Bob, G3VVT wrote: ...Opted like Vic for a balanced antenna with balanced feeder. Using 100W have not seen any problems unless I try to operate on a band that the antenna is
not resonant on. Using 75 ohm balanced twin as the feeder as the use of 300 ohm ribbon or higher would be impractical from my shack. The 75 ohm twin seems quite docile in use and can be installed in close proximity to metallic objects which is not recommended for 300 ohm or higher impedance twin feeder. ------------------------- All of the 75 ohm stuff that I've seen wasn't rated for high voltages anyway, so it's likely to melt at the voltage loop when hit with high power. AT the very least, it'll be rather lossy. The "rule of thumb" for any balanced line is to keep it at least 3 or 4 times the wire spacing from objects. With the narrow-spaced 75 ohm line that's much easier than using twin lead or open wire lines as you noted. I'm lucky to have my operating desk right at a window directly below the center of my doublet, so my 2.5-inch spaced open wire feeders can come right down to a header mounted in the window and into my balanced ATU. That's about the only way one can deal with wide-spaced lines effectively <G>. Even with the balanced line, if you happen to have a voltage loop - high RF voltage point on the feeder - at the ATU output, you may see the whole rig tend to "float" at the high RF voltage, producing some undesirable effects: RF in the shack and RF feedback in nearby equipment in the house. One approach is to cut the feedline to a length that makes the feed low-impedance. Adding or subtracting up to a 1/4 wavelength on a problematic band will do that. Often less will do fine. Another approach is the time-tested 1/4 wave long wire attached to the rig chassis and insulated at the far end. The far end will be very hot with RF and must be well insulated, but it will show a low impedance and low RF voltage at the rig. If by "resonant" you mean your antenna is 1/2 wave long, then feeding it in the center will produce a decent match for 75 ohm line and the RF voltage should be low all along it. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
