Passive short range X-band frequency doubling transponder

Various radar methods exist to identify and distinguish stationary and moving objects. Well known methods include synthetic aperture radar (SAR), millimetre-wave real aperture radar (RAR) and interferometric synthetic aperture radar (IFSAR) systems. In specific cases, one is interested to recognize a unique signal from objects. For example, in search and rescue operation radars, unique signals are identified from the Search and Rescue Transponder (SART) that is attached to the object of interest. However, unwanted clutter echoes can severely limit the detectability of the target. Clutter can be caused by obstacles, rain and surface reflections, is unpredictable and it cannot be easily removed. This paper presents the development of a low-cost, passive X-band transponder using frequency doubling to eliminate the problem of clutter. The passive transponder comprises of two rectangular patch antennas, a bandpass filter and a passive frequency doubler. Theoretical calculations show that such a passive transponder is able to operate at a range of about 1 km away from the radar.