"826" wrote in
:

Hi,
Sorry I didn't rely sooner. The weather hasn't been too good hear.
This
morning I disconnected the coax from the shack and measured the
antenna with the six foot length of coax. It looked OK. I then
connected just the shield of the long coax going to the shack and
still measuring with the short coax it shifted just like measured
before in the shack. I have then made up a choke balun and installed
it at the base of the antenna and it improved everything by about 60%.
I will now make another one for the shack end. I think everything is
under control now. Thanks to all for heading me in the right
direction. Vern M0WQR

Vern, whilst the choke made a difference, the fact that there remains a
difference in the feedpoint impedance with the shield connected and
disconnected, even when using the choke, indicates that there is some
effects observable from common mode feed line current in your scenario.

If you want a measurement of the antenna as it would be used, measure it
as it would be used... make sure the shield current path is as it would
be used.

Owen