Just an extra comment Chris.

The "non reflective path" is also real important. As a circular
polarised signal reflects off objects, it changes polarisation to the
opposite sense. You'll find that "opposite polarised" in circular has
far more (in practice) attenuation than for plane antenna cross
polarisation.

If your path has an odd number of reflections you have to use the
opposite polarisation at each end.

Cheers Bob VK2YQA

Chris W wrote:

I am reading the 19th edition of the ARRL Antenna Book. I am unclear
which polarization to use if I want 2 Helical antennas to talk to each
other. In the book it says, ". . . when two stations use helical
antennas over a non reflective path, both must use antennas with the
same polarization sense.."