Home > Daily News, HF, Home Brew > Fine Tuning the 80m Delta Loop

Fine Tuning the 80m Delta Loop

Earlier I wrote here and here about construction of my 80m delta loop which I’ve been using for a few days now. Though originally intended just for 80m DX, a reorganisation of the shack has meant that I can have one of my HF rigs connected to it at all times. This rig is connected to my PC (rather than my iMac) so is the rig of choice for PSK due to Ham Radio Deluxe’s DM780 being a better program to use for PSK than what I have on the iMac. That means the 80m loop now has to operate on multiple bands.

Let’s take a step back. Originally I constructed the antenna using the usual formula 300/F (MHz) corrected for insulated wire velocity factor ( a guesstimate). I then used a 1/4 wave 75 ohm coax stub to match impedance to 50 ohm. Since I used the antenna with a Z11Pro auto match,  that is as much as I did. The antenna worked quite well on 80m though I had not really checked if it was resonant, or what the VSWR was.

With the changes to my shack, I decided to use the antenna for digital modes. Someone suggested I give the Olivia mode a try. This mode is apparently great for low S/N ratio. When I tried this mode on 40m with the 80m delta loop, I noticed some very minor TVI on our new digital television. Since the neighbor’s TV antenna points right through the loop, I immediately shut down and decided to have a better look at what was potentially the cause. I have always been proud that I run a clean operation causing no TVI. Of course I have had to do a lot of work to ensure this is the case. Most of the time at least at this QTH if I have TVI, it is because I’ve neglected to properly isolate the antenna from the feed line, and I suspect this is the problem in most ham shacks right across the world. My G5RV (later modified to ZS6BKW) was a a perfect example. Feed line radiation was completely eliminated once I installed a well built 1:1 current balun.

So suitably equipped with all the bits I needed, I removed the 1/4 wave stub of 75 ohm coax and replaced it with a home brew 4:1 balun where the coax leaves the house. I attached this to some 450 ohm ladder line and ran this line to the antenna bottom corner feed point. I also ensured the metal balun housing was well grounded. I then decided to check the VSWR of the antenna. I was surprised to find it quite a bit worse than I expected. My calculations told me that in order to be 1:1 or thereabouts at 3600 kHz I would need to add 7 metres of wire!. I added the 7 metres and voila, right on 1:1 at 3600kHz. This makes it about <1.2 : 1 at the 3580 for PSK. and about the same at 3700 kHz which is the upper limit for my license.

The result- no more TVI! I have not had a chance to check the antenna performance, but I will bet it is improved.

73 David


Categories: Daily News, HF, Home Brew
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  1. December 10, 2013 at 8:46 am

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