20110050446 | PROXIMITY SENSORS - 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. | 03-03-2011 |