In a message dated 11/13/07 3:19:30 PM Eastern Standard Time, [EMAIL PROTECTED] writes:
> Almost universally, there seems to be > no recognition that even though the feedline is matched at the radio, > it is indeed NOT matched beyond the tuner. The feedline can see > humongous mismatch (high SWR) and the operator doesn't know because > the tuner hides it. That's only true if the load impedance is far from 50-75 ohms, or is highly reactive. In these cases, the feedline will see high > > voltages (perhaps very high), which causes heating in the dielectric > and will eventually destroy the coax. *If* the mismatch is considerable and the power is high enough. But at the hundred-watt level and SWRs under about 4 to 1, that's not going to be an issue with most coax. > So why is hardline so much preferred? It has next to no non-air > dielectric to be heated/destroyed. > The reason hardline is preferred is that its loss is less. All the TV hardline I've seen has foam dielectric, btw. > And last, what is the best solution? Most likely to remote the antenna > tuner so that it always provides a good match on the local feedline > for the transmitter to see. The ultimate, of course, is to place the > tuner at the antenna. Why doesn't everybody do this? Because it is > rather difficult to do. Ham radio, along with the rest of engineering, > is all about finding the best compromise that provides the best results. And knowing what is really going on in the system. An SWR of 2:1 at HF on 100 feet of most coax types isn't a big deal. 10:1 is a different thing completely! > > So, if you can assure a good match between the antenna and the coax, > or perhaps even one that transforms the antenna feed impedance to the > transmitter's at all operating frequencies, then you have found the > right solution for you and should go forward with whatever coax fits > that solution... Agreed - but a good match doesn't have to be a perfect match. 73 de Jim, N2EY ************************************** See what's new at http://www.aol.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
