[TowerTalk] Low band antenna problem

[John Geiger]

An interesting problem developed yesterday morning during the CW SS that
cost me a few QSOs on 40 and 80.  My antenna is a homebrewed somewhat
G5RV.  It is 100 feet long, fed in the center with Radio Shack 300 ohm twin
lead (probably 20 feet or so) which runs to a MFJ 1:1 current balun, and
the a run of RG11 75ohm coax which goes into the shack.  I use it on 80,
40, 30, 20, 17 and 12 meters, and have used it on 15 and 10 before I put up
monobanders for those bands-it didn't hear so well on 10.

Anyways, yesterday morning I tried to tune it up in 40 using my MFJ 941D
manual tuner, and it kept "losing its tuning".  What I mean is that I would
tune it for a 1:1 SWR, and as soon as I transmitted, the SWR would just to
infinity.  It kept doing this over and over on 40m and 80, but not on the
other bands, which puzzles me.

Later in the day it was tuning fine on 40 and 80.  I haven't been running
much on the lower bands recently, but I seem to remember it doing this one
time before in the morning.  The only difference I can see between the
morning and the afternoon was that it was cold enough Saturday to get some
dew, so maybe something in the balun or somewhere got wet, which dried out
during the day.

If that is so, why would it affect only 40 and 80 and not the other bands?
Looking for ideas or experiences that others have had with something like
this.

73 John AF5CC