To get a full wavelength apart on UHF you will only need to be about 30 inches. Now I may not be completely correct here, but my assumption has always been that antenna spacing was only critical if you were pointing the two in the same direction for increase gain. I have used two antennas before, in separate directions, and had them quite close together, without problems. As a side note, I've used many of the Stellar Labs 30-2155's and love them. Especially the new design released earlier this year.