Mode diversity of weakly modulated cavity antennas.

The radiating mode of a cavity antenna at a particular frequency is fixed. However, by actively modulating the permittivity inside the cavity, the radiating mode may be changed. Using time-independent perturbation theory, we derive the modes of a cavity perturbed by many modulating elements. It is found that with a sufficient number of modulators of sufficient strength, the number of unique fields radiated by the cavity may reach a limit determined by the number of unperturbed cavity modes. The number of addressable radiated fields increases exponentially with the number of modulators; however, perturbations involving the interaction of several modulators become progressively weaker. For antennas at millimeter and terahertz frequencies, such cavity antennas can realize a great diversity of radiation patterns using fewer active devices, better exploiting the diversity achieved by each added modulator.