About a month ago, I decided to trim the trees near the tall push-up pole I use for the center hoist of my antenna. The pole forms the center of an inverted vee mutli band 80-meter half-wave wire antenna installation. It’s guyed at three sides (120 degrees each) and at each of the five levels of the pole (it’s a galvanized telescopic push-up pole).
Hacking away
I picked up a new (80v lithium battery operated – Woo Hoo!) pole-saw to prune all the tree branches that were encroaching on the antenna guy lines and, on the antenna wire itself. Wouldn’t you know, Your’s Truly wasn’t careful enough and accidentally destroyed the antenna wire during the trimming process. Disaster! Well, maybe not. really…
Lucky me, I got a spare
See, a couple of years back I bought a 75-meter antenna wire to eventually replace my 80-meter wire. It sat on my desk for a couple of years, though. I suppose that I internally dreaded the idea of having to bring the pole down. It was a job and a half just putting it up in the first place, especially with all the guying that had to be rigged.
The reason I bought the 75-meter replacement antenna wire is that my current 80-meter wire is designed optimally for the low end of the 80-meter band, ~3.5Mhz, and this is CW territory. I don’t do CW, but I do a lot of voice in the low band area of around ~3.9 MHz, i.e. the 75 HF meter band That’s a minor problem for me.
At 80 meters, the resonant portion of a half-wave antenna is very narrow and doesn’t cover the entire amateur 80-meter band of 3.5 to 4.0 MHz. The center of resonance is around 3.5Mhz. As you get get closer to the voice frequencies nearer to 4.0Mhz with this antenna, the SWR (standing wave ratio) will climb precipitously. You need to use an antenna tuner to operate at these upper frequencies to protect your radio from excessive SWR or risk damaging it.
The 75-meter half-wave antenna wire I purchased is identical to my 80 meters antenna except with the addition of a capacitor at the antenna midpoint to slide the center of RF resonance (electrically shortening it) toward the voice frequencies (~3.8-3.9 MHz). So, the 75 meter antenna allows you to use the antenna at voice frequencies without a antenna tuner. (We tend to call these voice frequencies “the 75-meter band” because that’s wavelength at those frequencies, but officially it is considered the upper part of the 80-meter amateur band),
Did I really need it?
Not really. Although not optimal, I have often used the 80 meter antenna on 75 meters with my heavy duty antenna tuner successfully, even with my 1000w linear amplifier driving it. I don’t tend to abuse my radio by talking key-down for minutes on end. The stress on the antenna is primarily in the antenna’s matching transformer balun.
Half-wave antennas have their highest voltages at its ends where the matching balun is and this can create excessive heat if the SWR at the frequencies being used is too high. Excessive heat approaching the curie point of the balun’s ferrite core will cause SWR to skyrocket and the balun to appear to be an electrical short.
An antenna tuner will correct the SWR your radio sees to something low and acceptable, but beyond the tuner, the SWR is whatever the antenna’s native SWR is at frequency in use. My balun happened to be meaty enough by being rated at 1000W ICAS, (Intermittent Commercial and Amateur Service) so it didn’t destroy itself even though it was out of resonance.
It’s a better choice.
The 75-meter antenna will run cooler at the balun compared to the old 80-meter antenna, since 75 meters (~3.9Mhz) is where I’ll be using it the most, and that’s where the center of its resonance will be.
Less stress.
No damaged radios.
Good things.
Guy ropes vs guy wire
The original push-up pole installation used galvanized guy wire because that what’s the pole manufacturer (Rohn) specified. Working with galvanized guy wire is a right mess and unless you have the right tools or are a professional antenna installer/climber type, you’re going to need a huge boatload of four-letter words just to get half the way through the job.
When bringing the pole down. I simply cut the guys because the Gripple(tm) tensioners used (also mfr recommended) are damn near impossible to remove. Fine. I’m sure the gripples are fantastic for wire fencing and all that, just don’t get them near antenna installations. Ever, Not sure what posessed Rohn to recommend them
Now I feel better.
I took a chance on using polyester guy ropes from Mastrant when putting the antenna pole back up. Wow! Best decision I ever made. They are way less trouble than steel wire and no special tools are needed. They are super easy to use and UV resistant. The tensioners they offer for these ropes are these ingenious plastic blocks which you can tighten by hand – easy peasy.
Since I had fifteen guys to make, I simply bought some bulk rolls of rope, some stainless-steel duplex-type rope cinches, and some proper size tensioners, I didn’t need to buy more stainless-steel loop thimbles as I have a big bag of them laying around that I bought from Amazon before.
I went with the 3mm rope since there’s not much of a wind profile for a wire antenna held up by a pole. The 3mm rope has a 450lb breaking strength and a working load about half that plus, damn: there’s fifteen guy points altogether Should hold anything short of a full-on hurricane. The high winds of New Mexico have nothing on it.