This report describes both a general methodology and some specific examples of passive radio receivers. A passive radio receiver uses no direct electrical power but makes sole use of the power available in the radio spectrum. These radio receivers are suitable as low data-rate receivers or passive alerting devices for standard, high power radio receivers. Some zero-power radio architectures exhibit significant improvements in range with the addition of very low power amplifiers or signal processing electronics. These ultra-low power radios are also discussed and compared to the purely zero-power approaches.
This is *exactly* what we need at work right now. A way for the master unit to tell all the field units to "wake up" and prepare for incoming transmission. And it uses zero battery power. Even some of the powered designs only use microwatts of power, which you can easily get from entirely passive devices like a solar panel or a thermocouple.