Feedhorns & Antennas

Corrugated feedhorns are essentially mode transformers that convert waveguide modes into desired free-space optical modes and are often an integral part of any quasi-optical system. They were developed over 50 years ago mainly through the pioneering work of Clarricoats and Olver, and their performance can be modelled extremely accurately using mode-matching techniques. They can be designed and manufactured with extremely low levels of cross-polarisation, low sidelobe levels and low VSWR over entire waveguide bands.

Surprisingly, there is still considerable scope for innovation in their design, and the Group has an active design programme for high performance feedhorns for a wide range of applications. Many of these have state-of-the-art performance in terms of beam quality and bandwidth and are used in virtually all our instruments and radar systems. The Group can design bespoke feedhorns which can be manufactured by our collaborator Thomas Keating Ltd. that has a long-established track record of supplying feedhorns for critical space and instrumentation applications. The list below illustrates various design requirements for common applications:

Feedhorns for Quasi-Optics: Here the design goal is usually to maximise the coupling to a fundamental LG00 Gaussian beam, characterised by low sidelobes. Our Group has manufactured and tested compact horns featuring -60 dB sidelobes and 99.99% Gaussian coupling efficiency at spot frequencies. We have also designed horns featuring -35 dB sidelobe levels with constant phase centres across full waveguide bands.

Feedhorns for Waveguide Measurement or Transmission Systems: Here the goal is to maximise coupling to the fundamental HE11 corrugated waveguide mode and we have designed horns with >99.9% coupling efficiency across full waveguide bands.

Feedhorns for Radiometry: Here the design goal is usually a narrow beam with respect to the -3 dB points and as little power as possible at larger angles to limit the effects of beam spillage. This can best be achieved by exciting and phasing an appropriate combination of LG00 and LG01 modes, which can be achieved over full waveguide bands.

Feedhorns for Radar/Comms and Telescopes: In this application the feed usually illuminates a larger reflector or lens antenna and the goal is to maximise the system aperture efficiency. This means creating an approximation to an Airy beam pattern either at the aperture of the feedhorn or in the far-field. Both are possible and aperture efficiencies >~90% over full waveguide bands can be achieved.

Aperture Array Horns: In this application the goal is usually to increase the aperture efficiency of the horn itself. This can be achieved by creating a “top hat” function at the horn aperture by careful excitation and phasing of higher order waveguide modes. There is usually a trade-off between efficiency and bandwidth but very high efficiencies can be achieved over bandwidths of a few %.

Constant Gain Feedhorns: Most corrugated feedhorn designs feature constant beam sizes at the aperture, which means that the far field beam aperture scales with wavelength. However, it is possible to construct horns where the far field beam pattern is independent of frequency over entire waveguide bands. Normally, there is a trade-off between beam quality (sidelobes) and bandwidth.

Smooth Feedhorns: For some vacuum applications (or for cost reasons) it is preferable to use smooth feedhorns. These do not have the same performance or bandwidth as corrugated designs (as usually more modes need to be excited and phased for the same optical output) but we have designed and manufactured wideband smooth feeds with performance similar to the best published results that are “good enough” for many applications.