Our recent NVIS (Near Incidence Vertical Skywave) experiments brought to light a discussion of antenna polarization. Does it matter? Does it impact NVIS? Absolutely and absolutely.
First, lets talk about polarization on VHF. We all generally run vertical antennas when running VHF-FM. It works well when reaching out to repeaters or attempting point-to-point simplex communications. However, if two operators are attempting to communicate VHF-FM and one is horizontal and one is vertical, they will have issues. The E-fields are oriented perpendicular ("cross-ways", so to speak), and the loss of 10db-20db, signal degradation to 1/10th to 1/100th, is the result. It's just the nature of how radio signals are picked up by the antenna. In this case, compatible orientation is vital.
Things are a little different with NVIS, and there are other considerations with HF. If you are running NVIS then horizontal polarization is effectively required. A dipole will radiate signals overhead and to the horizon, and the closer to the ground then antenna is the less towards the horizon the signal goes, deflecting a larger and larger percentage of radiated signals vertically.
NVIS is typically a 80M-40M bands method, because the higher the frequency the less likely you'll be to get the signal to reflect off the overhead atmosphere. If you run a vertical antenna then your signal is going to be vertically polarized and be headed towards the horizon, not straight up. Verticals just won't work for transmitting NVIS. But, unlike VHF-FM, polarization isn't a complete show-stopper.
An operator using a vertical can communicate with someone operating NVIS. Once the signal hits the atmosphere polarization goes out the window. The reflected signals are scattered and disorganized - oriented in all directions. Therefore, a more distant station will be more likely to pick up a transmission once it has been reflected back to Earth. Likewise, that vertical-antenna station will also have its signals scattered in such a way that a NVIS/dipole operator could receive their signals! It isn't necessarily optimal, but it happens.
In close quarters (less than 50 miles from each other), as most of our stations operated in our NVIS test, you see more effects of attempting horizontal-to-vertical communications. With enough power and/or other obstacles for a signal to bounce off of, we will see "random" local contacts that don't seem to follow the rules. But these will be the result of reflections off ground objects or ground-wave as a general rule.
So, while HF is more forgiving of polarization differences, it is not immune. The fact that polarization becomes less of an issue once the signal reflects off the Ionosphere is great for our purposes; it extends our ability to communicate to more stations at the outer reaches of our NVIS coverage area, allowing us greater ability to communicate during an emergency.
