I had forwarded this message to Ron Hranac, N0IVN. Ron is a member of the
ARRL RFI Task Group and a Director on the Society of Cable
Telecommunications Engineers.
73,
Ed Hare, W1RFI
ARRL Laboratory Supervisor
225 Main St
Newington, CT 06111
Tel: 860-594-0318
FAX: 860-594-0259
Internet: w1rfi@arrl.org
ARRL Web: http://www.arrl.org
ARRL Technical Information Service: http://www.arrl.org/tis/
From: N0IVN
To the group,
The described cable modem interference problem is a new one! I suspect we'll
be seeing more of these in the future. My initial suspicion is the 10base-T
Ethernet connection between the cable modem and the user's PCs. It is
carrying data between the cable modem and PC network interface cards (NICs)
at 10 megabits per second (Mbps). A 10 Mbps data signal contains a lot of
energy in the frequency range where the interference is occurring. That
Ethernet connection should use _good_ quality Category V cable--it may be
necessary to go to shielded cabling if the Ethernet connection proves to be
the culprit and Cat V cable doesn't solve the problem. As well, common mode
choke(s) may be needed at the Ethernet (10base-T) inputs to the PC NICs and
the modem output.
With regard to other possibilities, the usual suspects are potential sources
of the interference: the cable drop to the modem. Make sure it's tri- or
quad-shield coax, and all connections are tight. As well, a high pass filter
on the CATV splitter output that feeds the home's TV sets, VCRs, etc, might
help, especially if the interference proves to be upstream (5-42 MHz)
leakage
from a cable-ready device that has less than adequate shielding.
The power lead to the cable modem may benefit from a common mode choke.
If all of these fail to solve the problem, the modem itself may have a
shielding problem. This is where you'll have to get the cable company
involved--but try the other suggestions first.
73,
Ron Hranac, N0IVN
-----Original Message-----
From: Peter D. Vouvounas [mailto:wb3fsr@home.com]
Sent: Friday, July 07, 2000 10:18 PM
To: rfi@contesting.com
Subject: [RFI] Cable Modem RFI
At last I've found the source of my 30db /S9 interference generator ---
ComCast Motorola Cable Modem ---
The interference completely wipes out 160M, most of 80M and can be detected
up through about 22 MHz
I could use any information and or experiences in eliminating this source of
interference short of unhooking the cable modem...
Cable modem located in Ham Shack coupled to 10bT hub that feed two
computers - One local in Ham Shack - Second approx. 200 feet away on next
floor.
I utilize the cable modem for cluster connections with my contesting
software (WriteLog) and DXing software (DX4Win) plus may other activities.
The computer and the 10bT hub can not be detected anywhere across the HF
bands -
When I remove the cable from the cable modem all is quiet with cable modem
powered - also with cable modem unpowered no noise can be detected.
XYL has her own machine that is used for university on-line courses around
the clock - she would not allow me to disconnect the cable modem anyway!
Regards,
PeterV {REN} WB3FSR
=========================================================================
Peter D. Vouvounas Ofc: 732-701-1130
Director Business Mgt. Cell: 609-306-5260
Technology S.A. InterActive 2way: peterv_gtei@bellsouthips.com
Genuity {GTEI} Mailto: wb3fsr@home.com
98 Meridian Dr.
Brick, NJ 08724 http://www.bbn.com
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