RFI From Computer Networks
your router shutting you down?
Submitted by Author, L.D. Blake, VE3VDC
About RF Shielding
I presently live in a medium sized apartment building
where I am surrounded by hundreds of electronic devices; televisions,
cordless phones, computers, fax machines, baby monitors and lord only
knows what else. Imagine my pleasure when I set up my 2 meter rig with a
simple antenna on my balcony and found the band was very quiet; almost
no noise level at all.
For quite a while I considered
myself lucky to be in such a fortunate location for good FM chatter.
From my 6th floor apartment in St. Catharines, I talked to hams all over
Southern Ontario. I could "hit" repeaters in Hamilton, Brantford,
Kitchener, Oakville and Toronto using 5 and 10 watts with no problem.
Hearing and getting back to simplex stations 100+ kilometres away was
common. I received many compliments on how well my little "cube dweller"
setup was doing.
For several months after
obtaining my landlord's permission to put up a small antenna, I checked
with him at least once a week to see if there were any interference
problems and none were reported. So I was doubly happy as everything
seemed good for my hobby.
Then in the beginning of
September, seemingly out of no place, I suddenly had an S7 noise spike
about every 50khz all the way from 138mhz to 162mhz. There were over 140
of them dominating my receiver so that I couldn't even hear some of the
stronger repeaters in my area. The noise was of a nature that it mixed
in with other signals and made conversation practically impossible.
Now I'm a relatively patient
person. I waited a bit because problems like this usually don't stand
alone. Whatever the source of this was, the equipment was almost
certainly experiencing other problems and in the process of fixing them
the noise would be cleared up too. Well, two weeks into it I'd had
enough so I went "Fox Hunting". This lead me to my next door neighbour's
After reluctantly knocking on
his door and explaining the intrusion, I discovered he had a haybail
mess of a computer installation that had to be seen to be believed. He
had 3 computers crowded onto a smallish desk with piles of stuff on the
floor all around it. There were cables, wires and parts scattered all
over the place. The computer cases had been "modded" with plastic
windows inserted into the sides, defeating any shielding the case
provided. In all my 30 years in electronics, I'd never seen such an open
invitation for trouble; a nightmare just waiting to come true.
Sniffing about with the HT I
found the loudest of several noise sources was his network router; a
Linksys wireless-g router like the one pictured on the left. It was
sitting right in the middle of the whole tangled up mess, with the cable
modem on top and spewing RF like a firehose.
My first step was to confirm
that it actually was the router. Ok, lets try this: remove power, noise
gone, reinstate power, noise returns. Off quiet, on noisy... Yep, it's
the router alright and it's sitting just a few metres down the hall from
So what's to do about this?
There were so many things wrong
with his setup I hardly knew where to begin explaining the problems it
would cause and, frankly, I really didn't want to be the one to give him
a laundry list of everything wrong with his pride and joy. He actually
seemed to like the mess he'd created and I immediately understood that
if I tried explaining why computers are in metal cases and why the
wiring should be routed in some organized fashion he would react as
though I was personally insulting him.
While I still had his
cooperation, I thought to try some simple corrective measures. We
quickly wrapped it in tinfoil, faraday cage style, and grounded a corner
of the foil to the connector shield; this actually made it worse. There
was no point to making coils or adding ferrite chokes to his cables in
such a totally disorganized setup where literally everything was
radiating, so I didn't even suggest it.
Next we tried disconnecting the
network cables one at a time; no difference until the last one was
removed. Finally with everything disconnected the router was sitting
there powered up and the noise was gone. The wireless (RF) part of the
router was actually dead quiet.
Next we reconnected his cable
modem and one computer. As soon as he accessed the router the racket
returned and did not stop. The noise was clearly the result of activity
on the ethernet ports and buddy tells me he's a downloader with stuff
coming in day and night. Oh joy.
He was complaining of his
network stalling all the time, I still had my noise problem. It was
beyond obvious the router is defective. At that point the only advice
possible was to take it back and get another one. Which, for some
reason, he refuses to do.
By this time his patience with
this intrusion was thinning and my mood was fading fast. So, as a final
step I took him over to my place and showed him the problem on my base
rig. He reluctantly agreed it was a problem but with no solution in the
offing we called it a night.
Update: August 2007
next door has moved on, taking his noise and interference with him. As
of right now, the problem is solved. Noise levels are back down in the
S1 region and no noise spikes anywhere. Time will tell what the next
tenant brings with them, but for now, all it's all good.
In recent years I have noticed a
significant reduction in care taken to prevent interference. Where I used
to open a monitor and find all kinds of RF shielding, now there is none.
Where I would open a simple clock radio and find the display logic
shielded from the RF sections, now I find everything on a single circuit
board. Often when looking inside new equipment I find a circuit board
inside a plastic box, with no attempt to prevent either ingress or egress
of radio signals.
