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[RFI] Cable Modem Interference

To: <rfi@contesting.com>
Subject: [RFI] Cable Modem Interference
From: wb3fsr@home.com (Peter D. Vouvounas)
Date: Sat, 1 Dec 2001 09:57:56 -0500

That's great news narrowing your issue down to the power supply!!!!  I'm
ready for the Comcast cable guy to arrive tomorrow with a new changeout
modem on my end.  Thought best to let them play modem swap on their ticket



It would be hard to imagine a 75 meter band crystal clear without 20 over 9
crude hihi

-----Original Message-----
From: rfi-admin@contesting.com [mailto:rfi-admin@contesting.com]On
Behalf Of Phil Duff
Sent: Saturday, December 01, 2001 09:35
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 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.

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 now.

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

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

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