Adaptive feedback frequency tuning for microstrip patch antennas

Antennas for hand-held portable equipment must be lightweight and compact. Microstrip patch antennas meet these requirements and, in addition, their manufacture using printed circuit technology allows easy integration with the transceiver's RF-circuitry. However, the main disadvantage with the low-profile microstrip patch antennas is their inherently narrow input bandwidth. Although, the input bandwidth may not be wide enough to cover all channels for a particular application, the bandwidth is usually perfectly adequate for a single channel requirement. For these applications frequency tunable antennas can be employed. Such antennas, using varactor diodes have been reported with operating bandwidths exceeding 20%. However, the antenna is a part of a system that will be operating in a time-varying environment. This environment may de-tune the feed-antenna match due to temperature changes or coupling and reflections from local objects. An adaptive feedback is introduced that is intended to optimise the feed-antenna match, and therefore act against any change in input impedance of the patch antenna. It is shown that the adaptive feedback gives a good match at any desired frequency inside the operating frequency range of the microstrip patch antenna, when the antenna is subjected to a local disturbance.