| 摘要 |
A proximity sensor is provided that can be used with a device such as a monitoring tag or portable tracking device that relies on GPS signals or other radio-frequency (RF) signals. The proximity sensor can be used to determine 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 (2), a directional coupler (4) and an antenna (8). A third port (6c) of the directional coupler acts as a coupled port when RF power that has been transmitted by the antenna (8) is reflected back to the antenna by a blocking object. The reflected RF power passes along a main line of the directional coupler (4) and a proportion is coupled to the third port (6c). A detector provides an alert if the magnitude of the power of the transmit signal that is reflected back to the antenna (8) in the form of a receive signal exceeds a threshold.
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