This Problem of the Day (PotD) post comes from a historically significant ECE test, from an RFID class that Manos Tentzeris and I developed 10 years ago. The question asks how a biased tunnel diode increases reflected power and range from a backscatter antenna. This idea of using an active, negative-resistance region device for backscatter has popped up in the patent literature from time to time, but Georgia Tech PhD Francesco Amato performed some of the first microwave backscatter measurements with his custom-built Quantum Tunnel Modulator (QTM) tag. This illustrated the feasibility of the idea, published in 2015. The use of a tunnel diode modulator for backscatter has been investigated by researchers in UI Chicago, Sweden, and more Sweden.