Far-Field Sensor for Adaptive Antenna Tuning

Tuning circuits are often used for electrically small antennas. However, the nearby environment can detune the matching calling for adaptive tuning. In this paper we present a new concept to adaptively tune matching circuits using a near-field probe that samples the instantaneous radiated power. For an electrically small VHF/UHF antenna, it is shown that a properly designed near-field probe is capable of sampling the radiated power up to 400 MHz. However, the presented design guidelines can be used to extend the frequency range of interest. Advantages over traditional vertical bar S-11 vertical bar minimizing topologies include: (1) lower cost as only one power detector is required, (2) lower insertion loss in the antenna tuner as no directional couplers are used, and (3) flexibility in design degrees of freedom.