http://www.earchi.org/proj_homebrew.html
According to the published design data, this antenna uses only a length of wire and a transformer that I could build on my own. The Emergency Amateur Radio Club refers to this component as "The Matchbox". The caveat of running this type of antenna is that it requires an antenna tuner, which my ICOM rig happens to come equipped with.
I made a few mistakes and learned some things along the way, so I wanted to share them so others don't make the same mistakes. I have seen these are available for purchase, but they also will be susceptible to the same problems I experienced using this type of antenna system.
Armed with a little knowledge and some simple directions I was able to construct a prototype 9:1 UN-UN Trifilar wound toroid.
I bought a weather-tight box to put the transformer in. The first problem was that I was getting a large amount of RF back into my radio room even when transmitting with very little power. This was completely unacceptable, so something had to be done. The first time I keyed up the telephone on my desk started ringing, and my computer monitor started going bonkers. Luckily I didn't receive any RF burns. I was directed by Bruce K1BG, to install a current choke in the feed line as close to the "matchbox" as possible. This is a sample image of what a current choke is, just a few wraps of coax feedline around a core. An air-core of a few inches didn't seem to reduce the reflected RF. The ferrite core cured the reflected RF so that I could apply full transmitter power to the antenna. This problem wasn't unexpected, and even appears as a question in the Amateur Radio exam. The solution wasn't apparent and wasn't discussed in the construction plans. (I don't think this problem would be noticeable operating this antenna at QRP transmission levels.)
Following the directions for construction, I even used the same color wire that the instructions called for. This was the second mistake. The problem wasn't that the wire colors were in question it was the type of wire that I used. I used some low voltage thermostat wire that I happened to have in the junk box. The problem was that this low voltage wire insulation did not protect the transformer against the voltages of high power transmission. During a transmission the SWR meter started to peg, which caused the transmitter to shut down through its self-protection mechanism. When I disassembled the box to investigate, I discovered that the insulation had failed, arced, and melted. The enchanted smoke had escaped.
The solution was to use the correct wire for the application. I obtained some expensive Teflon-coated wire of comparable gauge and re-wound the toroid transformer. (I broke the original ferrite during dis-assembly). The Teflon insulation temperature rating and voltage rating was well over what my transmitter is capable. It won't melt again. At this point the antenna was back to being operational, but I was still getting some RF from the antenna itself in the house. The way that my 67 foot length of antenna wire runs from the peak of the house to the nearest / tallest tree means that the antenna is fairly close to the roof of my home.
Re-reading the construction document it referred to the use of an additional counterpoise. K1BG encouraged me to droop an additional counterpoise wire connected to the ground lead of the matchbox. I wasn't able to droop it straight down due to the proximity of my garage doors, so I ran it along the edge of the roof line of my home. The benefit here seems to be the additional counterpoise acting as a reflector. The RF energy is now directed skyward. Adding this additional counterpoise wire seems to have eliminated all the RF energy that I was getting in my radio room.
The result is a very cheap, basic antenna proven tune-able to the 15, 40 meter bands. Probably other bands are tune-able if I gain spectrum permissions once I pass my General. Changing the antenna length will also likely yield better tuning on other bands. At 67 feet of antenna only 25 feet off the ground I have confirmed CW contacts in Europe, North America, and South America. I just wish it tuned better on the 6 meter band.
73
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