On Sun, 2009-04-26 at 10:33 -0700, Jim Brown wrote: > On Sat, 25 Apr 2009 17:29:15 -0400, Don Wilhelm wrote: > > >My reason: The paralleled line certainly will work, but the loss is the > >same as a single coax of the same material, > > I don't agree, Don. Loss in coax at HF is all I squared R. If you match to > twice the impedance, you'll have half the current.
Actually you'll have 1/sqrt(2) or 0.707 the current. So you'll have 1/2 the power loss in each coax, but there are two coaxes so the total power loss is exactly the same as with a single coax. As long as everything is matched it doesn't matter how many coaxes you use. The loss is always the same. > This means that the > fraction of the loss in the center conductor will be 6 dB lower at the > higher impedance. On the other hand, the loss in most coax shields is lower > than the loss in the center conductor, so I would expect losses to be about > 3 dB lower, not 6 dB. > > >and in a multiband > >situation, it certainly will be run at a high SWR. > > Agreed. This is not a good solution for antennas of random impedance. But it > IS a good solution for antennas that match to about 2.5:1 or better. The same could be said about using a single coax as well. One advantage to using two coaxes is that each carries only half the power (0.707 the voltage), so you get twice the power-handling capability. One issue with making parallel-conductor line with two coaxes is that they must have the same electrical length. If they are cut from the same roll of high-quality coax, then identical physical length should equate to identical electrical length, but it still might be worth checking to be sure. > 73, > > Jim K9YC Al N1AL ______________________________________________________________ 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
