Found this,
"Radar-Jamming Devices are Illegal (SB 1964) Use of devices capable of jamming, scrambling, neutralizing, disabling, or otherwise interfering with a radar, laser, or any other electronic device used by law enforcement to measure speed are illegal. In addition, such devices cannot be purchased, possessed, sold, manufactured, or distributed in California."
But you, know, it's only illegal if you're caught and punished
Owen