Topband: Shunt Fed Tower SWR Troubles

[Dale Drake]

Here's my problem. I'm experiencing SWR problems
with 160M shunt feed on my tower.  Using a gamma
match with HV air variable caps I am able adjust
the tap point to get a very nice SWR curve using
my AA-35 Zoom analyzer.  The problem I having is
that when transmitting the SWR goes up with power
and if I key-down for very long the SWR rises
rapidly and takes off.

My set up is 70ft Rohn 25 with about 22 feet of
mast above the top of the tower.  At the top of
the mast is a Diamond 2M/440 vertical.  At 15 feet
above the tower top is an XM-240 and 2 feet above
the tower top is a 4el SteppIR.  The elements of
the SteppIR are fully retracted.  All of the coax
and control lines are run from ground level inside
the tower up to the service loop. On the mast
above the tower the coax is tywrapped to the mast.
There are 60 ground radials with an average length
of 90ft.  The reflector of the XM-240 is insulated
from the boom and the driven element is fed
through a Comtek 1:1 balun. All of the coax
shields are grounded at the tower base.

I have an 80M gamma matched shunt on this tower
that plays FB with no weird SWR stuff going on.

I suspect that what is causing my 160M trouble is
that RF is coupling through the 40M coax, through
the balun and into the driven element. Somehow the
coupling changes with power, which I don't
understand.  I suspect that when I key-down and
the SWR takes off, the core in the balun is
getting hot and the magnetic properties are
shifting until it cools off and returns to normal.

My plan to try to correct this problem is to
connect the center of the XM-240 reflector to the
boom and to mount a box at the XM-240 feed point
with 2 vacuum relays that I will use to connect
the driven element to the boom when I'm on160.
This will be a considerable effort and expense so
I'm looking for the group's input as to the
soundness of my plan or if there may be other
approaches that would be less difficult to try
first.

Thanks,

Dale, AA1QD