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Bob Miller wrote:
Cecil, I have a 44-foot dipole with 42 feet of ladder line -- I can tune it for 30m-10m easily with my tuner. I can't get it to tune on 40m, though. If I shorten the ladder line to 33 feet, or lengthen it to 99 feet, would that give me a near-perfect match on what is a 3/8 wave dipole for 40m? Nope, it wouldn't, Bob, because the SWR is too high. EZNEC says a 44' dipole fed with 450 ohm ladder-line will have a current maximum feedpoint resistance of about 10 ohms. I define "near-perfect" as a feedpoint resistance of 25-100 ohms giving a 50 ohm SWR of less than 2:1. However, you can probably match that 10 ohms with your tuner. In general, to use the variable ladder-line length approach that I use, a dipole needs to be at least 1/2WL on the lowest frequency of operation. For dipoles shorter than 1/2WL, as the dipole is made shorter, the resistance goes down and the reactance goes up, causing the resistance at a current maximum point to go lower and lower with increasing SWR. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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