[RFI] Cable Modem Interference

dgsvetan@rockwellcollins.com dgsvetan@rockwellcollins.com
Thu, 29 Nov 2001 08:14:45 -0600


Yes, meeting FCC specs still leaves a considerable emissions spectrum when
talking sensitive receivers.

A couple of checks might be worthwile.  For one, disconnect the antenna
from your receiver.  If you still have the "junk" from the modem, then you
know it is getting into the radio either by direct radiation (modem cabinet
and/or cables to radio cabinet/non-antenna cables) or by power line
conduction.  If the noise stops when the antenna is disconnected from the
radio, then you need to determine how the antenna is "seeing" the modem;
very likely, it is picking up signal from cables and/or the power lines
(which themselves can be excellent antennas, especially in the 1 to 10 MHz
region).  Relocating the modem, extensive use of beads on cables, tight
bundling of cables, etc., all become possible cures.

If the radio still gets noise with the antenna removed, then you need to
much of the same sort of thing to determine the coupling mechanism.  (Be
certain to disconnect auxilliary cables, one at a time, that go to/from the
radio, as you might have one that suffers more pick-up than the other
ones.)  Consider the possibility that the modem may have to reside in
another, more distant, part of the house.  Good luck.

73, Dale

Phil Duff <na4m@arrl.net>@contesting.com on 11/28/2001 06:02:03 PM

Sent by:  rfi-admin@contesting.com

To:   rfi@contesting.com

Subject:  Re: [RFI] Cable Modem Interference

At 10:51 11/28/01 -0500, Peter D. Vouvounas wrote:
>Now that so many hams have cable modem I'd like to ask for feedback
>regarding experiences with noise generation into the HF bands.
>3.5 - 4 MHz.

Yep - I get crud off my cable modem also - especially noticeable on the low
Mine is provided by Cox Communication and is a "Terayon" unit.  I feed my
PC card directly from the modem - no hub, etc.

>  If I remove power from the cable modem the same major drop in
>noise. Tried wrapping the modem in aluminum foil and grounding with little
>or no effect.
>I've tried ferrites on the cables to no avail.  Should I move the cable
>modem from the Ham/Office location it would only bring it closer to the
>antenna systems on the roof.

I've tried similar remedies all to no avail.  Ferrites on power supply
input/output cords, cable modem coax, etc.
  My cable modem is located in the radio shack and the only effective thing
I've found with mine
is to unplug the modem power supply.

It's possible these things meet FCC specs but can still generate enough
noise to cause
interference in closeby sensitive HF radios?

73 Phil NA4M

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Phil Duff NA4M     na4m@arrl.net   Georgetown, Texas

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