Over the last few weeks I’ve spent quite a lot of time reading about different options for wire HF aerials and one kept coming back into my head as being something I could easily try at this QTH – A full wavelength horizontal ‘delta’ loop for 40m. My garden is the right sort of size to make this kind of aerial and with the available support I knew I could get it at the right height to take advantage of NVIS propagation. I really like the idea of being able to put a good signal on 40m into the whole of the UK and a large chunk of Europe.
I knew it would be quite straightforward to do this, the far end would be supported by a rope from the top of the 20ft pole I have mounted at the end of the garden and I could use a couple of screw in insulators on the eves of the house spread as far apart as possible for the other two anchor points. There are a three common ways to feed such a loop, either balanced feeder back to a tuner, by using a measured quarter wave stub of 75 ohm coax or by using a 4:1 current balun and 50 ohm coax into the shack. I chose the latter because the only balanced feeder I have is currently at the Martello Tower and I had a 4:1 current balun I recently bought from M0CVO Antennas handy.
The screw in insulators I used are more commonly used for electric fences but they’re ideal for aerial supports. I had these from my original long wire installation so it was a simple matter to unscrew them from the top of the fence at the bottom of the garden and relocate them to the side of the house.
At the top of the pole I’ve got a pulley I bought from a local boat chandlers and through that is a run of rope which ends in a standard dogbone insulator of the kind one can pick up from any radio rally for a pound or so.
I used flexweave copper wire, bought at a rally for the actual aerial itself and cut a length to around 45 metres which I knew would be too long for the loop but it gave me the flexibility to be able to shorten it to the correct length.
Once everything was in place it was a simple matter of attaching the coax to the balun and then adjusting the aerial until it matched. I used an MFJ-269 to check it and trim the wire – Once I’d shortened the wire by around 1.5 metres I got a perfect match on 40m and was very pleased to see that I can now operate on the entire band with an SWR of <1.3 without the need for an ATU.
As a bonus it also tunes the whole of 20m with an SWR of <1.8, 15m with an SWR of <1.5 and on 10m it’s <1.2 in the SSB/CW section of the band.
Because the loop is so new and conditions have been so poor on HF over the last week I’ve not really had a chance to test it thoroughly yet but my initial impressions are good. I’ve had QSOs out to over 850 miles already using it with good signal reports each way.
I’ve had some modelling done of the radiation pattern of the loop by Martin, G8JNJ and I’m very pleased to be able to post those models here. As can be seen, it has a fairly good omni pattern on the HF bands with not too many deep nulls and some gain at low ‘ish’ elevations. The plots show that it should have very good NVIS performance on 40m and that’s what the aerial was designed for so I’m happy.
North is at the top of all the plots.