On Thursday, June 21, 2012 11:35:29 AM UTC-5, Irv Finkleman wrote:
I envisioned a pair of reasonably close spaced conductors (like open wire
line) and no short -- in other words it would
be used with a sliding tap on each side to control the length -- thus
simulating a variable length for the 300 ohm feeder.

As I explained in my last posting, that won't work as expected because it leaves an open-circuit stub in the circuit. I previously tried just shorting out a loop to decrease the length. It didn't work because it created a reactive stub.

What I mean is, for example, say the optimum length for the feedline on 80M
is 32 ft, on 40M 34 ft. and on 20M 33 ft. If I were to
use the average 33 ft. and operate only on those three bands would I notice
any significant difference -- would it make much difference
in my signal at the receiving operators shack?

My 20m rotatable dipole works that way with the internal autotuner on my IC-756PRO. With ~70' of 300 ohm ladder-line, the tuner will find a match on 20m, 17m, 12m, and 10m. I was never lucky enough to achieve a satisfactory SWR on more than two bands without a tuner. However, there are certain combinations of dipole length and feedline that work on multiple bands. The ZS6BKW antenna with a dipole length of 90' and a ladder-line length of 40' is said to work without a tuner on 5 HF bands if one is lucky.

Here's a DOS-based program that I wrote that locates the current maximum points on a transmission line. It runs under XP but DOSBox is needed for later versions of Windows.

http://www.w5dxp.com/IMAXGRAF.EXE

The accompanying article is:

http://www.w5dxp.com/goodbad.htm

P.S. Two weeks ago at a neighbor boy's birthday party, I spent about three hours in the hot Texas sun playing on a large water slide and I am still paying for it. But if I die, I'll die happy.
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73, Cecil, w5dxp.com