RTL-SDR can receive signal from host device, but only when antenna is plugged in - Page 1
Forum section: electronics / rf-microwave

author: Ben321
author_rank: Frequent Contributor
timestamp: November 25, 2019, 08:47:02 pm
content: I have no way to test this with a desktop computer, since my faraday cage is so small, but I have tested it with a cellphone as host device (using a USB-C to USB-A adapter cable). I'm not talking about interfering signals from the USB device interfering with what I'm trying to receive. I'm saying one of its main signal pathways for receiving ordinary signals can be the host device. If I put the RTL-SDR dongle, and antenna and antenna cable all inside a faraday cage, and also keep all of the USB cable in the cage, and keep only the host device outside, I can still hear the signal I tuned to (I chose a local FM radio station for this experiment, as it's a known good signal).However, there's also something else weird going on. Assuming that the signal can also be received though the USB cable (or even the tracings on the PCBs in the phone) acting as antennas (and then the signal working its way from the USB side of the RTL-SDR over to the RF side), I disconnected the normal antenna from the receiver. However, after doing this, the signal disappeared completely, even though the phone and USB cable were not inside a faraday cage during this part of the experiment. So somehow the USB side and the antenna side of the RTL-SDR are working together in a way to receive the signal.Antenna shielded + dongle shielded + phone unshielded = signalAntenna unshielded + dongle shielded + phone shielded = signalAntenna unplugged + dongle unshielded + phone unshielded = no signalWhat is going on here?

author: TheMG
author_rank: Frequent Contributor
timestamp: November 26, 2019, 02:17:07 am
content: The device and USB cable are acting like a receive antenna, the RF signal is carried into the faraday cage via the USB cable while the RTL-SDR dongle itself and any portion of the USB cable/connector penetrating into the faraday cage is re-radiating that signal into the cage, where it is being picked up by the RTL-SDR dongle's antenna.It's actually fairly tricky to have cables of any kind penetrating into a faraday cage without carrying external RF signals into it. You'd need chokes and pass-through capacitors on each individual wire going into the faraday cage, while being careful not to compromise the integrity of the USB data signals.So, the FM radio station signal is not being coupled from the USB cable to the RF side side of the dongle itself. What's happening is it's being re-radiated inside the faraday cage, then being picked up by the antenna.

author: Ben321
author_rank: Frequent Contributor
timestamp: November 27, 2019, 05:40:41 am
content: Quote from: TheMG on November 26, 2019, 02:17:07 amVery interesting. I had no idea that was possible. Not sure if this has an impact, but the antenna in my test was actually touching the inner surface of the faraday cage. I tried to have it not touch, but I really had no way to mount the antenna where it was, so I just let it lay on the metal floor of the cage, which of course put it in electrical contact with the cage.

author: radiolistener
author_rank: Super Contributor
timestamp: November 27, 2019, 02:08:13 pm
content: Ben321, Faraday cage cannot remove RF interference completely. It just can attenuate it a little.For better protection you're needs very thick solid metal casing, which don't have any hole (even very little). And metal case should be very thick. For about 1-2 meters of thickness of solid metal without micro-gaps and without micro-holes. Completely solid metal. Then it will works good. But it still cannot eliminate RF interference at all, just a good attenuation

author: Ben321
author_rank: Frequent Contributor
timestamp: November 27, 2019, 07:45:38 pm
content: Quote from: radiolistener on November 27, 2019, 02:08:13 pmIf you read my description it clearly stated that it was blocking completely, depending on which parts of the system were inside it. The answer provided by TheMG was a LOT better answer, and actually explained very accurately why what was happening was happening. Your answer though, Radiolistener, is incorrect.