On Sat, 8 Sep 2007 18:16:12 -0400, "Bob D."
wrote:

Any solutions out there? Is it possible to build a 1350 KHz filter that
won't throw off the impedance reading at 1800 KHz?

Hi Bob,

Call the station, ask for the Engineer. Then ask when they go off the
air for testing. You may be up late into the wee hours, but for only
10 minutes or so.

If you are going to filter the AM out, you have to look through that
filter to see the antenna. Your experience has already borne out the
futility of that. However, I've seen just such advice in the past
with recommendations of charting things through a Smith Chart. I
can't say I have much confidence in that outcome - still, it may beat
out the alternative of waiting a month for the scheduled down time.
Even then, I would still get up that night and get the benchmark.

73's
Richard Clark, KB7QHC