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On Mon, 11 Apr 2005 23:47:51 GMT, Lancer wrote in
. com: On Mon, 11 Apr 2005 13:46:07 -0700, Frank Gilliland wrote: On Mon, 11 Apr 2005 08:03:21 GMT, "Pete KE9OA" wrote in : snip On a final note...........at 37 ohms, you will have a VSWR of 1.3 to 1.................at 75 ohms, you will have a VSWR of 1.5 to 1. What is the difference here? For a transmitter with a tube output and an internal matching network, you wouldn't really see much effect. For a typical solid state transmitter, there would be some difference between the two antenna impedances, because the broadband solid state transmitter would be called upon to deliver more current to the antenna. It probably wouldn't have any effect, unless the ALC circuit was aggressive in its operation. In this case, power foldback would occur into the 37 ohm load. Would it happen? Probably not. .....uh, what? Both tubes and transistors use matching networks, so I don't know what distinction you are trying to make there. Power will be reflected from an antenna/coax mismatch -regardless- of whether you have a tube or transistor final. And what does an ALC circuit have to do with transmission line propogation? This pertains to solid state amateur transceivers that don't have an adjustable output matching network.........most of today's units have fixed-tuned bandpass filters after the output stage. I am not referring to transmission line propagation; I am referring to the fact that, with a fixed-tuned output network that expects to see a 50 ohm characteristic impedance, the ALC can fold back the power. Usually, that doesn't occur until a VSWR of 2 to 1 is reached. My explanation is for illustrative purposes only. I understood that much. My point was that whenever an antenna/coax mismatch occurs, tuning the output tank (as with a tube final) doesn't cure the mismatch or the resulting signal loss. All it does is protect the final from the reflected power. The output tank should be matched to the characteristic impedance of the coax whether the final is a tube or a transistor. If the coax shows standing waves, the mismatch should be fixed at the point of mismatch, not at the radio. I realize that this is not always practical, but people should know that such a conjugate match just dissipates that reflected power somewhere else, usually from the outside of the coax or the radio. Frank; Why should I care about the antenna/coax match? If I run a 1/2 wave of 450 ohm line to my dipole, my transmitter will "see" 75 ohms, same as a 1/2 wave of 75 ohm coax. And that's fine if your transmitter output is also 75 ohms and you operate on only one freq. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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