[RFI] Home networking birdies

Stu Benner w3stu at myactv.net
Tue Sep 19 22:46:52 EDT 2006


I believe you may have missed the "smoking gun" or maybe I don't quite
understand how you did the tests. It appears that the Netgear hub, not the
router, is the primary source of your problem. You demated the cable from
the router end and the noise dropped significantly indicating that there is
likely (common mode) noise from within the hub being coupled onto that
cable. I'd recommend trying the test again of applying a ferrite to that
cable at the hub that will give you several hundred or a thousand ohms at
the frequency of interest. In my experience, the common mode noise is often
coupled out of both the data cables and the DC power cable. Try ferrite
loading all cables at once on just a single box. 

Remember, this is a "war of attrition." For example, assume that you have
two cables that are contributing equally to the total power being emitted by
the noise source, if you eliminate one of them, your noise level will only
decrease 3 dB (very roughly 1/2 S-unit). If you then eliminate the second
cable as an emission source, the noise may go away completely. From your
description, I'm not sure if you only did one thing at a time and looked for
a big difference or if you "layered" the ferrites on at each step.

As for shielded Cat 5, I've never had a need to go to that at home but have
needed it in my professional work. As for shielding the boxes with foil,
that is not likely to yield any improvement. If you do it properly and the
shielding of the box is the problem, then you may help. Remember, those
cables are potentially acting like antennas connected directly to an
undesired transmitter within the box. Shielding the box won't help much if
you're coupling energy out to those "antennas."

Also consider the proximity of the network cables to your feedline.
Improperly terminated connectors or poor/damaged coaxial cable could allow
noise currents on the outer surface of your coax to couple inside the
shield. Either move the cables around with respect to each other or
terminate the feedline with a load at the antenna.

I hope this helps. These things are touchy to troubleshoot remotely. Very
subtle changes can be indicators of where the problem lies and these can be
easily overlooked if the proper troubleshooting techniques are not followed.

Stu Benner

-----Original Message-----
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of D. Kemp
Sent: Tuesday, September 19, 2006 1:47 PM
To: rfi at contesting.com
Subject: Re: [RFI] Home networking birdies

Thanks guys for all the good suggestions.

My RFI is on 14.030 and some smaller ones up and down
from there. It seems to be getting on through the
antenna; if I disconnect the coax from my rig it goes

I have done the following:

1. Put some toroids of my cat5 cable on both ends.
These are the rectangular split type. No help, same
level of interference.

2. The cable from the D-link router seems to be the
RFI generator. If I unplug it from the hub there is a
great decrease in the level of the RFI signal. 

3. If I unplug the power from the Netgear hub and then
run the feed cat5 from the d-link directly  into the
computer there is a good reduction in the level of RFI
from an s5 signal to an S1 signal.

4. I covered the Netgear hub with foil as per one
suggestion without any difference in RFI signal

5. I wrapped the dc power cable of the D-link router
onto a toroid. No help.

6. Wrapped the power cable of the Netgear hub onto a
toroid. No help.

7. Wrapped the D-link router with foil. No help.

I am thinking of 2 direct runs from the router using
shielded cat5.

What about shielded cat5? Anyone have experience with
using it?

The large toroids may help, but they are expensive, I
don't have any, and it might be better spent on cable
if shielding would work.

Any more suggestions?

Thanks again for all the help.


Don, NN8B
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