The goal of RF Shielding on most
electronic devices is twofold:
First we want to prevent "Ingress", keeping outside signals
such as strong local radio signals or signals from other devices in close
proximity from getting into the high impedance inputs of audio and CMOS
Second, we want to block "Egress" by trapping the switching
noise caused by high frequency oscillators and switching logic within the
device itself. If it is allowed to radiate into free air, the risk of one
device interfering with another in close proximity can and often does
become a reality.
RF bypassing in the form of ferrite beads, small coils and
capacitors is used inside these devices for sound reason:
Ingress along exposed cables is blocked as unwanted RF
signals are shunted to ground before reaching the sensitive inputs inside
Extraneous RF that can leak from cables is blocked before it
leaves the confines of the source device.
We run cables in organized groups
for several reasons:
Most obvious is the cosmetics of not having a workspace that
looks like a bomb went off.
Grouping cables together often brings about considerable
phase cancellation that can and does reduce the field strength of any
Finding the source of RFI in a "dog's breakfast" setup can
be nearly impossible.
Computers and other digital
equipment pose a real problem. They are almost entirely switching logic,
tens of thousands of square waves dancing about a printed circuit board
millions of times a second. If precaution is not taken --if the device is
not shielded-- these signals can and do radiate as low powered radio waves
that will bother nearby equipment.
Add to this the matter that most
PC cabling is totally unshielded. Video cables, running between computer
and monitor, carry frequencies well into the upper HF regions. CAT5
network cables carry signals that can actually get into the VHF
frequencies. Printer cables, USB cords and external drive cables all carry
signals that get well into radio frequencies. Each and every one of these
has the potential to become a source of RFI that can invade nearby
equipment. Each of them is an antenna that will carry any strong local
signal into it's host device.
As we move to the "wireless
world" so often touted by the WiFi industry, these problems will become
increasingly prevalent. Networks sitting only inches apart on opposite
sides of apartment walls will begin shutting each other down. Cellphones
already do cause so many problems they are banned in many RF sensitive
locations. Cordless phones will shut down entire computer networks.
Wireless devices of all kinds will increasingly cause erratic behaviour in
nearby electronics. We are already running out of frequency space for them
and the problems of physical proximity will only get worse as more and
more of these devices flood the airwaves.
Computer "geeks" with their case
mods and increasingly complicated setups are seldom aware of RFI or the
problems it can cause. Many don't even know that ham radio exists, never
mind taking it into consideration when hacking great huge holes in the
manufacturer's metal covers and grounded cases. They are broadsided by
problems that appear to defy explanation and most often the cause is
co-interference within their own hardware. As with my neighbour, the very
idea that a computer could cause interference to other devices is foreign
to them and, often, they simply don't care when alerted to it. He's doing
his thing and to heck with anyone else... not good.
Radio frequency interference
--the mystery invader-- may well become the "noisy stereo" of the new
millennium as neighbours increasingly come into conflict because of
electronic interference instead of loud music.
On Further Investigation
I'm a curious sort, so I wanted
to know the exact cause of the problem with my neighbour's router and I
began researching this on the web. I found a ton of links to the problem,
all kinds of people --not just hams-- were experiencing interference from
Linksys products. I found pages detailing how Linksys routers are shutting
down nearby networks, interfering with cellphones and messing up WiFi
connections. And, of particular interest, I found quite a bit about
interference to radio receivers.
A couple of large forum threads
told of interference problems from Linksys routers. This Thread on the EHam website was damning; the correspondents
were actually organizing an FCC complaint against Linksys. Also This Thread was particularly interesting as people talked about
essentially the same problem from a different set of Linksys products.
From the computer user's side,
people aren't exactly happy about this router either. This Review on the Small Net Builder website lays out how the
router has problems with low throughput and multiple connections. The
users on the Toms Hardware seem just as unhappy with these routers as they
talk about constantly loosing internet connections and entire batches of
them have had to be swapped out by home network installers when
inexplicable communications failures started showing up.
It was slowly becoming clear
there was a larger problem so I started researching the routers directly
and found the pictures of Linksys router internals shown on the right.
The first thing to notice is
that these are simply circuit boards housed in a plastic box. There is
no shielding either over or under the printed circuit boards and there
is no conductive coating on the inside of the enclosure. No shielding at
On closer inspection, you can
also see that aside from a tin box over the connectors and a cover over
the wireless portion there is no shielding or RF bypassing on the
boards, either. Nothing stops any source of RF from either escaping or
invading these boards.
Also, Please note that these
two completely different circuit board designs have both been marketed
under the same model code: WRT54G. The latest version is on top, the
oldest on the bottom.
