11. These units were equipped with " Veeps " : jeeps having Very High Frequency SCR-193 crystal sets . 12. The foxhole radio is like a crystal set , in that it does not require an external power source. 13. Traditional crystal sets use half-wave rectifiers. 14. The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 20 kHz. 15. Early radio listeners, often using home made crystal sets and long wire antennas, found radio stations few and far between. 16. In recent years, the number of such believers has swollen, spilling from the Birkenstock-and-crystal set into the culture at large. 17. Some crystal set users added a carbon amplifier or a mechanical turntable amplifier to give enough output to operate a speaker. 18. This later gave way to the famous and more sensitive galena crystal sets . spark coil from Ford Model T automobiles. 19. It was used in the earliest days of radio, when only one or two stations were within a crystal set 's limited range. 20. The early earphones used with wireless-era crystal sets had impedance, often 75 ?, and required more current than a crystal radio could supply.
