Receiving Unintentionally Radiated Signals from the Computer System Bus with an RTL-SDR
Back in 2018 we first posted about "System Bus Radio" which is code and a web based app that allows you to transmit RF directly from your computer without any transmitting hardware. It works on the principle of manipulating the unintentional RF radiation produced by a computers system bus by sending instructions that can produce different AM tones. The idea is to demonstrate how unintentional radiation from computers could be a security risk.
Recently the creator of System Bus Radio has uploaded a guide on receiving the generated signals with an RTL-SDR. He recommends using an RTL-SDR with upconverter, balun and an AM loop antenna. He then shows how he was able to receive the signals from his MacBook Pro M1, noting that he was able to receive audible signals from several inches away at frequencies between 63 kHz to 5.5 MHz.
This made me chuckle remembering myself at age 12 (1985). The family bought a Commodore 64 and I quickly discovered that if I put a transistor AM radio near the C64 (about 6-10 inches) and played with the dial it would would pick up the sound sent from the computer to the monitor wirelessly LOL. …. no wired speakers necessary for sound.
At 5.5MHz near field (within 1 wavelength) would be about 55 meters (~180 feet)
His large loop of wire, almost in direct contact with the machine IS part of the circuit, it is acting like the secondary coil of a transformer! There is direct inductive coupling.