One of my first goals for the new QTH is to get an antenna up on HF. There are two nicely placed trees about 65 feet apart. Since one of the trees is close to the house, an end-fed random longwire might just work for me.
But how long should my longwire be? There are plenty of articles on this topic to be found, but the gist of it seems to be that thing important thing is to avoid lengths that are resonant in any of the bands—or their harmonics—where you want to operate.
For 40m through 10m, a long wire of 60 feet should work!
But how high can I get my feed point?
In his page, Random Wire Antenna Lengths (to avoid), Mike Markowski (AB3AP) has provided a very nice explanation, the C source code for a program that makes the necessary calculations, and a chart that visualizes the lengths to avoid for various combinations of bands. As you might expect, if you only care about 40m, 30m, 20m, and 15m, you have a lot more options that if you hope to work everything from 160m to 6m. (Spoiler alert: you can’t.)
For the heck of it, I ported the code to Python and was able to reproduce the calculations on my own. My first question: how long would my wire need to be to work everything from 80m to 10m?
It turns out, this isn't too much of a stretch. I could fold the end of the wire if it ends up being a little too long, but here’s the catch. The counterpoise needs to be at least one quarter wavelength on the lowest frequency where I intend to operate. On 80m, that ends up being about 69 feet. If I use the shield of the coax between the unun and the choke as a counterpoise, then the feedpoint of my wire is going to have to be 70 feet off the ground! That’ll never work. So, as I suspected, I have to forego 80 meters. That’s a sacrifice I’m prepared to make.
Okay, that leaves me with 40 meters and up. (I’m skipping 60m. Maybe that’s a mistake, but I’ve lived without 60m for almost 38 years.) Taking 80m out of the mix gives me a lot more options. 60 feet fits my space quite nicely. The 35 foot counterpoise that I need could still be a problem, though. I’m not sure I can get the feed point up that high.
If push comes to shove, I can keep the wire at 60 feet and shorten the counterpoise to 24 feet (for 30 meters) or 17 feet (for 20 meters). A tuner may help too. The next step will be to throw some wires up and test it out. More to come.
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