Powerless radio - Page 1
Forum section: electronics / rf-microwave

author: ve7vie
author_rank: Contributor
timestamp: January 25, 2023, 03:56:19 am
content: Seehttps://theconversation.com/device-transmits-radio-waves-with-almost-no-power-without-violating-the-laws-of-physics-196271Including the 'ambient backscatter' video linked there.Opinions?

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 25, 2023, 06:24:48 am
content: Sounds bogus, but if it did work as described, it's range would so short that it'd be useless.

author: szoftveres
author_rank: Regular Contributor
timestamp: January 25, 2023, 06:32:32 am
content: 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.Alsohttps://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.

author: Berni
author_rank: Super Contributor
timestamp: January 25, 2023, 08:47:54 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2023, 08:50:55 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 25, 2023, 09:37:01 am
content: https://en.wikipedia.org/wiki/The_Thing_(listening_device)

author: yl3akb
author_rank: Regular Contributor
timestamp: January 25, 2023, 10:06:33 am
content: Quote from: Berni on January 25, 2023, 08:47:54 amMy first thought is that this experiment works because of good coupling between the directional horn antennas they use. It forms kind of beam waveguide, so that receive side is relatively strongly coupled to TX side and we get similar effects as TX and RX side would be connected with transmission line. And of course, if load is modulated at TX side, that slightly modulates gain and NF of RX LNA due to input mismatch which is detected as amplitude modulation. Input matching even doesnt have to be modulated - one can modulate noise temperature at TX side and that also would be detected as amplitude modulation (see the test with liquid nitrogen cooled resistor they mention in publication).All this would not work in case of low gain antennas such as dipoles and other.

author: tszaboo
author_rank: Super Contributor
timestamp: January 25, 2023, 10:21:09 am
content: That's good, because a CR2032 battery that costs like 10 cents in quantity, and makes an RF transmitter work for years was really the bottleneck of such systems.

author: jonpaul
author_rank: Super Contributor
timestamp: January 25, 2023, 10:29:01 am
content: 1960s USA Moscow embassy...Soviet "gift" of seal OF USA, contained passive UHF cavity microphone eavesdropping device.When illuminated by a beam of radio transmission at the cavity frequency, a réfection was emitted with sound modulated.This the KGB, GRU, GPU, Spied on USA for many years before the mendacidty of the Russkis was exposed.Similaires to the laser beam off the window spy microphone.Jon

author: Berni
author_rank: Super Contributor
timestamp: January 25, 2023, 10:36:52 am
content: Yeah the issue with backscater like this is that the reflected signal sits right on top of the 'carrier', so it only works if the reflected signal is large enough to meaningfully change the amplitude of the large carrier signal back at the antenna transmitting it. Only way to get that is to have very good coupling between the antennas.This is why RFID is very short range as it uses the same kind of principle of applying a load on the antenna coil. Sure there is a limit set by how much power the RFID tag electronics needs to harvest to actually work, but you can't actually build a massive 1 kW RFID reader and read tags from the next building. The tags electronics could harvest enough power, but the backscater signal would be so tiny that you couldn't make it out from the main power carrying signal that is being sent out.The better way to do it would be to connect the backscatter antenna to a RF mixer and feed it a high frequency LO tone, this would mix the power carrying signal into side bands around it that leak back out of the mixer and back out the antenna. On the other side you can then filter out the power signals frequency and amplify up just the frequencies in the sidebands for much better receive sensitivity. But at this point this is more of a scavenged RF power powered active transmitter.

author: yl3akb
author_rank: Regular Contributor
timestamp: January 25, 2023, 10:59:00 am
content: I dont think their experiment can be explained completely and compared with RFID, backscatter, 'The Things', etc. because all of them requires actively generated external RF signal, but their setup dont.

