Hi Brian, not sure where the 20dB figure comes from?
With my little 23cm PCB antenna, it is possible to null out strong signals by tweaking the polarisation, though it is a very sharp null - in practice, you would probably be very unlucky to experience this by chance. Even if the antennas were exactly at 90 degrees, there would likely be some path rotation / reflections etc. Maybe the 20dB is a practical expectation…?
This reference suggests that the theoretical loss is infinite:
It does feel “wrong” to use SSB with a vertical antenna, but the potential benefits in the context of SOTA activating (assuming a single element dipole / Jpole etc) include: simple universal antenna, omnidirectional, easy to mount high on a fishing pole.
Maybe this thread will encourage me and others to give it a go…