In the early 90's I lived in a condo and couldn't put anything outside for an antenna. The MFJ Isoloop was fairly new but I wanted to work the lower bands more than 10-20.
In either the ARRL Handbook or the antenna handbook from the 80's (Don't think it was Bill Orr's handbook) I found instructions written by a Ham in Florida I believe, to make an octagonal short transmitting loop out of 1" copper pipe and 45° junctions. An extremely high Q antenna, it was center fed at the bottom horizontal pipe with a gamma match made of flexible copper tubing running from the center of the coax to the center of the first 45° section. It had a very narrow bandwidth of maybe 5KHz but it was a perfect 1:1 match at its resonant frequency. You could adjust the resonant frequency by changing capacitance at the feed point. There were several ways of doing this, the most straightforward way was to use air variables that were wide enough not to arc. I wanted to use my SB-220 with this antenna so I decided to use an on-hand 15KV 5-500 pF vacuum variable. I wanted to make a geared stepper motor control to run it remotely but that was new technology to me at the time and opted instead to use a pair of servos which worked perfectly. I put a Groth tuner on the servo in the control box by the radio and kept the numbers to get tuned quickly in a notebook. I made three of these antennas and placed them on the wall going east west with the largest octagon 7 feet from top to bottom, a smaller one about 4.5 feet high inside and inside of that, another one 3 feet high. Using all three of these antennas I could cover 160 through 10. According to the literature at its resonant frequency it was extremely efficient approaching something like 95% to that of a dipole at the same height(I never compared one of these antennas to a dipole at the same height but I will say it worked wonderfully for an indoor antenna). I built small shelves to hold the vacuum variable and servo assembly and used quick disconnects to allow me to switch between antennas without unscrewing anything, just relocating and reattaching that assembly. The XYL was willing to let me run the coax and control cable from the basement to the second floor of the condo where we had an unused spare room. I never had any arcing issues of any kind using the vacuum variable with that Heathkit amplifier. Since I was not in the room and didn't want her to touch the antenna while I was transmitting, she understood the reason not to go near the antenna and as an extra precaution, when the servo was engaged, an attached lightbulb was lit to resolve all doubt. It would be difficult to use such an antenna to chase DX that operates split because the band with is not as wide as some of the splits I'm seeing today. There were some crossover frequencies between the different antennas where the Q was lower at the bottom end of a larger loop and that would allow much larger splits. It was interesting to tune the antenna because you would turn to the desired frequency and then adjust the capacitor to get there and it was dead silent in the headphones until it was resonant where you were listening and instantly the band would become alive and loud. Extremely steep skirts. One of the most attractive aspects to me was that it would provide a good vertical polarization from your signal and that made it a good choice for DX. When I look through my log at the DX I was able to work with those antennas, I'm surprised at the number of entries. 73, Gary KA1J --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
