On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Try rearranging your network, try pi series C, pi shunt C, etc.

Also make friends with someone with a vswr analyzer ;-)