Chris W. wrote:
"I am unclear which polarization to use if I want 2 helical antennas to
talk to each other."

Unless you are bouncing the signals off a reflector, use the same
polarization for both.

The axial-mode helical antenna is circulaly polarized and fires along
its axis away from its ground screen. It can be either left handed or
right handed depending on which way the helix twists. It responds to
vertical, horizontal or diagonally polarized waves. It does not respond
to waves with the opposite twist to its helix. Circular polarization of
the opposite twist is rejected.

This is like machine bolt threads. A bolt can screw into a nut from
either side if it has the same itch direction, but a left hand bolt
won`t screw into a right hand nut. Circular polarization of the opposite
rotation is rejected.

Refer to any edition of "Antennas" by Kraus for the details. Kraus
invented the helical antenna after other experts said it could not work.

Best regards, Richard Harrison, KB5WZI