Wednesday, May 13, 2009

Moxon Rectangle for 2m

Ever since getting a 2m transverter to use with my K3, I have had thoughts of putting up a horizontal 2m antenna to try SSB.

From my location here in the far north western corner of the UK, with the Cumbrian Mountains between me and the rest of England (who are all beaming the other way towards Europe in any case) I have never heard any 2m SSB activity. So it is certainly not worth the cost of putting up a commercial beam and rotator outside, even if I could get away with it. What I had in mind was a small addition to my attic antenna farm, that might enable me to make a few contacts during tropo or Sporadic-E openings.

I was considering an HB9CV, but its feed arrangements are a bit complicated and I wondered whether a simple two element Yagi would be a more practical choice. However there are so many permutations of 2 element Yagis that I became confused. Then I happened across the Moxon Rectangle, which happens to have not only its own website, but even a little software utility written in Visual Basic that will print out a design for you.

The attractions of the Moxon Rectangle over any other small two-metre beam is that it is small - narrower than a conventional Yagi with a boom only a foot long - and is a perfect match to 50 ohms, eliminating fiddly matching arrangements. It is also claimed to have a very good front-to-back ratio. However, when the design utility spit out dimensions in fractions of a millimetre, I started to wonder whether this was superfluous accuracy or whether the design is so finicky it would be impossible for someone with my limited DIY skills to replicate accurately enough for it to work.

I used some wood that I had in the garage for a boom, while the elements were made from four 6mm diameter lengths of aluminium tubing that had once formed a crossed dipole antenna for the weather satellite band. Two of these elements had to be joined at the center for the director of the antenna, the other two formed the driven element. To mount them to the wooden boom and provide attachments for the feeder I used 30 Amp electrical "chocolate block" connectors. What I didn't realise until later was that the brass connectors aren't a tight fit inside the nylon insulating housing, which made for an antenna with slightly wobbly elements.

When I first connected a very short length of co-ax to hook the antenna up to the AA-200 antenna analyzer I got a nice sharp dip going down to a perfect 1:1 SWR - at 135MHz! For a moment this caused me to wonder whether the elements had somehow remembered the original frequency they were used on! I then connected a longer length of RG-8X Mini, and got a much broader SWR curve with double-dips either side of the intended center frequency of 144.3MHz. Fiddling about with the gap between the director and driven element didn't seem to make too much difference either to the position of these dips or the SWR.

With this odd and rather broad SWR curve I'm not sure if the antenna is working properly. With so little 2m activity in these parts it's difficult to test it. I can open up GB3AS, the local repeater, and when I wave the antenna around there appears to be some directivity, but whether it's as good as it could be I can't be sure.

For the moment my 2m Moxon Rectangle is attached by a G clamp to a top shelf in my shack, pointing out the window in an east-south-easterly direction, in the hope that a Sporadic-E opening will occur and I can find out if it works or not. Only then will I decide whether it is worth putting it in the loft, if I should get a commercially made small 2m beam, or whether I should give up the whole idea.
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