Low-loss tuning circuits for frequency-tunable small resonant antennas

Minimization of power loss in the tuning circuits of frequency-tunable small resonant antennas, such as microstrip patches, is studied. First, the frequency shift and the associated power loss in certain tuning circuits are theoretically calculated based on approximate circuit models of fairly narrowband resonant antennas. By investigating the ratio of frequency shift and loss, an optimal configuration for the tuning circuit can be determined. The theoretical results can be used to provide optimal starting values and design curves that facilitate the final design with a full-wave electromagnetic simulator. To support the theory, a design procedure is demonstrated with two example antenna structures, for which both simulated and measured results are presented.