Full characterisation of varactor-loaded, probe-fed, rectangular, microstrip patch antennas

A thorough investigation of the radiation characteristics of a probe-fed microstrip patch loaded with a varactor diode is presented. The theoretical analysis used to model this structure incorporates the full-wave spectral domain integral technique in conjunction with an equivalent circuit model of the varactor diode. Attachment modes are used to accurately model the current through the feed and load. Very good agreement between theory and measurement was obtained. From this investigation, the advantages and drawbacks of using varactor diodes to improve the radiation characteristics of a microstrip patch antenna were established. It was shown that large effective bandwidths can be achieved using varactor diodes, although at the expense of reduced antenna efficiency and increased cross-polarisation levels