Tunable antenna system with controllable reactive circuit element - has level detector circuit producing signal which contains component at second harmonic frequency of dither signal

摘要

The antenna (12) is tuned by the controllable reactance (e.g. varactor diode) (14) and tuning circuit (18) in which typically a local oscillator (24) is tuned by a frequency synthesiser (26) to produce a superheterodyne signal (28). The tuning signal is the sum (68) of a frequency approximation component (67) for coarse tuning to the desired signal, an error component (56) for fine tuning, and a dither component (62) which is generated in a second-harmonic detector circuit (50, 52, 54). ADVANTAGE - Length and breadth of antenna are substantially less than a quarter of the wavelength throughout range of operating frequencies.