Building a 40m (7MHz) horizontal delta loop

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.

Electric fence insulator

Electric fence insulator

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.

Dogbone insulator

Dogbone insulator

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.

My 40m delta loop installation

My 40m delta loop installation – Click the image for a larger version

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.

40m Loop Azimuth

40m Loop Azimuth

40m Loop at 10 degree elevation

40m Loop at 10 degree elevation

40m Loop at 20 degree elevation

40m Loop at 20 degree elevation

40m Loop at 30 degree elevation

40m Loop at 30 degree elevation

Click here to rate this blog entry at dxzone.com

This entry was posted in Amateur radio, Construction, HF and tagged . Bookmark the permalink.

7 Responses to Building a 40m (7MHz) horizontal delta loop

  1. Bas PE4BAS says:

    Hello Keith, I use a 80m horizontal loop in winter and have good results as well on all bands. Especially NVIS is great. 73, Bas

  2. g6nhu says:

    I’d be interested to see how an 80m loop would work here, it would have to be a square and I’d have to put a couple of coils in the corners to get the full length of wire out. I might have to do this because I had a quick try a couple of days ago and couldn’t persuade the MFJ ATU to tune the loop on 80m and it’s a band I do want to be able to operate on.

  3. Scotty 2E0OZI says:

    Hi Kieth, interesting about the loop, I built one for 10m and it worked, at least out to 5 miles! As 10m might be coming to life, I shall have to put it up again. The whole 40m loop is a bit of a no-go here due its visual impact and my wifes opinion!

    Scott

    • g6nhu says:

      10m is one of those bands where when it’s open, you can work the world on a bit of damp string, much like 6m. I reckon that if you put it back up again that you’ll do much better than 5 miles with it Scotty – My indoor 6m dipole is rubbish when the band is closed but when there’s some Es kicking around I’ve worked over 1,100 miles using it.

  4. Ellis says:

    Hi,
    Nice clear plan drawings. I put a Delta Loop (skyloop) up about 2 moths ago using exactly the same configuration as yourself. It works great and the best thing of all, it works well at low height. My one is up at 7.5m agl, (1:4 voltage balun & 1:1 air cored current balun).
    Nice one. If you manage to get your Delta set for 80m I would be interested in how you do it and what sort of results you get.
    Cheers, SV0XBU

  5. Paul Collins M0BSW says:

    This is exactly the same amount of room I have here at my location, so just got to have a go at this one,
    73′s
    Paul M0BSW

  6. Jorge says:

    Hi Kieth, interesting about the loop, tks for share!!
    73` from Jorge,LU8HRW

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>