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To receive OSCAR13 or possibly the new AMSAT Phase IIID satellite a simple parabolic dish with a circular polarised feed is described. The dish is a standard low cost commercial unit with a diameter of 60 cm. A home-brew feed is proposed. The overall gain and pattern have been measured.
The focal length has been determined with 
(see Fig. 3) to be 281mm. The subtended angle 
was calculated to be 56°.
The F/D ratio is 0.47.
Fig. 1 shows the helix feed. This is compromised of 2.5 turns of 3 mm copper tubing. The inner diameter of the helix is 40 mm and the pitch is 25 mm. A rod (Acrylic or similar) provides a support for the helix. Matching is done by a 6 mm wide and 45 mm long copper strip, which is soldered at the one side to the helix and at the other side to the N-connector. Small adjustments are possible by changing the distance between helix and reflector.
Fig. 1: Helix-Feed for dish
A low noise preamp as described in [2] can be connected to the feed. Even long run* of coaxial cable can be used with only a small deterioration in noise figure.
* Given a noise figure of 0.45 dB at a gain of 40 dB, a subsequent attenuation of 20 dB, which Is equivalent to a run of 44m of RG213/U, the deterioration of the total noise figure Is only 0.1 dB, provided that the the converter has a noise figure of 3 dB.
The 3 dB angles of the helix were measured to be 71.5° in E and 67.5° in H. The 10 dB angles were determined with 130 and 122° respectively. The subtended angle of the dish is 112 ° (2x56). This gives quite a good illumination profile for the dish.
Fig. 2 shows the complete dish with feed. Figs. 3 and 4 show the geometry of the dish and the feed.
Fig. 2: View of Dish
Fig. 3: Geometry of dish
Fig. 4: Position of the Feed
The measured VSWR is shown in Fig. 5. It's below 1:1.2 from 2.15 GHz to 2.45 GHz. The distance of the feed has been optimised for maximum gain on 2.4 GHz. The focus of the dish eventually was in the centre of the helix. Patterns both in horizontal and vertical can be read from figs. 6 and 7.
Fig. 5: Measured VSWR
Fig. 6: Horizontal Pattern
Fig. 7: Vertical Pattern
Total gain is 21.3 dBi.
Literatur
- J.R. Miller: "6Ocm-Parabolspiegel für Mode S", AMSAT-DLJournal, Jahrgang 20, Nr.2, 1993
- R. Bertelsmeier: "PHEMT LNA for 13cm", DUBUS, VOL. 24, 1/1995, S. 34 - 37
DC6GC, Dieter Briggmann.