[RFI] Cable Modem

Karin Johnson karinann at tampabay.rr.com
Fri Nov 13 16:23:54 EST 2015

Hello All:

I've recently debugged an RFI problem I was having with a cable modem.

I reluctantly did not write down the model number but it was made by

Arris.  It was of the type that includes battery backup for a cable

as well as the normal Ethernet via cable.  It is powered from the AC line.

I noticed three distinct broadband humps around 3510 kHz that were about

20 dB above the noise floor.  Unplugging the modem and the humps/RFI

I tried wrapping the AC line cord, Ethernet cable, and input coax around a
type 43 toroid core.

I did try a different number of turns from four up to ten and all in
between, and could not

get rid of the RFI.  I might note that the antenna (40 meter dipole) I have
picking up the

RFI was only about 15 feet away from the modem and the cable from the
distribution point

on the street runs only about six feet away from the 40 meter dipole.

I have two other antennas located about 60 feet away from the modem and the
RFI was still

observable on the spectrum analyzer but only down to about 10 dB above the
noise floor.

I had the cable company come out to replace the modem with another type that
had a separate

walwart.   The new modem is made by Ubee, model U10C022.  Initially when it
was installed I noticed

that overall noise floor as observed on my spectrum analyzer rose about 6 dB
compared to when the 

Modem was not powered.  So different modem same type of problem, Bad RFI.  

Here was my cure with the new modem.  I took a type 43 toroid core and
wrapped 7 turns around the

coax input to the modem.

This eliminated the broadband noise observed at the lower end of 80 meters.
Check other bands 

I noticed a discrete broadband hump around 7040 kHz, about 20 dB above the
noise floor.  The new modem

comes with a separate walwart switching type power supply.  I replaced it
with a linear supply and wrapped

the DC lead supplying the modem around another type 43 ferrite.  Now all is
quiet.  No observable RFI 

as far as I can tell.  Now the question to the assemble multitude.  Anyone
know of a QUIET cable modem?

I am actually paying the cable company about 2 dollars a month to "rent"
their junky modem.  If I can find

a quiet modem I will purchase one myself and save the extra 2 dollars a

So for now I've solved my RFI problem with the cable modem.  This exercise
sounds like the proverbial 

"Pin 1 Problem" that K9YC talks about.  



Karin Anne Johnson  P.E.   K3UU

Palm Harbor, FL 

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