Using 2 900Mhz Omni's for Mimo questions

There was a post on these forums quite a while back, can't find it right now and I have since heard of other WISPs doing it, about using two Omni's for 900Mhz MIMO (an H and a V) on 450i.   I'm am fully aware of all the downsides to Omni's as we have been using 900Mhz Omnis on Canopy and 2.4 Ghz Omnis with Ubiquti (now using 2.4Ghz Omnis with ePMP/Sync)  for many years. However we have some places were sectors simply are not possible.

Is there some math someone could point me to for determining how far apart these Omni's should be mounted from each other or is it possible that we could mount them side by side, maybe even so close to each other that they touch ?   In the Canopy days we use to buy 900Mhz Yagis from M2 and they made a kit for bonding 2 Yagi antennas. The kit came with a Y coax and a bar to mount the Yagis on at the correct distance from each other (we bought to test and I still have it around here somewhere).  I believe the distance was some fraction of the 900Mhz wavelength BUT that was for joining two antennas of the same polarity that the radio would see as 1 antenna. Does this same rule apply when the antennas are different pols and being used as 2 separate antennas ? 

Most good 900Mhz Omni's are around 10-11dBi gain. I see that one company actually makes a 900Mhz Omni ( but it is only 6dBi gain. Any opinions on which would be better, two 10's  or  one 6 ?  

Since we already have several Old 900Mhz Canopy sites with Omni antennas I already have the H Omnis (Last Mile Gear , I forget what company actually made them). What is a good  V Omni for something like this ?


I dont have the math available but I know that if you have at least 15dB of cross isolation than the antennas can be side by side. This depends on the antenna more than the location. Cambium gear does not to my knowledge use simultaneous up/down radio steams. The tdd style of mimo allows same stream dual polarity or dual stream dual polarity in the same direction. This means the transmitters are both active at the same time on the ap and the receivers are not getting anything until the tx window is finished. KPPerformance has a dual pol omni in the same rad-tube for ubiquity/ePMP gear. These antenna elements a centimeters apart!

And yes omni antennas are fine as long as they are used correctly and thought is given to the locations they are used. My use for them is an isolated area with fewer than 30 clients and less than 6miles away. Omni antennas do not have much gain.

Forgot to add that the old waverider 900mhz omni was virtical pol and 13dB. Dont know if they are still available though.

"the old waverider 900mhz"

Now there's a name I have not heard in a very long time.    The first 900Mhz system we tested was waverider. The omni was almost almost 10ft tall !