Avoid the Loopy-ness of HF Loops
In reworking a wire antenna around the edge of my shingled roofline that was installed 10 years ago as a (mostly) horizontal HF loop, I discovered a couple of things. After applying them successfully and preparing a draft of the principles, procedures and results, Don G3XTT decided to publish my work in the March issue of Practical Wireless magazine. Thomas K4SWL, Kirk NT0Z, and Scott K0MD gave me valuable advice on this journey.
One is that the current ARRL Antenna Book mainly parrots the wisdom of “just use 450-ohm ladder line and you’ll be fine.” Usually. OK, that’s the Cliff Notes version and they do have more to say than that. But they repeat that Philosopher of Knoxville, Larry Cebik, in his well done set of antenna models for HF loops as the main source of this wisdom. L.E. Cebik, to the surprise of many hams, was not an engineering faculty member at the University of Tennessee but a Professor of Philosophy. A good one at that.
The new English translation of Rothamel’s Antenna Book had references! It’s a more thorough and deep-dive treatment of HF loop antennas. It’s “grounded” in the published literature which “bonds” things together in terms of the reader’s comprehensive of what has been published about these HF antennas. (Sorry for the Ward Silver puns here!)
A second is that the problem with doing what most antenna texts say, measure the impedance of an antenna at the feed point, is that the loop needs to be up in the air where it’s going to be installed. This is a far climb on a ladder with an antenna analyzer!
It dawned on me that the new era analyzers with bluetooth connectivity that are light weight offer a way to measure the feed point impedance at the “bare wire” ends of the loop while keeping one’s feet on the ground. Here I am on a step ladder with my Rig Expert Stick Pro with a banana plug adapter to the bare wire ends of a loop being installed at Clay AC5Z’s house among four tall pine trees. I’m making sure that my iPhone has a good bluetooth connection to the analyzer before I attached the second bare wire end to the banana plug and let AC5Z hoist it up to it’s intended height above ground.
We used Clay’s IC-7300 set at the lowest power output (5w) to run a WSPR beacon for 12 hours on most HF bands. Below are displays from WSPR.Rocks for the fundamental 80M band (pounding Eastern Europe over night!), 20M (Europe, Australia and vicinity, Hawaii) and the 10 & 12M bands (getting to Australia and Japan).
This particular loop is nearing our final optimization. It can be optimized a bit more by raising the balun wind from 4:1 to 6:1 and taking about 3 feet off of the bare wire. This should situate the 80M band to Clay’s liking and bring the higher harmonics more into line with those bands, too. It seems to be working from the test phase.
I’ll cover it and perhaps another one being planned for Mike N5DU’s house soon. Read about the process I’m using in the March issue of Practical Wireless magazine. I cover it in detail for the loop at my HOA-based residence that didn’t just follow the conventional wisdom. It got a much improved optimization. Even being matched to 160 meters for an 80 meter fundamental design frequency!
Good thinking. Being a lazy guy I attach a 1/2 wavelength or multiple thereof,
to the feed point, hoist the antenna into the air and then sit down in my lawn chair and get the same reading that I would if I had climbed up to the feed point. It takes
about 240 feet of line on 160, but works out pretty well on the higher bands. I just
have to take the velocity factor into consideration if I am using coax cable.
Of course the real reason is because I can’t afford a Rig Expert analyzer !
Thanks for the good work on that article.
Thanks Larry! Yea, I’m a measurement person…taught statistical measurement models for about 25 years so I enjoy applying these ideas to the hobby.
Or just put the auto-tuner at the feed point and use coax to connect to radio.
Yes, that is done by some. I’ve thought of it in my own case. Kirk NT0Z (see The Spectrum Monitor for his column on this) has used a remote ATU. In MN, he actually has to heat the ATU so the relays and such do not fail, lol. So it’s not for every environment or pocketbook, especially if you use more than 100 watts.