Quite true, David, but that energy flowing along the feed line induces currents in the feeder wires as the currents produce the field.
Those currents can be considerable at the current loops, even at low RF power levels and, considering that the current is flowing along the surface of the wire, the resistance of the wire at RF is much greater than at dc. It's my understanding that it's that resistance that cause feeder losses you see on the loss vs. frequency "curves" as the SWR rises above 1:1. That's why the "curves" are straight lines. They represent simple resistance. Of course, what makes open wire feeders superior to coax is that the practical impedance of coax is limited to fairly low levels, meaning much higher SWRs when terminated at a voltage loop on the radiator. In practical antennas that voltage loop may show as much as 4,000 ohms, resulting in a very high SWR on a 50 or 75 ohm line but a rather modest SWR on 450 or 600 ohm line. Of course that holds down the current maxima. I have not done a numerical analysis of the loss of open wire line using small vs. large conductors. Obviously, large conductors offer lower resistive losses but I, too, have used small diameter feed line conductors for light weight when I had to let the antenna carry the weight of the feedline. I'd not panic over using 450 ohm or, in a pinch, even 300 ohm "twinlead" if that was what was fit the situation. But when I can, I use larger wire. Even so, in my overflowing notebook of "experiments I'd like to do" is to set up, say, 100 feet of open wire line and terminate it in an impedance that produces an SWR of, say 10:1, then measure the loss with different diameter conductors. Antennas are like any other part of our gear: if we stick with what the manufacturers offer us, it's pretty simple. But manufacturers are trying to address the biggest audience with the most fool-proof designs. They can be good performers too, but that's not their most important criteria. Us O.T.s learned that with the Gotham vertical decades ago and the newer Hams are seeing it in the absurd claims of gadgets like the "magic" antenna. In both cases "performance' is a distant concern to the manufacturer compared to a broad appeal based on the idea that they are very easy to set up and use. At the other extreme are a very few options requiring specific installations and environments for proper performance. If we have the room and money for a tower or some really high poles, we're in luck. If not, we're pretty much "on our own" designing the best antenna for our individual situations. I find that part of the fun -- and part of the frustration when I'm convinced I did it wrong, Hi! I generally install my doublets "inverted V" style if I have only one support and run the open wire feeder up the support. If it's metal I keep it several line spacings from the support, of course. That way the weight of the feeder is unimportant and it's easy to install it so it's stable and free from twisting. I can use rather large wire easily. 73, Ron AC7AC -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of David Cutter Sent: Tuesday, July 14, 2009 3:01 PM To: .hank.; Phil LaMarche Cc: 'Elecraft_List' Subject: Re: [Elecraft] The use of a doublet I thought the wire gauge was of little importance with parallel feeder, since the energy flow is between the wires not in them. The insulation is of more importance. You might want something stronger for mechanical reasons. I used that pink tv twin many years ago and learnt that the terminations need to be well sealed to prevent water ingress. It is lightweight and thus adds little to drag the feedpoint down (catenary), however, I had to take a lot of care at the joints to relieve wind stress which otherwise causes breakages. David G3UNA Subject: Re: [Elecraft] The use of a doublet ______________________________________________________________ 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
