[RFI] Router RFI

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Mon Apr 28 12:13:04 EDT 2008


There are two sources of radiated interference with rouiters:  the power 
supply (usually a switcher), and the actual wiring carrying the Ethernet 

Eben though Ethernet uses a balanced line scheme (like the telephone 
system), there are numerous opportunities for differential mode currents 
to develop.  (Slight variances in the twist of the cable pairs, impedance 
mismatches at the connector interfaces, and common mode propagation of 
clock frequencies from within the router that couple onto all interfaces 
are all ways that radiated emissions develop in routers and similar 
hardware.)  When they do, radiation usually occurs.  Aerospace 
applications are very sensitive to Ether net radiated emissions, 
especially now that several major aircraft are "fly-by-wire" and the 
"wire" is Ethernet.  From testing that I did several years ago, I can tell 
you that 100 Base-T is a much worse (by 10 to 20 dB) radiator than 10 

My house was bult in 2000, and in keeping with the times, I pre-wired it 
with many runs of Cat. 5E cable and plenty of jacks around the house. (The 
wireless routers were just coming onto the market and were rather pricey 
at the time.)  Although I do not have all of my lines active, I have had 
minor problems with noise from my system at just one frequency:  145.39 
MHz, which happens to be the output of the local repeater for which I am 
trustee!  My base radios don't hear the noise, but if I have my HT on my 
person, it will pick up the digi noise if I walk near where the Ethernet 
lines are run.  I don't have any problems on HF, and I credit that with 
the fact that all of my HF antennas are 50 to 250 feet away from the 

I can tell you that equipment running 100 Base-T Ethernet will NOT pass 
aerospace requirements to MIL-STD-461E, RE102, by using unshielded twisted 
pairs or RJ-45 connectors.  (One problem with RJ-45s is that they do not 
provide a truly low impedance means to terminate the cable shields.)  The 
best way to meet these requirements ( and to get rid of RFI from your 
router or LAN) is to use shielded twisted pair CAT 5e cable and connectors 
that offer a backshell termination for the shield.  This would mean having 
to hack out the RJ-45s and replace with sub-Ds or other connectors that 
can offer better shield termination to chassis ground.  It also means NO 
plastic cases and, as noted by others, a well shielded and filtered power 
supply, as well. 

It's the old story of victim (your radios via their antennas) and source 
(the routers via their wiring).  You have to either increase the 
separation distance between source and victim (which is what I have done 
by placing my antennas well away from the house) or reduce the emitted 
levels from the source (painful, but doable, if you don't mind some 
serious work and possible mods to the equipment). 

Always remember:  when making for faster digital equipment, you are also 
building a better transmitter, whether you want it or not.

73, Dale


"Andy" <ingraham.ma.ultranet at rcn.com> 
Sent by: rfi-bounces at contesting.com
04/26/2008 10:19 AM
Please respond to
Andy <ingraham.ma.ultranet at rcn.com>

<rfi at contesting.com>

Re: [RFI] Router RFI

Why do routers cause RFI ...?

Wired routers do not intentionally radiate.  But some do, probably because
they are poorly designed.  Just like many other modern electronic devices.

Wireless routers are intentional radiators, so by definition they send out
RF, though they are supposed to be on frequencies that don't interfere 
other services.  Some do anyway, in part because of inadequate bandpass

> My question, if I go with a completely wireless setup for 4 computers in 
> house, will I have a good chance of eliminating the RFI??

All other things being equal (which they aren't), the wired router ought 
have less RFI, and might be easier to improve by wrapping all cables on
toroids or through clamp-on filters.  The wireless router must be allowed 
radiate so fixing it might prove harder.  There's no good reason to 
that a wireless router would eliminate RFI ... unless you know the
brand/model you get is RFI-free.


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