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



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