Abstract: A proximity sensor can be used with a device such as a monitoring tag or portable tracking device that relies on GPS signals or other radio-frequency signals. The proximity sensor determines if the signals are being deliberately blocked by an object such as metal foil being placed in proximity to the device. The sensor includes an oscillator, a directional coupler and an antenna. A third port of the directional coupler acts as a coupled port when RF power that has been transmitted by the antenna is reflected back to the antenna by a blocking object. The reflected RF power passes along a main line of the directional coupler and a proportion is coupled to the third port. A detector provides an alert if the magnitude of the power of the transmit signal that is reflected back to the antenna in the form of a receive signal exceeds a threshold.

Abstract: The invention provides a monitoring tag for wearing about the appendage of an individual. It includes a self-contained transmitter module that is located in a housing and transmits radio signals to a base unit, for receipt by the base unit if within a predetermined range. If the individual wearing the monitoring tag moves outside this range then the base unit can trigger an alarm or send a warning signal. The housing has an opening for receiving the end of a flexible elongate strap to secure the tag around the appendage of the individual. The strap is attached to the housing inside the opening using a clip, which has structure for fixedly securing it to the strap and structure for fixedly securing it in the opening. Once the clip is attached to the housing it cannot be removed unless a weakened or frangible region between the securing structures is broken.