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SSB Antenna connection
"Chuck" wrote in message ...
Bruce, I am asking why there is apparently such difference between feeding an ungrounded dipole with coax from an ATU (my shore station) and feeding an insulated (hence ungrounded) backstay from an ATU? I work Alaska bareback in the summertime with that setup and I just can't understand what GTO-15 does that hardline doesn't. If you could explain or reference a document that specifies the reasoning I would try to correct my misunderstanding. Thanks, Jack Painter Virginia Beach, Va If I can jump in, the quick answer is that the coax is approximately the same impedance as the center of your ungrounded dipole, at least at the frequency for which it is resonant. Thus, from the perspective of the transmitter and the antenna, the transmission line is "invisible." I'm exaggerating, of course. In the case of a backstay used as an antenna, the feedpoint impedance can be anywhere from a small fraction of an ohm at low frequencies to thousands of ohms where it approximates a half-wavelength. In those cases, the coax will most certainly not be invisible and will most likely either burn up or greatly attenuate your signal (incoming as well as outgoing, actually). If you tried to end-feed your half-wavelength dipole with coax, you would see a similar problem because the impedance at the ends is in the thousands of ohms range. Hope that helps. Chuck, as with Meindert's answer, yes that helps, thank you. I do end-feed a long wire as I said earlier, but it uses a 4:1 Balun, and additionally, has one side of that Balun shorted to ground. This is a noise-limiting design, and while the nice folks at Radio Works (Portsmouth, Va) maintain that it cannot possibly work this way (their Baluns), the CG aircraft I worked in Ecuador with it thought otherwise. So does it's designer, whose name slips my mind at the moment but he was a primary contributer to "Proceedings", and a Phd in EE with many patented antenna designs. Anyway, it would be interesting to see some modelling done with backstay antennas using various feedline approaches. I suspect the difference varies greatly with wavelength, height above ground (water), angle, and frequency. 73, Jack Painter Virginia Beach, Va |
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SSB Antenna connection
On Fri, 28 May 2004 19:10:14 -0400, "Jack Painter"
wrote: "Chuck" wrote in message ... Bruce, I am asking why there is apparently such difference between feeding an ungrounded dipole with coax from an ATU (my shore station) and feeding an insulated (hence ungrounded) backstay from an ATU? I work Alaska bareback in the summertime with that setup and I just can't understand what GTO-15 does that hardline doesn't. If you could explain or reference a document that specifies the reasoning I would try to correct my misunderstanding. Thanks, Jack Painter Virginia Beach, Va If I can jump in, the quick answer is that the coax is approximately the same impedance as the center of your ungrounded dipole, at least at the frequency for which it is resonant. Thus, from the perspective of the transmitter and the antenna, the transmission line is "invisible." I'm exaggerating, of course. In the case of a backstay used as an antenna, the feedpoint impedance can be anywhere from a small fraction of an ohm at low frequencies to thousands of ohms where it approximates a half-wavelength. In those cases, the coax will most certainly not be invisible and will most likely either burn up or greatly attenuate your signal (incoming as well as outgoing, actually). If you tried to end-feed your half-wavelength dipole with coax, you would see a similar problem because the impedance at the ends is in the thousands of ohms range. Hope that helps. Chuck, as with Meindert's answer, yes that helps, thank you. I do end-feed a long wire as I said earlier, but it uses a 4:1 Balun, and additionally, has one side of that Balun shorted to ground. This is a noise-limiting design, and while the nice folks at Radio Works (Portsmouth, Va) maintain that it cannot possibly work this way (their Baluns), the CG aircraft I worked in Ecuador with it thought otherwise. So does it's designer, whose name slips my mind at the moment but he was a primary contributer to "Proceedings", and a Phd in EE with many patented antenna designs. Anyway, it would be interesting to see some modelling done with backstay antennas using various feedline approaches. I suspect the difference varies greatly with wavelength, height above ground (water), angle, and frequency. 73, Jack Painter Virginia Beach, Va Jack, Using a balun to feed an end fed wire may help and it may hurt the situation. It depends on the length of the wire verses frequency. If the wavelength is greater than a quarter wave length and the impedance of the wire is high, the balun will transform it down to a sometimes easier to match impedance. However if you use the antenna on different bands and you chose a band where the impedance of the antenna is low, then the 4:1 balun will step the impedance down even lower than the already low impedance of the antenna. It may well be that it is too low to match efficiently. As a general rule that type of balun is not a good idea when using that type of antenna on multiple bands. The only good a 1:1 balun would do with that type of antenna would be to decouple the feed line from the antenna (assuming coax feed line) and keep the feed line from radiating and or picking up unwanted signals. Regards Gary |
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