Automatic Tuning of High-Q Antenna For VLF FSK Transmission

A solution to the problem of increasing the bandwidth of a VLF antenna without decreasing the efficiency is to automatically tune the antenna to the transmitter frequency. Automatic tuning is achieved by means of two nonlinear inductors connected in series with the antenna tuning coil. The average inductance of these inductors are made to vary in accordance with an error signal derived from the phase difference between the antenna current and transmitter voltage such that this phase difference is minimum. This feedback system automatically resonates the antenna to the transmitter frequency. A small-signal transient analysis shows that the bandwidth of this feedback system is limited to where ωais the bandwidth of the power amplifier that drives the nonlinear inductors.