Unexpected Ethernet QRM

I made a quick-and-dirty directional Yagi antenna out of PVC pipe and measuring tape for some future Radio Direction Finding (“fox hunt”) fun. (Instructions here)

2-meter tape-measure Yagi antenna

Since I spent an evening sticking it all together, I had to test it out. So I connected it to a 2-meter (144MHz) radio, tuned it to an unused frequency, opened the squelch so I could hear the noise, and waved it around my apartment, watching the signal meter go up and down.

What I found was kinda surprising. Apparently I have a hot zone of radio frequency noise in a corner of my living room, right where the cable modem, router, and Gigabit Ethernet switch sit on a shelf. It’s typically not a problem when I’m working nearby repeaters and stations on FM (which is largely immune to this sort of noise), but if I’m doing SSB weak-signal or am using a handie-talkie at my desk, then it’s definitely going to be a problem.

So I asked the Internet what frequency the 1GB Ethernet standard (802.3ab 1000base-T) operates at. Turns out the signalling frequency is 125MHz, really damn close to the 144MHz passband of my radio’s receive filter. Thankfully, the voltage and current levels that exist in the cable and termination circuitry are relatively low, and are mitigated by the hashing effects of 4 differential twisted pairs of copper wire inside the cable keeping their electromagnetic impacts contained. But if I’m pointing a highly-directional antenna at a device that has an Ethernet port, I’m gonna hear something.

At this point, I’m not sure if wrapping every cable in a clamp-on ferrite or putting the entire networking stack inside a metal box will fix anything. But it’s something to consider if I intend to do weak-signal work with an omnidirectional antenna near that corner of the room (both of my 2m/70cm and 1.25m omni groundplane antennas outside are up on a pole 10 feet away through a wall).

If you look hard enough, you really can find QRM in surprising places.

Published by Shawn

He's just this guy, you know?