The AM Forum

To-day was warm and pleasant outside, so I did some serious rf sniffing, and located the source of the mysterious tone-modulated carrier every 30 kc across the spectrum from 3 to 30 mc!

It pays to check your own house first.  It turned out to be my no-name cable modem.  While nothing else affects it, unplugging the modem from the ac mains completely kills the signal.

So far, that's the only way I can kill the noise, though.  I tried an ac line filter on the power cord, and tried clamp-on ferrite chokes on both the power cord and coax cable, with no noticeable effect on the interference.  I have the model number and serial # of the unit, but no manufacturer's name on the unit itself or the box it came in.  I sent a letter to the  local cable company, and will wait to see if they offer any assistance.

One problem is that my computer room is located upstairs in what used to be my hamshack, so I suspect that every electrical conductor in the vicinity of the computer acts as an antenna. The UPS unit generates its share of digital rubbish as well.  

There may be no solution short of  building a completely shielded computer room, on the ground floor, lined with copper screen.  (Hey Marty, can I call on you for a contribution?)

Don,
Why not put it in a shielded box with beads on the leads. Feed through filters on the power leads. Sounds like switching power supply noise.

Don,
Is there an FCC ID tag on the modem?  If so, you may be able to find the manufacturer that way via the FCC database which is on the FCC website.

Don, the signal could be radiating from the data lines.  Try a bunch of chokes on everything.   In my experience one small choke doesn't do it.  You may want to try a brute force L/C line filter - some of those "line strip" line filters are worthless.

73 John

My name brand (Motorola) cable modem broadcasts like crazy on 3880 and 3880/2. I wrapped the power cord  through a RS snap-on, as many turns as I could get then did it two more times with two more chokes.  This helped a lot but not enough.

I built a special Ethernet cable sans insulation so I could get a lot of turns through a mix 77 torroid.  This helped not a whit.  It's very pretty, though, with all those colorful twisted pairs wound on the donut.  

I'm thinking of shielding the box with copper screen.  I'm also thinking of modifiying it with a sledge hammer.

Moto gave me a legalistic repsonse when I finally got through to them.  I should have saved it for everyone's amusment.


Jon

Aluminum screen will work just as well.

Had the same problem with two Motorola Cable Modems 20 over 9 noise all across 80 meters. Shielding with aluminum screen and grounding the screen to the cable shield helped a lot also had clamp on ferrites on the power cord and 10 base T cable to computer droped noise to S5 over 9. Fortunately the modems are provided by Time warner cable and after making a lot of noise they changed it to a Moto Dx-100 no more interference then that one failed so now I have A moto SB4100 and no noise with this one either.