Early form of battery-free radio receiver powered by radio waves?

Answer 1

Among the earliest radio receiving devices, the Crystal set, sometimes called a cat's whisker detector, utilized a small piece of Galena crystal [a Lead mineral] that converted high frequency radio waves into audible sound. The device was self-powered and did not require any external energy. In its most basic form, the radio consisted of a set of headphones, the crystal, the "cat's whisker", a wire antenna, and a ground wire. The cat's whisker (a fine metal wire mounted on a lever) was positioned over the crystal and various contact points were tested, until the crystal produced the loudest reception of the incoming radio signal. The crystal set had very limited selectivity (the ability to separate stations from one another) and generally the strongest station could be heard on top of other stations. The addition of a tuning coil provided for limited ability to separate these stations from one another.

In the late 1950s/early 1960s, I recall a do-it-yourself project in the now-defunct "Popular Electronics" magazine, to construct an updated crystal set using dual diodes (the modern replacement of the original crystal) that was capable of self-powering a small speaker, something the original crystal sets were unable to do.