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This feed is a variation of the feeding system described in(1). It consists of a open circular waveguide and a subreflector. The form of the subreflector has an influence an the gain of the antenna (1-2 dB). The most simple form is a plane metal disc of 30 mm diameter. In this case the gain is similar to that of the commonly used waveguide feeds. More complicate subreflectors(2) increase the gain about 1.5dB. The subreflector I used for the 60 cm dish (f/D=.35) is a compromise in gain and mechanical expenditure.
Illumination efficiency can be calculated with the differenc dG of the theoretical gain G1 and the measured gain G2 as follows: dG=G1-G2=20×log(π×D/λ)-G2. The efficiency (%) is: 100×10
At the UHF-meeting in Notzing near Munich the gain of the simple reflector disc was measured with 31 dB. The gain of the here described subreflector is then 31.5 dB and that of the corrugated subreflector(2) is 32.5 dB. The corresponding efficiencies are 35%, 40% and 47%
The adapter at the end of the brass tube consists of a SMA-socket and a capacitive probe (4 mm diam. 4 mm length. 25 mm from the end of WG). The exact matching can be adjusted with the variable short circuit at the end of the tube, which is a simple cylindrical piston. For better electrical contact, the tube can be slotted.
Many stations use the WR-90 or R100 rectangular waveguide. An adapter from circular to rectangular waveguide can be simply done by bending the tube and fitting it to a rectangular flange. The mismatch loss caused by the incorrect corners is negligable. For this type of adapter the tube with the greater diameter can be better fitted than that with the smaller one.
The waveguide is a copper or brass tube with 20 mm OD and 18 mm ID or 22 mm OD and 20 mm ID. The tube can be mounted and adjusted easily with a simple turned part in the dish. The subreflector is a 60 mm disc with a 7 mm rim. It is soldered by 2-4 2mm wires to a clamp, which can be moved on the tube.
The feeder must be adapted to the dish. Therefore focus point and distance from subreflector to waveguide must be optimized successively. The latter is very critical and was for the 60 cm dish 7mm. Then the short is adjusted to the best VSWR.
DK2RV, Ulf Hülsenbusch.