Electronics > RF, Microwave, Ham Radio

Powerless radio

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See https://theconversation.com/device-transmits-radio-waves-with-almost-no-power-without-violating-the-laws-of-physics-196271 

Including the 'ambient backscatter' video linked there.


Kim Christensen:
Sounds bogus, but if it did work as described, it's range would so short that it'd be useless.

The idea of a passive device communicating by modulating the surrounding RF field is not new, this is pretty much how RFIDs work. The RFID tags however are supplied by substantial RF power (relative to their power consumption) in order to be able to function and communicate.
Also https://en.m.wikipedia.org/wiki/The_Thing_(listening_device)

The LED demonstration in the video is 100% snake oil, they've essentially created two RFID devices, just don't show the RF power source in the video.
The small dipoles have to be supplied by a nearby RF power source (outside of the camera frame) transmitting at the resonant frequency of the dipoles for this demo to work, as opposed to the suggested "ambient backscatter" they claim to be using.

This is same thing as backscatter radio. Nothing new there.

By switching a load resistor to the antenna terminals they are changing how much RF energy the antenna is reflecting. With the resistor connected the antenna takes in the radio waves and turns them into heat on the resistor, when the resistor is disconnected the RF power has nowhere to go, so it bounces back into the antenna and gets radiated back out. The other side then just measures how much power is being reflected back to it, getting the information out of it.

This is indeed what RFID does.

Seems like I read about a Cold War era passive espionage transmitter, I don't recall the details on how it worked but I'm sure there are some pages about it scattered around the web.


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