Brian Howie, GM4DIJ, wrote:
The local 2m beacon ( about 80 miles north and
shielded by a local hill) here is horizontally
polarised. However if I beam south it is much
stronger on vertical polarisation. I have no idea
why.

Nor do I, really. It could be that a close-by
piece of metal is oriented just-so to act as a
passive re-radiator for both horizontally and
vertically polarised waves (ie: it's at a
45-degree angle to both). It could be that your
beam actually has a relatively large, horizontally
polarised response in its back lobe. Or that
there is some interaction between the beam and its
(randomly polarised) feedline. Or that you live
in an anomalous neighborhood. ;-) ;-) ;-)

The sweepingly general statement I made about
radio waves not changing their polarity with
distance was made with the unstated assumption of
a propagation path relatively free of scattering
objects (re-radiators) and having a direct
line-of-sight path between transmitting and
receiving antennas.

Jim, K7JEB