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A Simple Waveguide Feed
for
Short Focal Length Dishes

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From RadCom October 1976
This article introduced this feed to the amateur community generally. At 10 GHz the dish diameter is approximately 30 mm in diameter, the same size as a pre-decimal UK penny. This then is the Penny Feed.
The dishes that amateurs inherit are often of the short focal length type; that is, the ratio of the focal length to the diameter of the dish typically is in the region 0.25-0.3. (Note that to check a dish, measure its diameter D and the maximum depth at its centre c, when this ratio will be given by the value of D/16c.) A suitable feed for these dishes is the dipole/reflector type. Although widely used, these feeds are not particularly easy to make, and consequently the much simpler design given above is of great interest. G4ALN, who supplied the design information, used a 10 GHz version during his recent G/ON and G/PA0 contacts on this band.
The feed is constructed by cutting two grooves in the end of a length of waveguide of appropriate size, and soldering on a circular end disc. The length of the slot formed, and also the diameter of the disc, are probably not critical within a few per cent, and the width of the slot even less so. Values for λ and λg for frequencies of amateur interest, together with details of suitable waveguides, are given in below. Signals having the standard horizontal polarization are produced when the broad faces of the guide are vertical.
The feed can be used without any attempt to improve the match-the vswr typically is about 1.5:1. The match may be improved by conventional matching screws which preferably are fitted behind the dish as shown in order to reduce unwanted resonances. An elegant alternative method, which at the same time can be used in weather proofing the system, is shown in the top half of the picture. In this, a Perspex sleeve is made a sliding fit on both the end disc and the waveguide. By adjusting its position, the right proportion of power in the correct phase is fed back into the feed to cancel that reflected by the mismatch.
 

Centre Frequency (MHz)

Suitable Waveguide

λ mm

λg mm

1,297

WG6

231

324

2,305

WG8

130

162

3,457

WG10

86.7

109

5,761

WG14

52.0

78.2

10,050

WG16

29.8

39.4

10,369

WG16

29.9

37.3

24,193

WG20

12.4

15.2

This exhibit was last updated on August 19, 2014.