This post will hopefully encourage you to experient with more aerials/antennas and try different types for your location!
The delta loop is a full wave antenna that is usually fed with a 50-ohm coax via a 4:1 balun or 75-ohm matching section, depending on the build. Also, where you attach the coax will affect the polarisation of the antenna, so you can make it vertically polarised or horizontal depending on the chart below:
Note, in the case of the vertically polarised feedpoint it is measured as 1/4 wavelength from the top. Another chart found on the internet shows the polarisation and radiation angle. So, for a DX loop, you probably want low angle of radiation:
Also, the delta loop shows a null to the broadside of the loop, which can be useful when trying to reduce stronger stations in the chase for DX!
Measuring up
So, armed with this information I thought I’d try my hand at building a Delta Loop for 10m. First off, how much wire do I need? well it’s a full wave loop, so:
Length of full wave in meters = 306 / frequency in MHz
L = 306 / 29 MHz (center of band)
L = 10.56m
Cut the wire to length, mark each third and test mount on a 5m painters pole:
Matching Section
Looks good! just need to make the matching section from 75-ohm coax:
Quarter-wave matching section length in meters = 75 * cable’s velocity factor / frequency in MHz
L = 75 * 0.84 / 29
L = 2.17m
84 is the velocity factor of air spaced TV coax (which I was using) but my theory is that if it it lower, I can always trim off the coax until I get a good match. I coiled it up and mounted it in a plastic tub to semi-waterproof it
Perfect Shape
I tried mounting it flat side to the top, my thinking was that it would help get the most of the antenna up in the clear. But unfortuately the plastic conduit I was using as a frame wasn’t stiff enough! A quick check of the SWR showed it to be 1.3:1:
I then mounted it like a traditional triangle, the conduit still drooped, but at least I could tape it to the painters pole. The result was a perfect Delta shape and 1:1 SWR !
Results
On air results showed it to be comparible or better to my OCFD, but with a definite null to certain areas. One noticable thing though, it was definitely picking up less noise – this is a good thing as you can now hear the weaker stations. A quick CQ on FT8 should give me a good indication of where I’m being heard:
As a final test I thought I’d measure the SWR on 6m and 12m – the two bands either side of 10m. It turned out to be 1.4:1! I was using a long length of coax into the shack and this could’ve affected the reading, but at least it’ll be easy on the output finals!