Sunday, November 8, 2009

Antenna Maintenance.....

...Is best done when your antennas are down, like mine are right now. My primary HF antenna is a Shakespeare Model 120 Military Antenna. It's a multi-section antenna, with a really stout "Flange Stand-Off Base" that's damn near indestructible. It consists of a Model 120-31, an eight section, 32' stationary antenna, with a Model 120-28 Flange Stand-Off Base. The individual sections are a fiberglass tube with a radiating element inside, and they screw together by means of metal endcaps. Although they're Genuine U.S. Military grade, mine was up in the elements for almost NINE years, and was showing some signs of deterioration when I took it down a few weeks ago. The gelcoat on the elements isn't very thick, and the sun and smog took their toll. It was *almost* beginning to delaminate, and I'm sure the military would have just scrapped it out, and put up a new one. Not wanting to toss something that's still electrically good, I scrubbed the element sections with denatured alcohol to clean off the crud, and then painted them with some Rust-Oleum outdoor paint. I gave them a fairly heavy coat, as I wanted the paint to penetrate into the 'glass, let them dry 48 hours, scuffed them with some ScotchBrite, and gave them another coat. They look pretty good, and as soon as I reinforce our patio roof a bit, I'll put the antenna back up, mount the antenna coupler and a choke balun, and run the cables.
The nice thing about this setup is that you don't have to run all the sections, but can tailor the total length to what frequencies you want to cover. All eight sections gives you 32 feet of antenna, a quarter-wavelength on 40 Meters (7.00-7.300MHz), exactly what you want for long distance ("DX") communications. Five-eights of a wavelength is actually better, but starts getting pretty "floppy" for a vertical antenna made like this one. The bad thing about running all eight sections is that you now have a FULL wavelength on 10 Meters (28.00-30.00MHz), which results in a less-than-optimum radiation pattern, with most of your signal going skyward, rather than out at a "low angle", which gives maximum ground-wave distance, and "Good DX" when your signal bounces off the ionosphere. My solution is to leave off the two bottom sections, which gives me 24', almost the magic "Five Eighths Wavelength" on 10 Meters, while still being completely usable on 40 Meters, although a bit short for best radiation and "Take Off Angle". There *are* times when you want a high take-off angle for regional communications, and I'll get into that (it's called "NVIS") as part of my "Wireless EMCOMMS" series.
What about all those frequency bands in between 40 Meters and 10 Meters? Well, I use an SGC Model 230 "Smartuner" at the base of the antenna, with 4 counterpoise radials cut for each band. This allows me to operate on any frequency I'm licensed for between 7MHz and 30MHz, and I've actually used it at 54MHz, although the tuner isn't rated for that frequency.
SGC has a wealth of information on their website about setting up low-profile or clandestine antennas for those times you really need to be in touch, but don't want your "neighbors" to know you have world-wide shortwave communications available.
I know I could have purchased any one of the various commercially available "All Band" verticals on the market, but the nice thing about using an antenna coupler/matching network (sometimes called an 'antenna tuner', but it doesn't really "tune" the antenna) at the base of a long radiating element is that you don't have the losses associated with the "traps" that are used to separate the various segments of the antenna by frequency, along with worrying if they were made correctly, weatherproofed correctly, and other things.

2 comments:

  1. Interesting, I hadn't thought about ANY of that, or the math in years...LOL Now my head hurts (again) :-)

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  2. That's why I'm proud to be in the Unorganized Militia Signal Corps!
    You need a message delivered, I'll get it there!

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