Applying a little logic to the
situation gave me the rest of the explanation.
The RF part of the router is
not the problem. It's at 2.4 ghz, far enough away from my gear that the
two should be unaware of each other This is confirmed by the router
going silent when all the cables were disconnected.
Next we have the noise being
triggered by ethernet activity on the router's ports. Linksys says it
only happens at 100 mbps, which provides the final clue.
These things use square waves
with really sharp rise and fall times (10ns for 100 mips). Given the
nature of ethernet transfers, this can happen over a very wide range of
frequencies ranging from 1/2 the data rate (50mhz for 100mips) all the
way down to a few kilohertz for sparse data exchanges.
Now add in the matter that a
square wave is actually a fundamental frequency plus all harmonics of
that fundamental frequency and you shouldn't have much trouble figuring
out how this thing could cause a noise spike every 50 khz along the
entire VHF spectrum.
There are 5 ethernet ports on
that router. Each one producing about a 3 volt square wave on the 50 ohm
load presented by the cables. A quick power calculation tells us that 3
volts on 50 ohms is actually 180 milliwatts. Five little transmitters,
each with enough power to transmit a signal over a mile and I have, in
fact, heard my neighbour's router on my handy talkie from nearly a block
Next add in the 5 ethernet
(cat5) cables. These are twisted pair cables with absolutely no
shielding. In theory, with differential drivers and inputs, the phase
relationships in each pair causes signal cancellation. But very few
network cards use true differential I/O. One wire of the pair is often
simply grounded, destroying any chance of phase cancellation. Connect
these cables to the source of Ethernet's racket and you have a bunch of
antennas radiating this RF hash out into the air, on its way to find a
nearby antenna... mine.
I was actually hearing the
switching "hash" from the ethernet ports in the router. Simply adding
ferrite beads to the ethernet signal lines inside the router itself
would likely have stopped the RF egress. With even minimal shielding in
the case, the whole messy problem would most likely have never come
Getting Linksys Involved
The morning after checking my
neighbours equipment I called the Linksys support line
(1-800-326-7114) explained that I am a ham radio operator and
I was experiencing a rather nasty problem with interference from one of
their routers. To my surprise they seemed quite aware of the problem, and
put me on with one of their senior technicians. After considerable
discussion we settled on the following advice:
Set all NIC card speeds to 10 mbps.
Coil up extra cable length on a "three
finger" form and tape.
Disable unused ports.
Make certain all connectors are clean and in
Keep the router at least 6 inches from metal
Disconnect unused cables.
Ok, this sounded reasonable so I
wrote it up on a note and dropped it in my neighbour's mail slot. A couple
of hours later the noise stopped as abruptly as it started. At least one
of the items on that list had stopped the noise so the next day I dropped
in another note thanking him for fixing the problem.
A couple of days later the noise
returned and, as of this October 2006 update, has not stopped. I am
effectively put out of "business" by it.
Other Routers Too?
In researching this, I looked at
reviews and webpages on other routers and found little or no information
about interference of this kind. There were a couple of comments about
D-Link's cheaper products being in plastic cases. A couple didn't like the
3Comm units very much. But I found nothing to match the dozens of pages I
found on the Linksys.
I seriously doubt this is an
"every single router" problem with Linksys products. I rather suspect it's
a compatibility issue between their products and some other network
products. I found a number of hams using Linksys products who had no
trouble. But the ones who did report problems were almost always Linksys
Summing It All Up.
Ethernet, itself, is very RF
unfriendly, and you may run into problems like mine with any setup where
the owner is not careful about wiring and shielding. Of course the closer
the router is to your antenna the worse the problem is going to get.
The tips above are not a perfect
solution. The old saws about coiling wires and keeping things tidy
absolutely do apply but I doubt they'd have much effect in this case. The
10mbps recommendation is unconfirmed: it may simply shift the interference
to a block of frequencies I can't hear.
Bottom line... this router is a
lousy design that never should have reached production.
With respect to my neighbour, the
most obvious first step is for him to take the router back and get one
from another maker. If that gets it we're done. If not, well, that would
take a diplomatic miracle.
These things can be worked out,
provided all remain cooperative. It is when communication stops that
You may also want to see the
article about this on Ham Universe. Don. N4UJW, has been researching
this same problem, is building a list of model numbers and offers good
advice on isolating interference sources. You can see his Article Here
Radio Amateurs of Canada (RAC)
has a page on interference with links to many sources of information and
guidance on dealing with RFI problems. You can access Their Page Here.
There is an excellent article
for hams dealing with RFI problems and their neighbors on Ham Universe.
It covers everyting from diplomacy to "fox hunting" for noise. The
article is written from an american perspective, but the principles
apply just as well in Canada. That Article Is
Thousands of Electronic Parts