John's homebrew pages

2m Lambda Loop Antenna

My location in the city is not a good one. First, it's not on the top of a hill - it's halfway down a north facing slope. Secondly, it's a flat in a fairly dense residential area, and consequently the level of electrical noise is severe - that's why most of my operating is /P - either portable on a hilltop, or at a quiet site away from the home QTH. There's an additional disadvantage: the property is in a "conservation" area, which means that it's difficult to do things that change the appearance of property - so a rotator with an assembly of Yagis on top attached to a chimney and sticking up above the flats would be a definite "no-no".

However I decided it's worth having a go, if only so that I have a reasonable setup for local chat. Since my preferred mode on 2m is SSB, I decided to go for horizontal polarisation. There's a neat antenna described in the RSGB VHF/UHF Handbook (Second edition 2007), which is a wavelength long conductor bent into a circle, with the two end folded in to form a matching stub. Here's the basic component made from 8mm copper small bore central heating pipe:

The matching stub is held at the design spacing using perspex sheet, with two supports attached to pipe fixings, so the antenna can be supported on a vertical plastic pipe. This in turn is mounted on a base so that the antenna can be free standing in the loft.

Here's a close-up of the support end of the stub. Unfortunately the first version I built based on a drawn circle did not match well - it had high VSWR at 144.3, though better in the FM part of the band (145.5). Not good enough though. I dismantled it and straightened it out, and found that the length was under a wavelength, being 42mm short of the desired 2078mm. So I straightened out another length and cut it to 2078mm, then bent it into the loop. Better this time - a very good low VSWR at 145.75, but still a bit high at 144.3. I attached a couple of bits of thick copper wire to each stub end, about 15mm on each, and that is much better; so I may build another with the initial length 30-40mm over the wavelength, if I am at a loose end sometime. Most unlikely!

This is what it looks like installed in the loft - it's a few metres away from the TV antenna in the background. The red wires in the middle are the (thick mains distribution cable) extensions to the stubs. The feeder has a 1:1 balun (Pawsey stub, again described in the VHF/UHF handbook) to match the coax to the balanced input of the lambda loop.

It has been tested out for local chat and gets good signal reports, so it's clearly working. Now I need to make a vertically polarised omni for local FM chat as well ...