I was in an antenna laboratory, and I saw a spiral antenna with a sort ground plane, about 0.5 cm behind the antenna (probably a reflector as the antenna was not connected to it.)
Is that a known technique? Is such a reflector thought to increase the gain of spiral antennas in their view direction?
In general, ground planes on antennas are used to effectively double the length of the antenna. This is done by the properties of what we call "Image Theory" or "Method of Images" and can turn a quarter wavelength monopol into a half wavelength dipole.
However, in the case of endfire helical antennas, such as the one you are referring to, the exact geometry of the ground plane can have a significant effect on the gain (to the tune of ~4dB) as documented in this paper.
