Microwave Attenuation with Photoconductive Materials

The phenomenon of photoconductivity has been used to control the transmission of microwave energy in a waveguide with a beam of light. Samples were made from a mixture of "sensitized" cadmium sulfide powder and polymerized methacrylate, one form of which is known by the trade name "lucite." When these samples were centered as slabs in a waveguide, an illuminated attenuation of seven decibels was possible for a sample length of 0.9 inch and a width-to-waveguide-width ratio of 0.25 at 9.5 Gc/s. The insertion loss is low, less than one decibel. For very thin samples, a variational approximation is used to predict the insertion loss from the dielectric constant and conductivity of the sample. The response time for the cadmium sulfide sample appears to be about the same as that observed at low frequencies. This means of attenuation could serve as a useful means of instrumentation for decreasing microwave energy when variability is required, and when it is desired to have no mechanical or electrical coupling to the attenuation element.