperture efficiency is even higher than predicted by CCWM. Results also indicate that implementing dielectric-to-air matching at the horn aperture is crucial to achieve good cross-polarization performance and input match for hard horns in general. The horn can also be designed to radiate a flat-top pattern. The horn could be useful as an element in cluster feeds or limited-scan arrays, as a feed for quasioptical amplifier arrays or in millimeter wave applications.

A new class of dielectric-loaded hybrid-mode horn antennas with selective gain: design and analysis by single mode model and method of moments

Hard horns or waveguide feeds with circular symmetry have been studied with respect to gain (aperture efficiency) and crosspolarisation. The corrugated or strip-loaded representations of these feeds give almost identical radiation properties when surface waves are neglected. The contribution from the wall region increases the gain and crosspolar bandwidth of the feed. The gain increases and the crosspolar bandwidth decreases with increasing waveguide diameter and wall permittivity. The hard horns may be useful in cluster feeds and limited scan arrays due to reduced size and weight.

Hard waveguide feeds with circular symmetry for aperture efficiency enhancement

The electromagnetic fields propagating up a cone having an arbitrary wall impedance are found using an asymptotic solution. Three special cases are then considered: the smooth-metal wall, the corrugated wall, and the metal wall with a lossy-dielectric lining. The last case, in the form of an absorber-lining is then shown to behave like a corrugated horn since it too provides a highly tapered E -plane and H -plane aperture distribution. Furthermore, it does this over a much larger bandwidth, over 3:1, with negligible gain drop.

On the fields in a conical horn having an arbitrary wall impedance

Theoretical radiation patterns are predicted and compared with those obtained experimentally. The measured peak cross-polarization level is less than -29 dB at 9 GHz. It is shown that aperture efficiencies of the order of 91-96 percent may be obtained.

A. Kumar

Papers

Dielectric-lined waveguide feed