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Retrodirective Microwave Backscatter Tag

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M. Alhassoun, M. Varner, G.D. Durgin. “Theory and Design of a Retrodirective Rat-Race-Based RFID Tag“, IEEE Journal on RFID, vol 3, no 1, March 2019, pp 25-34.

Movement toward mm-wave backscatter communications in radio-frequency identification (RFID) systems necessitates seeking designs that compensate for the path loss introduced by the radio channel. A viable, simple, and power-efficient solution is to equip RFID tags with retrodirective arrays, which guarantee reflection with maximal gain in the direction of incidence. In this paper, we build upon a previous work to design and implement an original retrodirective tag in which its feeding network is a properly terminated rat-race coupler. We start by deriving the required terminations that ensure retrodirectivity. Then, we build a retrodirective tag and experimentally compare its radar cross section to that of a single-antenna tag. The measurements reveal that the radar cross section of the proposed retrodirective tag is, on average, approximately 6 dB more than that of a single-antenna tag while both tags have the same field-of-view. From the promising results in this paper, the proposed design is a potential candidate for next-generation microwave and mm-wave RFID tags because it is compatible with low-power reflection amplifiers (e.g., tunnel diodes), it can implement multiple modulation schemes without changing the circuit layout, and it can be implemented using only a single RF switch.

2014: The Microwave Anti-Skimmer

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2014: A team of GT students develop an anti-skimming solution for payment terminals using micro- wave backscatter. Successful demonstration is shown on various payment devices.

In this application of the 5.8 GHz backscatter system, a tag is placed near a card swipe machine in order to sense for the presence of foreign “skimming” electronics — devices that crooks put on payment terminals and ATMs to read card information from them.   The system works by using a reader to sense the return amplitude and phase of the backscattered signal, which is static as long as the environment around the tag does not change.  However, the introduction of even a thin nearby electronic wire will change this received amplitude and phase signature, which triggers an alarm.

Fun fact 1:  This work was tested at NCR headquarters in a giant room with over 100 different autonomous teller machines.

Fun fact 2:  A senior design team at Georgia Tech used the concept to make a through-wall museum alarm system.  Their real-time demo at the senior design expo was a crowd pleaser, as a large wooden “T” was placed on a table while observers were invited to steal it without setting off the equipment (which was nowhere in site).  The uniformed should consult the following Wikipedia page on Tech Tower.

Happy Spring Break, Students!

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The week on campus is very quiet, as most students have left for their Spring break week … hopefully to warmer, ocean-front locales. Safe travels, students!