Antenna resonant circuit

A parallel antenna resonant circuit, operable to transmit and receive, and having a selected transmit resonant frequency and a selected receive resonant frequency, comprising:

• an antenna coil for transmitting and receiving HF signals, said antenna coil also operable as a first resonant frequency-determining element;
• a second resonant frequency-determining element connected in parallel with said first resonant frequency-determining element;
• a series combination of a third resonant frequency-determining element and a threshold value switch, said series combination in parallel with said first and second resonant frequency-determining elements, said threshold value switch operable to be electrically conductive in response to said transmitted HF signal, during a transmit mode, thereby determining the selected transmit resonant frequency to be a function of the parallel combination of said first, second and third resonant frequency-determining elements and said threshold value switch operable to present a high resistance to said received HF signal during a receive mode thereby effectively eliminating the effect of said third resonant frequency-determining element from said resonant circuit, and determining the selected receive resonant frequency to be a function of the parallel combination of said first and said second resonant frequency-determining elements.