Abstract: An arrangement to determine whether a tennis ball is "in" or "out" on a playing surface, which includes both a transmit coil and a receive coil beneath the playing surface and a modified tennis ball to have a non-zero magnetic permeability, so that an alternating signal can be transmitted from the transmit coil and re-radiated from the tennis ball, which can then be detected by the receive coils, and can be synchronously demodulated and resistive and reactive components of the signal analysed to discriminate the ball from other influences. The receive coils can be wound to avoid electro-magnetic interference from far fields.

Abstract: A means and arrangement to assist in determining the location of a ball relative to a line of a game surface is disclosed. The arrangement utilizes transmit and receive coils buried beneath lines on a game surface. Overlapping transmit coils operate at different frequencies and include filtering means to make each transmit coil substantially independent of the other transmit coil.