Radome to active element separation

Could any point me towards some guidelines for separation between active elements of UHF antennas (helix, yagi etc) and their radome? I was assuming half wavelength but I could do with coming in closer. Thanks.

Do you have a way of monitoring the S11 (reflection) from the antenna or signal detected by a receiver? You do run some quick experiments to see. I am pretty confident you could get away with a quarter wavelength or less.