Tune Around!

RFI From Computer Networks
Is your router shutting you down?
Submitted by Author, L.D. Blake, VE3VDC

 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 apartment.

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.

Linksys Router 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 my antenna.

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
Buddy 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.

About RF Shielding

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 digital devices.
  • 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 the device.
  • 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 leaked RF.
  • 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.

Linksys internals
Linksys internals

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 all.

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 away.

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 up.

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 good condition.
  • Keep the router at least 6 inches from metal objects.
  • 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 users.

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 trouble starts.

More Information

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 Here.


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