Sensor nodes distributed over a large area can be used to collect environmental information such as chemical pollution. The sensed information can be communicated to a central location, without significant power consumption by the sensor nodes, by utilizing backscatter modulation. In this approach, a central source such as a radar illuminates the sensor nodes by microwave radiation. The reradiation from each sensor can be modulated by switching a load on the sensor antenna. This permits design of low-power sensors having a long battery life. To avoid interference at the radar from unwanted objects, harmonic reradiation can be used on the sensor nodes and the reradiated harmonic can be processed. A passive harmonic reradiator is designed to reradiate an incident 10 GHz electromag- netic wave at 20 GHz. The design is for low incident power density. The design comprises of receiving and transmitting integrated patch antennas, a low-barrier Schottky diode and double stub impedance matching. Measurement of the reradiated power, with a represen-tatively input low power density, showed promising characteristics for the use of harmonic reradiator as described above.

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