Approximate boundary conditions are a means for simulating material and surface effects in scattering and propagation. A number of conditions are discussed, and criteria are given for their validity.

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Approximate boundary conditions

A millimeter-wave dual frequency circularly polarized feed for an offset reflector antenna is being developed for a portable satellite ground terminal. The two frequency bands are Q - and K -bands for transmit and receive, respectively. A compact feed design consisting of a single corrugated horn with two circular waveguide concentric openings at the horn throat is described. Good aperture efficiency and low sidelobes in both of the frequency bands are achieved.

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A compact Q-K-band dual frequency feed horn

We investigate possible configurations for dual-band waveguide diplexing assemblies appropriate for use as feed systems for dual-reflector antennas. Specific applications are where the bands are well separated and, individually, have relatively wide bandwidths. Two crucial problems are addressed: the demanding bandwidth requirements of the feed horn and the subsequent means of diplexing the signals. We present results arising from the analysis of candidate systems and discuss a preferred option in detail.

Diplexing feed assemblies for application to dual-reflector antennas

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

This paper proposes overview of the multiband feed system for reflector-type antennas. Coaxial, multimode Potter's type structures, multiband corrugated and dielectric loaded feed horns is discussed below.

Recent progress in development of multiband feed horns (Review)

Based on a systematic analysis the design and experimental studies on a composite dual-frequency feed with a single radiating aperture are presented. The criterion for selecting the ratio of the diameter of the corrugated guide excited at the lower frequency in the dominant mode to that of the dielectric rod, axially located within the guide and excited at the higher frequency in the dominant mode, is discussed. The constructional features of a typical dual-frequency feed designed for operation at S and X bands (with 2.68 GHz and 9.375 GHz as center frequencies, respectively) are outlined. The measured radiation patterns show good symmetry and sidelobe suppression over 20-percent bandwidth in S and X bands and are in good agreement with calculated patterns.

Propagation and radiation characteristics of dielectric loaded corrugated dual-frequency circular waveguide horn feeds

The far-field radiation patterns of conical horns of arbitrary flare angles excited in the TE_{11} mode are obtained employing the geometric theory of diffraction (GTD) based on the theory of Kouyoumjian and Pathak [3] and the slope diffraction technique [4]. The analysis presented enables one to predict accurately radiation patterns over the main beam, near and far sidelobes, and the becklobe of the horn. Validity of the analysis is established by satisfactory agreement between the calculated and measured patterns of an experimental conical horn. The radiation patterns of wide-flare corrugated conical horns excited in the HE_{11} mode of operation have also been calculated over the main beam, which contains most of the radiated energy (up to -40 dB with respect to boresight field), employing slope diffraction technique, and a good agreement is noticed between the calculated and measured radiation patterns.

GTD analysis of the radiation patterns of conical horns

The E -plane radiation patterns of wide-flare corrugated conical and E -plane sectoral horns ( 2\alpha_{0} ) excited in the dominant mode are obtained employing the theory of Kouyoumjian and Pathak [12] and the slope diffraction technique [11]. Besides the other parameters of the horn, the finite thickness of the corrugated edge has been taken into account with a view to improve the accuracy of the pattern computation. The analysis presented enables one to predict accurately radiation patterns over the main beam, near and far sidelobes, and the back lobes of the horns. Validity of the analysis is established by satisfactory agreement between the calculated and measured E -plane patterns of a few typical experimental wide-flare corrugated conical and E -plane sectoral horns.

M. Sheshadri

Papers

Propagation and radiation characteristics of dielectric loaded corrugated dual-frequency circular waveguide horn feeds

GTD analysis of the radiation patterns of conical horns

GTD analysis of the radiation patterns of wide-flare corrugated conical and E-plane sectoral horns