HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on June 27, 2020, 1549 UTC
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We all like to post our antenna success stories. How about the failures? Or at least what didn't work out as well as you hoped. I'll start.
The antenna in question is a giant MW loop, some details (at least on the initial build) here: https://www.hfunderground.com/board/index.php/topic,36639.0.html
It worked, but I never really used it much. Last year I got the the idea to try remotely tuning it with a varactor diode, and to change the wiring to coax cable. The plan was to use the shield of the coax as the conductor, as it was relatively large in diameter, so theoretically good for high Q.
Well, it didn't really work so well. I think in reality the resistance of the coax shield is not very low, for starters. It was resonant, but performance was nothing to write home about.
I still have the PVC frame, and may resurrect it (again) in some form.
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Well, my first attempt at a VHF/UHF diplexer didn’t turn out too well. The calculations and simulation was for a crossover freq of 500 MHz. As you can see, that didn’t happen! I think the inductors I wound were the issue, so will give it another shot.
(https://i.imgur.com/qEmRLaj.jpg)(https://i.imgur.com/mG4EHUV.jpg)
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Decided to "upgrade" my MW transmission aerial from the inverted L that I was using to a short vertical.
Plan was to be about 65 feet tall using aluminum poles of different diameters that slot into each other, like a massive telescopic aerial, sitting on insulators.
Couldn't get it above about 50 feet high though so that would have to do. The 3 top stay wires were acting as loading.
Managed to match it to 50 ohms and switched on the 1KW TX. SWR was perfect........for a few seconds and then rapidly rose to beyond an acceptable level.
Water had got in to where the pole was sitting on the base insulator and the RF was literally boiling the water. No problem. Quick redesign and that was fixed.
A few weeks later I noticed that the tension of the top wires had altered slightly so the uppermost section was bending slightly to one side.
No problem, I'll sort it out in due course. Famous last words!
High winds took out the top 20 feet. It remaining 30 feet is now used to hang an inverted V HF antenna and the MW TX is back on the inv L.
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Oh yes, there have been failures. :)
My failures have tended to be more mechanical than electronic -- though I am decidedly not a genius in mechanics or electronics -- and while I have no especially spectacular or amusing tales of failure, I can certainly sympathize with the feeling of hearing trombones when the material you thought would work suddenly doesn't (an outdoor MW loop frame suddenly goes flying in a minor breeze off the deck and into the neighbor's lawn 40 yards off), or when the clever hook-up suddenly lets out the magic smoke (who knew those electrolytic caps would be THAT picky about polarity? Not young me!)... and etc., and etc..
Still, any experiment that teaches one how to do -- or how NOT to do -- a given thing is successful enough in my book, and worth the effort... juuuuuust as long as you don't kill yourself (or a neighbor) while you're learning!
Mike
N0TLD
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For me??? I had to learn the hard, and fairly expensive, way that high gain GND-Plane antennas that kicked-ass in fairly flat terrain just did not work at all in hilly terrain and deep in a valley. Co-linear antennas for VHF / UHF are pretty much useless where I live today. Even a 5/8-Wave GND-Plane is too high gain for here. 1/4-Wave, maybe even 1/2-Wave, and that less than Unity Gain Discone, seem to work best here. Live, learn, develop buyer's remorse.