A-Tech wrote:
. . .
Roy Lewallen, W7EL, concurred with Dale and suggested
a "phased array" design. Unfortunately, this is beyond
my capabilities. Roy and Walt, W2DU, both agreed that
using "null tuning" might help. Roy also suggested a "choke"
for the feedline to reduce feedline pickup. - But it seems to me
that 95% shielded 75ohm coax should not be a
contributing factor.
. . .

With the added knowledge of your antenna and your technical
capabilities, the approach I'd encourage you to try is to simply turn
your antenna while listening. It might not take much turning to get the
interfering signal into a null, leaving the desired signal close enough
to the peak of the front directional lobe to be strong enough.

The problem with feedline pickup has nothing to do with the shielding,
it's caused by current on the outside of the shield. But it's not likely
to be a concern with your commercial antenna and coax because of the way
these antennas are typically fed.

Roy Lewallen, W7EL