author: mawyatt
author_rank: Super Contributor
timestamp: January 25, 2023, 02:26:56 pm
content: DARPA had a related program called N-ZERO.https://www.darpa.mil/program/near-zero-rf-and-sensor-operationshttps://www.darpa.mil/news-events/2015-04-13https://eri-summit.darpa.mil/docs/ERIPoster_Applications_NZERO_UVA.pdfBest

author: mawyatt
author_rank: Super Contributor
timestamp: January 25, 2023, 02:39:19 pm
content: Quote from: jonpaul on January 25, 2023, 10:29:01 amThe "gift" feature was eventually uncovered because the US embassy was periodically being "illuminated" by high levels of RF/MW energy for unknown reasons, causing concern and eventual discovery. The technique was a coherent radar where the "audio" was encoded as phase modulation for the reflected return signal. Clever rendition of the well known remote laser "microphone".Many devious and clever techniques were employed on both sides, likely still areBest,

author: jonpaul
author_rank: Super Contributor
timestamp: January 25, 2023, 03:01:39 pm
content: "the Thing", invented by Léon Theramin, the same man with the 1930s valve touchless electronic instruments.Pix of seal gift of Soviet schoolchildren ( and NKVD)pix of Averel Harriman exposing the 7 years of Soviet violation of US Embassy at United Nations.Jon

author: Bud
author_rank: Super Contributor
timestamp: January 25, 2023, 05:37:56 pm
content: Is that a cooking oil container that they in the article used for the cavity ?

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 25, 2023, 09:10:40 pm
content: This is not ambient backscatter.This is similar to what goes on in "cold source noise figure measurement".Changing the source impedance (by using the switch) is just creating a mismatch that changes the noise power received.

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 25, 2023, 09:33:21 pm
content: Consider measuring the noise power out of an LNA with the input connected to a cable. Measure the noise power with the cable connected to 50 ohms. Then measure with the cable shorted. The noise power will change. So replace the cable with an antenna link. That's all that is happening here.

author: Bud
author_rank: Super Contributor
timestamp: January 26, 2023, 07:44:50 pm
content: Where do they get the power required to operate the switch?What is the point of having a powereless transmitter if the power needed to operate the switch could instead be used to power the transmitter?

author: antenna
author_rank: Frequent Contributor
timestamp: January 26, 2023, 08:00:29 pm
content: There are posters like this in some cities. There is a small detector that harvests a small amount of RF power from the antenna to run an audio chip that shorts the feed point of the antenna at an audio rate. When you are listening to the same radio station the poster is tuned to, you can hear the audio from the poster over the broadcast signal. I have been wanting to build one of these for years now.

author: antenna
author_rank: Frequent Contributor
timestamp: January 27, 2023, 09:09:31 am
content: Are they heating the resistor with DC current or are they freezing their receiver? The thermal noise in their receiver would be the same if not more otherwise, then add path loss....the dB's don't add up unless energy is put in elsewhere like cooling. Are they dreaming of some miniaturized mesh network embedded in everything or something? What else could be the goal there?

author: yl3akb
author_rank: Regular Contributor
timestamp: January 27, 2023, 12:50:05 pm
content: No need for heating or cooling. As already mentioned previously, it is a effect of modulating the source impedance of receivers input amplifier detected as slight amplitude modulation. The 'original' part here is that modulated source is 'connected' to receiver using closely coupled horn antenna link instead of transmission line.I would not agree with this this statement:Quote from: antenna on January 27, 2023, 09:09:31 amThey use relatively high gain antennas, which effectively attenuates signals, including ambient thermal noise from directions other than bore-sight. See classic integral formula here:https://www.antenna-theory.com/basics/temperature.phpBy placing such antennas directly facing each other, most of the above integration in case of RX antenna would be dominated by aperture of TX antenna, and that means that we can actually cool and heat the TX resistor, and that would also be detected as amplitude change even if ambient thermal noise is say 300 K. As matter of fact they did such experiment (see their paper) using liquid nitrogen cooled resistor.All this is nothing new, papers/'discoveries' like that slip through all the time.