[Antennas] Antenna Feedline Question

[Richard Brunner]

I have a multiband dipole, 40/20/15 (almost) and 10M, made with 450 Ohm
ladderline, One wire full length for 40 and 15 (almost), with a ¼ wave stub
on the end for 10M, and one wire half length for 20M.  This is fed with 45
feet of 450 Ohm ladderline to a balun, and coax in the shack, and gives very
good swr and performance.  Recently I moved it and added 30 Feet of
ladderline, and then it had no usable resonances and I was unable to
resonate it with a tuner.

 My theory is that with a 45 foot feeder on 40M the feeder looks like a
matching section, and on other frequencies it looks like n-quarter wave
feeders. The ARRL teaches that feedlines shound not be exactly n-quarter
waves to avoid feedline plus antenna resonances, and any length feedline
should be usable.  I am confounded...  What is happening here?  Anyway, what
is wrong with feedline plus antenna resonance?

I also have a WARC dipole covering 10.1/18.1/24.9 Mc, (doesn't need a stub)
with a 41 foot 450 Ohm feeder, which also works FB.

The nominal 15M resonance on the 40 etc. dipole actually falls at 18.5 Mc,
and is useless.  I am thinking of another multiband dipole for 15M and
another part of 10M.

Richard Brunner, AA1P