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

435 MHz 9-turn helix

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.

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.


2.4 GHz dish

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.

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:

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°.

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

Updated 29th August 2001

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