Whenever people do weak-signal work on VHF and UHF frequencies, it's always done with horizontal polarization. I'm trying to understand exactly why in the context of some long distance packet radio experiments I'm currently performing. Looking around the Internets, I find answers ranging from "you use horizontal polarization for weak signal VHF/UHF because that's what everyone else is doing" to various justifications for its superiority. Note that I'm discussing weak-signal work via e.g. troposcatter and diffraction, and not Es, meteor scatter, or EME.
Here are some of the answers that I've seen:
It doesn't really matter. ISTR one paper discussing troposcatter stations in the North Atlantic that didn't see much difference between horizontal and vertical polarization. Similarly, it appears some commercial troposcatter systems communicate using both horizontal and vertical polarization as a form of diversity.
Ground gain. Horizontal polarization gives you a few db (up to ~5dB) of ground gain over vertical polarization.
Lower takeoff angle. I've seen some claims about this, but I'm not at all sure if this is the case.
Better diffraction. Horizontal polarization may diffract better around obstacles like hills, mountains, etc. I don't know if this is analogous to how sunglasses are often vertically polarized because glare off of objects (e.g. water) tends to be horizontally polarized.
Less QRM. Apparently interference (both natural as well as man-made) on some frequencies such as 2m is vertically polarized, so horizontal polarization avoids reception of it.
I was wondering if anyone who was familiar enough with the theory behind troposcatter/diffraction communications could key me in on how much better horizontal polarization is vs. vertical polarization and whether the info I've seen is correct.
