Abstract: A circuit mounted on a rotatable wheel of a vehicle varies its resonant frequency in accordance with tire pressure or another parameter measured on the wheel. The path traveled by the resonant circuit as the wheel turns extends along a coil mounted on the adjacent non-rotatable structure of the vehicle. A magnetic field is produced by the coil when excited by a cyclically varying interrogation signal and the force field interacts with the resonant circuit to the extent that its frequency matches that circuit's resonant frequency. The combined impedance of the coil and the resonant circuit thus vary with the frequency of the interrogation signal to indicate the inflation pressure. The coil may be a ring formed by a circular array of loops each wound about an axis that extends toward the path of the resonant circuit, by one or more elongated loops or a combination of coils that produce a similarly confined and directed field.