I have always set up a cluster in a small round ring. I am now looking at putting it out on the points of a triangle platform. The platform is 12 feet point to point. If I put two at each point covering 120 degrees, about how much deadzone will I have till they cross with the 120 dregree ones on the other point.

Assuming there is NO signal outside the 60 degree beamwidth, there will always be a 12 foot gap between the signals and they will never cross. This is of course also assuming that the devices are perfectly aligned at the corners of the triangle and make a perfect 120 degree arc. - Drawing not to scale.
The pic above is my quick Microsoft Paint version of what I think your connection will be like.


I don’t think this will be a problem.

The 60 degree number is the angle of radiation of the -3dB point (half power) from the azimuth, or about 30 deg on each side of the center line. The actual radiation pattern is much wider, decreasing as you get farther from the C/L and determined by the radiation pattern of the antenna, the distance the antenna is mounted from the tower etc. So there will always be signal between sectors, even with 12 feet separation.


rbobbitt is right on this one. You will not have a problem at all.