A dipole can have a center impedance anywhere between 50 and 90 ohms, depending on height above ground. Off-center feed impedances are subject to wild variation depending on height. A guaranteed 300 ohm feed at a certain offset is simply a myth.
Further, with off-center feed the issues with the feedline shield are exacerbated, and switching short end to shield and long end to shield connections make the antenna behave quite differently. This MIGHT improve your feedline current problem, just depending on the particulars. If that were not enough, baluns which can behave with 50 ohm feed Z and near resistive impedance become ineffective to the point of invisible when confronted with largely reactive impedances in the 500 ohm plus range. To be blunt, your setup is practically guaranteed to have high current on the feedline. You would never get me to bet real money that it did NOT. Sucker bet. If you want to have a "hanging" network off center feed to multiband, use a stepdown transformer THAT HAS NO CONNECTION between the high and low side (isolation transformer) and use balanced feedline to the ground. Use a second isolation transformer to go over to coax. Unfortunately, nobody makes those, and they have to be fabricated at home. 73, Guy. On Tue, May 3, 2011 at 1:26 PM, Ray Spreadbury <[email protected]> wrote: > The antenna is currently fed with RG213 cable and there is a 4:1 balun at > the feed point so as the impedance at the dipole is supposed to be about 300 > ohms I thought it would be a reasonable match for the 50ohms cable? > > Anyway the antenna was OK until yesterday, when I raised it a bit, but > obviously changed something to now cause the rf in the shack. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
