Ask Dr. SETI ®

Phil, Australia

The Doctor Responds:
The rule of thumb for any parabolic reflector is that the surface inaccuracies should not exceed a tenth of a wavelength at the operating frequency. If, for example, you are going to search at the hydrogen line (21 cm), that means you cannot deviate from the parabolic curve by more than 2 cm at any point. (That's not too difficult a tolerance to hold, which is why some L-band dishes are made out of hardware cloth or poultry wire). We have people searching at much higher frequencies; their tolerance is correspondingly tighter. For example, at 10 GHz, the wavelength is 3 cm, so the dish has to be accurate to 3 mm. (This is compatible with a window-screen surface, but be sure to use copper, not plastic, screening!) And at 24 GHz (1.25 cm wavelength), it's necessary to keep all points on the surface of the dish accurate to within about a millimeter, to achieve full rated antenna gain. So you see it's easier to achieve reasonable antenna performance at the lower SETI frequencies.

Dish material doesn't much matter, as long as it's RF reflective. It's normally a matter of what's available, and what materials you're skilled to work with. But bear in mind that fiberglass-structure dishes still need a metallized reflector surface -- this is often sprayed on over the fiberglass.