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[RFI] Cable Modem Interference

To: <rfi@contesting.com>
Subject: [RFI] Cable Modem Interference
From: dgsvetan@rockwellcollins.com (dgsvetan@rockwellcollins.com)
Date: Mon, 3 Dec 2001 10:29:37 -0600

Finding that your problem is the wall wart is probably one of the best
"cures" to have.  Recently, I was having problems with excessive noise on
10m, and traced that issue to a noisy wall wart that powers a video box.

There are plenty of alternatives to using your big Astron to power the
modem.  Since you seem to have a "clean" situation when powering the modem
with a linear power supply, any linear wall wart should be usable.  The
trick is to find one that is regulated (most are not - relying upon
regulators within the powered product to do the job of voltage
stabilization).  One surplus equipment house that I have had success with
on a variety of wall wart requirements is Marlin P. Jones Associates.  They
have a website.  There are others, of course.  Also, Radio Shack had a neat
little 2.5 A power supply (not a wall wart, but small desk unit) that is
rated for 13.8 VDC output.  I have one, and I believe there is an internal
pot to adjust the output voltage.  I have seen these frequently at hamfests
for $5 to $15.  One catch:  the metal case is riveted together, so you have
to drill out the rivets to gain interior access.  I paid $5 to a friend for
mine, and had to open it because it quit working.  The problem was a poorly
soldered connection.  I re-assembled with sheet metal screws.

One other caveat when buying a wall wart from a surplus house or hamfest:
beware the polarity of the output plug.  Make certain it matches the
polarity requirement of your modem (usually molded into the case right
adjacent to the power jack).  If there is a mis-match, you will have to cut
off the existing plug of the wart and install a new one with correct
wiring.  Since linear regulated warts are not the most common critters, I
would mainly concentrate on finding the proper wart and not worry about the
connector too much.  (You will find that most linear wart supplies are
"brute force" devices, containing just a transformer, diodes, and a high
value cap.  They typically produce 18 to 20 volts with no load, and fall to
the rated 12V only at or near full rated load.  If all else fails in your
search for a regulated wart, you could find a non-regulated one that
produces at least 15V at the required 1.25A, and then add a separate 7812
(or similar) regulator chip - with associated by-pass caps on a small perf
board -  to protect the modem.)  Issues such as the connector type and
polarity can be addressed after you get the supply.  Radio Shack, and
others, carry a variety of DC power plugs.

Incidentally, if you are into experimenting, you probably could make things
work with the present switcher wart, but you would need to open it up,
remove the guts, and integrate it into a metal enclosure (such as a
mini-box), complete with filters for both the AC line input and DC output,
and equipped with a 3rd prong grounding pin.  A fair amount of work that is
probably more than you want to do (and certainly more than the wart is

Good luck.

73, Dale

Phil Duff <na4m@arrl.net>@contesting.com on 12/01/2001 08:35:11 AM

Sent by:  rfi-admin@contesting.com

To:   rfi@contesting.com

Subject:  Re: [RFI] Cable Modem Interference

At 08:14 11/29/01 -0600, dgsvetan@rockwellcollins.com wrote:
>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
>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.

The cable modem crud is coming in the antenna system.

Last night I determined that the crud from my cable modem is actually
coming from it's power-supply.   It's a small 12V 1.25a. output deal that
is manufactured offshore and is supplied with the cable modem.  Obviously
a switching supply.   With it removed and using my Astron
13.8VDC supply to feed the modem there is no crud in my FT1000MP
on 75m.   I was a little concerned feeding 13.8V into the modem not
knowing if the modem would have a problem with the extra 1.8V input.

I made a small "sniffer probe" of a few turns of bare wire on the end of
some coax attached to my FT1000MP and when placed next to the
plastic case of the power supply the crud jumps to 20db/S9
over a small 75m frequency range - i.e. 10-20khz that wanders
up and down the band.  The crud usually drifts across the 75m DX window
just when
some weak DX is transmitting of course

I tried ferrites on input/output lines, noise filters on AC input, and
even wrapping the entire power-supply and input/output cables completely
in aluminum foil.  None of these helped.

I may visit my cable provider office soon to see if they have a different
to try.  The risk here is that it may be worse than this Terayon I have

It may be best to just feed the cable modem 12VDC from a good clean
non-radiating power supply like my Astron RS35M.  The modem is supposed to
stay powered on continuously and I'd like some alternative to leaving my
Astron being powered on all the time to feed this modem.

73 Phil NA4M

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....- --
Phil Duff NA4M     na4m@arrl.net   Georgetown, Texas

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