Will a voter switch fast/often enough to keep up with the flutter style fading?
Good question. Simple answer: no, generally speaking, a regular SNR voter and two separate cross-polarized antennas won't be effective in combatting mobile flutter.
The rate of mobile flutter is primarily a function of two things; the relative velocity of the stations (in the case of a repeater, only the mobile is moving), and the wavelength (the shorter the wavelength, the faster the flutter for a given velocity). A voting comparator designed to specifically deal with mobile multipath flutter could be designed, and back in the AMPS cellular days, such designs were effective, but the voting was not based on comparing relative SNR's in the audio domain like a typical two-way voter does.
To others that have commented: at any instant of time, there is no guarantee that that, at any instant in time or space, multipath experienced at the receiver is being caused by cross-polarization. That is, don't assume that simply having two linear cross-polarized antennas, or even a single circular polarized antenna, is going to magically cure multipath. CP helps to address polarization skew caused by reflections, but the reflections are still there - there can, and will, still be destructive cancellation that can't be avoided regardless of whether the receiving antennas is linear or cpol. And of course, whenever a CP signal is reflected, its polarization sense is inverted, so a mobile CP antenna absolutely does not help deal with multipath if the repeater is also CP.
The US is one of the countries that regularly uses CP on FM broadcast, and to a lesser extent on TV, but it's certainly not a worldwide standard. Many countries, including most of Eu, use linear polarization. There have been a lot of studies done on the subject of reception of Vpol vs Hpol vs Cpol vs slant-pol transmitted signals using linearly-polarized receiving antennas, often with conflicting results.
Bottom line: CP can help in some situations, but it's not a magic cure. I'm not trying to dissuade anyone from experimentation, just have reasonable expectations on what you'll achieve.
--- Jeff WN3A
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