Sunday, 8 March 2009

DXing from a small (or no) garden

This a is a presentation I gave to my local club. It talks about finding DX, the modes to use, what you can do with QRP and CW and also looks at a number of antenna designs and what I found.

I have made it into a 3.8Mb PDF and you might find it interesting, even without the narrative that obviously went with it.

Download the presentation

Rybakov 806 vertical revisited - a stealth tree antenna?

In an earlier post I talked about the Rybakov 806 vertical – basically, a 7.6m vertical end fed with a 4:1 un-un (unbalanced-unbalanced transformer), so giving a reasonable match across a number of bands.

In practice you MUST use an ATU to bring it down to 1:1.
Anyway, while playing with the design I tried wrapping the wire around an 8m fishing pole and had an idea.

If you push the pole up through the branches of a tree, taping each joint as you go, do you end up with a stealth antenna?
I think the photograph says sit all!

The pole is practically invisible and the leaves aren’t even out yet. Next to the matching box is a single earth stake and then two 20ft radials go left and right on the ground – you could bury them. I think it would work better with more radials.

My FT-2000’s built-in ATU was able to match it to 1:1 on 40-6m. The SWR on 80m is too high and the rig wouldn’t match it. Mind you signals were well down on 80m as the antenna is too short.

So how does it perform. On 40m it was either equal to or one S point down on an 80m 132ft OCF Windom on EU signals – you can just see the long leg of that in the photograph. Given the length is under a quarter wave this is quite good.

On 30m it equalled or outperformed the Windom slightly. This doesn’t surprise me as it is virtually a quarter wave on that band. The measured SWR was 3.6:1, which if you think about it is roughly 1:1 into 50 Ohms before it goes through the 4:1 un-un.

It was a similar story on 20m – equal to or just slightly down on the Windom.
The RR90 (Russian) beacon was down about 1 S point, the 4X6TU 9(Israel) and 4U1UN (New York) beacon was the same. VY2 (Prince Edwards Island) was down 2 S points. KQ2M (CT) was equal or slightly worse. K1RX (NH) slightly better.

It has to be said that compared with a dedicated half-wave horizontal dipole signals were generally 1-2 S points down, but that antenna is not multiband.

18MHz was barely open, but it was a similar story to 14MHz – roughly the same, some signals better, some worse.

21, 24 and 28MHz were not open, although as I have said before, it hears CB stations very well out to about 12-15 miles so I have high hopes for 10m.

So there you are - a stealth antenna that works from 40m – 6m for about £10 plus the pole. If you string the wire up into the branches you don’t even need the pole to be honest.