Horns for the 1 Square Kilometer Array?

 N. Roddis, C. Baines, A.Pedlar (NRAL, Jodrell Bank, UK)

1. Introduction

Horn antennas are worthy of consideration as elements of the 1 square kilometer array, rather than, for example, using small (~10-15m) paraboloid reflectors. Paraboloids are clearly well established in the radio astronomy world but have several drawbacks as far as the 1 square kilometre array is concerned. Some areas of concern are considered below.
 

2.Simplicity of feeding

Multi-octave feeds can be designed for paraboloid reflecting antennas, generally utilizing a log-periodic structure. They would necessarily be quite complex, needing some kind of wide-bandwidth balun connection, and would require careful alignment. At least some of the receiver equipment would need to be mounted adjacent to the feed, at the focus of the antenna, presenting an access problem for installation and maintenance. For moderate size dishes the aperture blockage could be quite significant, given the low minimum operating frequency of the feed (and hence the need for the feed to be quite large). Furthermore at the lower end of the operating frequency band there could be significant spillover noise and susceptibility to interference, owing to the wide beamwidth of the feed. Sidelobe levels are determined largely by the amplitude taper produced by the feed and the blockage which it causes, and could be as high as -12dB for a 10m dish with a log-periodic feed. A wideband horn of the type described below would be extremely simple to feed via coaxial probes (one per polarization) into the waveguide near the apex of the horn. Access to the feed point, and hence the receivers, would be much more convenient. Sidelobe levels would be determined only by the geometry of the horn, with some designs capable of producing maximum levels of -30dB, and interference susceptibility would be much less at the lower end of the band.

 3.Aperture efficiency

Typically a prime focus paraboloid might have an aperture efficiency of 50 to 60%, depending on how well it is illuminated by its feed; often lower efficiencies are obtained at UHF frequencies, where efficient feeds are difficult to construct. Horns, on the other hand, can have aperture efficiencies up to about 80%.

 4.Standing waves

One problem which can occur with reflector antennas is the excitation of standing waves between the feed and reflector. These severely degrade the bandpass response of the instrument and create difficulties when observing broad, weak lines (eg primordial recombination lines etc). Standing waves on paraboloids can be reduced to some extent with a vertex plate (narrow band) or reflecting pyramid. However in the case of horn antennas the standing wave problem should not be present in the first place. \vskip 1.0pc \noindent
 

 5.Mechanical Considerations

Mechanical construction of the horns would be simple, when compared to  paraboloids of similar aperture, being largely constructed from flat plates and tubing. For elevation adjustment the horn could be pivoted at its centre of gravity, so there would be no need for counter-weighting, with azimuth adjustment performed in a manner similar to paraboloid antennas, ie kingpost, wheel and track etc.
 

 6. Short Axial Length Broadband Horns

Short axial length horns have been produced with the bandwidth 200 MHz to 2 GHz [1], albeit of relatively modest dimensions. This bandwidth is achieved by using double-ridged waveguide techniques. If the broadbanding were achieved with a quad-ridged structure then dual linear polarization would be available over the entire frequency range via two probes into the waveguide. It is conceivable that the design principle could be extended to give, say, a horn with a 100 square metres aperture with an axial length of just 10 metres, although it would present a considerable technical challenge to achieve sufficiently constant phase excitation across the aperture at the high frequency end of the band. At a frequency of 1420 MHz a horn of the dimensions mentioned above gives a path length difference between its axis and edge of almost 6 wavelengths. To optimise the performance of the horn this would have to be corrected with some kind of lens [2],[3]. The greatest technical challenge in the horn antenna approach would be the design of a suitable phase correcting lens to allow it to work efficiently at the higher frequencies in the band. Of course there would be some trade-off between horn axial length and lens size and complexity and such a lens would have to be both low-loss and low-cost if this approach is to be viable.
 
 

7.Conclusions

It is worth investigating the possibility of using horn antennas to form the elements of the 1 square kilometre array. If the differential phase problem can be solved for a broadband horn, then the simplicity of construction and superior low frequency sidelobes, spillover levels and aperture efficiency could make it very attractive.
 

8. References

[1] JOHN L. KERR, 'Short Axial Length Broad-Band Horns', IEEE Trans. Antennas Propagat., vol. AP-21, pp. 710-715, Sept. 1973.

2]H. PAUL WILLIAMS, 'Antenna Theory and Design', Volume Two, Sir Isaac Pitman \& Sons Ltd., p. 238.

[3] SAMUEL SILVER, 'Microwave Antenna Theory and Design', McGraw Hill, pp. 395 ff.