Adaptive tuning of microstrip antennas

The effect of the hand or body on printed antennas used in pagers and cellular telephone handsets is of concern as significant reduction of antenna efficiency can result due to changes in the input return loss. This change is due to the local conditions loading the antenna and changing its resonant frequency. Similarly effects may also be observed in fixed equipment when mounted in different environments. Varactor diodes can be integrated with printed antennas to change the resonant frequency. Diode tuning has been integrated with adaptive feedback to produce automatic retuning systems. In these arrangements the detuning is sensed by a directional coupler. The directional coupler needs good isolation to give adequate detector sensitivity. In principle this directional coupler could be associated with the antenna but more easily integrated into the transmitter circuit. Receive operation requires a directional coupler located within the receiver. In this paper we present an automatic retuning method in which the patch detuning is sensed by a detector diode integrated within the patch to form a more compact arrangement. Thus in principle our arrangement is suitable for transmit only. However it may also be appropriate in time division duplex systems or those where the transmitter can be periodically switched on. In arrangements where this is not possible or receive only systems, then a feedback signal from the receiver will be needed