A few quick photos of antennas at G4HLX for AO-40 mode U/S:
435 MHz 9-turn helix | 2.4 GHz 70cm prime-focus dish
| Based on the dimensions given in The Satellite Experimenters Handbook (ARRL, 1984), but with a feed match system derived by scaling-up the G3RUH 2.4GHz design (Oscar News, April & October 1993) - see below. |
| It's actually 9.25 turns, including the quarter-turn impedance transformation to get a 50 ohm feed. The frame is wood, with several coats of gloss paint, the reflector is an aluminium mesh, and the turns are of 10mm copper "minibore" central heating pipe. |
| And here are a couple of pictures of the quarter-turn feed match. The copper strip is 32mm wide. I adjusted the spacings from the reflector until I got a perfect match at 50 ohm, using an MFJ antenna analyser. This was with 5mm spacing at the N-type feedpoint and 22mm at the end of the quarter-turn match. |
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Does it work? Well, yes, it seems to. I don't have any quantitative gain measurements, but at low squint angles, when AO-40 is at ~50,000 km, about 50W triggers Leila. When I first constructed this antenna (different feed arrangement) and used it through AO-13, I noted that the performance was similar to a 19-ele Tonna long yagi (but without the spin modulation). Of course, it's a heavy antenna, but since I've only ever used it on a low mount (as pictured) and adjusted az/el by hand, that's not a problem. |
| I removed all that, and the heavy mounting brackets at the back. I constructed a 2.25 turn helix feed, following the design of G3RUH (Oscar News, April 1993). The F/D of this dish is 0.35. The photos below show the completed dish and feed, with a DB6NT preamp mounted on the feed, and the Drake 2880 downconvertor behind the dish. |
A couple of pictures of the feed:
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Does it work? Oh yes. A definite improvement over the 16-turn helix. Compare these two 10 second audio clips of the S2 beacon, near apogee (> 62,000 km), squint angle about 15°. |
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| Signal received with 70cm dish | Signal received with 16-turn helix |
| In each case the DB6NT preamp (0.7 dB NF) and Drake downconvertor were in use. | |
Neill Taylor
G4HLX
Updated 29th August 2001
In the photos above, you'll also see my 16-turn helix for the S-band downlink. Although this worked, it was really only barely adequate, so I jumped at the chance to get a dish when my local club acquired some surplus 70cm diameter prime-focus dishes. Right: this is what the unit looked like when I got it. Under that radome is a waveguide and feed for the original frequency, apparently around 23 GHz.