If there is a definitive answer to this I wouldn’t mind knowing it.
From way back in the day I remember the rule of thumb being the length of the antenna. If it was a 10ft Omni then they needed 10ft of separation. But I know I also remember it being 1/2 a wavelength, 1 full wavelength and three wavelengths depending on who and where you ask.
I had asked once why it was that home wifi router’s antennas were obviously not one antenna length or wavelength apart and it was explained to me that the distance you want to reach is also relevant while someone else said it was the gain of the antenna that mattered… however no one could explain how either worked into how far apart they needed to be.
Somewhere I remember seeing someone on a forum somewhere post a rather complex mathematical formula for figuring it out but I’ve long forgot/lost it and don’t know that it was even correct.
Someone at KPP once told me the KPP sector I was asking about needed 4ft of horizontal or 10ft vertical separation due to the way the side lobes were… so maybe some antennas need to be farther apart than others.
I was also told once it didn’t matter as long as there was 20Mhz separation between the channels. And I believe KPP told me that a Dual 5Ghz sector I was asking about required 20Mhz channel separation to work correctly. So those are basically 2 5Ghz sectors right on top of each other in the same radome.
From personal experience almost all of our antennas are on water towers and many of them are stand pipe water tanks with an 8ft diameter so no V separation and very little H. On two we have to mount our antennas to the ladder rail where it comes over the top of the tank. In these cases vertical separation is impossible and horizontal separation is about 2ft and I’ve never noticed them behaving any different than Omni’s on other towers with 4ft or more separation (mostly kpp and ubiquiti 5ghz omnis).