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Transmit Radio Signals via Ethernet

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Re: Transmit Radio Signals via Ethernet

#3

Post by zsellera »

As someone recently participated in an EMC measurement, I really don't understand how anyone passes these tests without some kind of cheating (using double-shielded, very expensive industrial cables + hacking with functional earthing).

Re: Transmit Radio Signals via Ethernet

#5

Post by zsellera »

_joel wrote: post #2

Would the cable length affect the tuning or is 125MHz specific to just the switching on the silicon?

I think what happens is the transformer inside the rj45 connector couples a litte of the differential mode signal (that should not radiate significantly) to all wires in the cable harness (common mode), which will radiate. You can select the cable length so it resonates well, acting like a good antenna, but bad antennas radiate as well (worse efficiency). A few mV of noise is quite well detectable from 10m of distance.

Re: Transmit Radio Signals via Ethernet

#6

Post by _joel »

zsellera wrote: post #5
_joel wrote: post #2

Would the cable length affect the tuning or is 125MHz specific to just the switching on the silicon?

I think what happens is the transformer inside the rj45 connector couples a litte of the differential mode signal (that should not radiate significantly) to all wires in the cable harness (common mode), which will radiate. You can select the cable length so it resonates well, acting like a good antenna, but bad antennas radiate as well (worse efficiency). A few mV of noise is quite well detectable from 10m of distanc…

Thanks for that explanation, makes sense.

Re: Transmit Radio Signals via Ethernet

#8

Post by jcims »

_joel wrote: post #6
zsellera wrote: post #5

Earlier quoted context omitted.

I think what happens is the transformer inside the rj45 connector couples a litte of the differential mode signal (that should not radiate significantly) to all wires in the cable harness (common mode), which will radiate. You can select the cable length so it resonates well, acting like a good antenna, but bad antennas radiate as well (worse efficiency). A few mV of noise is quite well detectable from 10m of distanc…

Thanks for that explanation, makes sense.

zsellera covered the mechanism of transmission very well. The 125MHz frequency is specifically coming from the Ethernet standard, as its essentially the operating frequency of each pair in typical hundred megabit and gigabit Ethernet. 10Mbps Ethernet operates at 10MHz.

By flipping the port speed between 100 and 10Mbps they are essentially toggling the 125MHz oscillator. I don't think anything is intentially generating the frequency of the tone you hear, it's just intermodulation products of the noise and the LO in the receiver.

Re: Transmit Radio Signals via Ethernet

#9

Post by skunkworker »

hansjorg wrote: post #4

Very nice, could be used for exfiltration with some tuning. The most advanced example of this kind of inadvertent transmission I've seen is Fabrice Bellards DVB-T transmitting with a standard VGA card: https://bellard.org/dvbt/

I was thinking the same thing, have it exfil a secret key out of a server room (theoretically).

I also got reminded about the method to send data from one computer to another over low frequency sound https://www.extremetech.com/computing/171949-new-type-of-aud...

Re: Transmit Radio Signals via Ethernet

#10

Post by coderjames »

Please don't try this at home! 118 MHz - 137 MHz is a protected Aviation band across the globe for airplanes to communicate with air traffic control. We already have enough industrial noise problems in this band; please don't contribute to pollution of protected spectrum. You will be interfering with the safe operation of the airspace.

See: https://en.wikipedia.org/wiki/Airband

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