Sunday, February 22, 2009

A Slim Jim for 2m

One of the benefits of having an attic antenna farm is that making and putting up antennas is really cheap and easy. You don't have to bother making your antennas look nice or weather resistant, and erection (if you'll pardon the expression) may require nothing more elaborate than a drawing pin.

I am wanting to get more active on 2m and in this VHF black hole a quarter wave telescopic on the back of the FT-817 in the shack doesn't cut it. I looked at ads for collinears costing £60 and then decided why not try to make a 2m antenna?

A Slim Jim made out of 300 ohm flat twin is probably one of the easiest antennas to make. It takes about ten minutes for the first cut, based on a web page from Southgate Amateur Radio Club. Unfortunately my antenna analyzer told me that it was resonant on about 141 MHz. So it took me a further hour to prune off a little at a time from each end, as well as to try slightly different points for the feedpoint.

The final result had a total length of 57 inches (144.7cm) with the long section above the cutout 37 1/4 in (94.6cm) and the short stub section 18 3/4 in (47.6cm). The cutout length is one inch (2.5cm). The feedpoint is 3 1/2 in (9 cm) up from the bottom. The SWR was below 1.3:1 at just under 145.0 MHz when I decided that was good enough.

Erecting it in the attic was a simple matter of making a small hole at the top, threading a bit of string through, and securing it to a convenient beam in the attic with a drawing pin. I then found that the minimum SWR point had shifted up to about 146.25MHz, which was a bit annoying. The SWR at 145.0 MHz is 1.5:1 according to the antenna analyzer, although my SWR meter shows it as 2:1 and the FT-817 gives it three bars.

Still, it seems to work quite well despite this. I can get both GB3AS and GB3DG at S8, and made a contact with Richard G1JTD/P using a hand-held on Blencathra, about 14 miles away, for my first Summits On The Air contact. Not bad for an hour's work and a cost of about two quid.
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