[RFI] ethernet EMI revisited - questions for the pros

aa8ia at aa8ia.org aa8ia at aa8ia.org
Sun Nov 16 13:38:00 EST 2008

Hi Roger,

Thanks for your response.

> This kind of interference may be from the equipment rater than the
> connectors or cables as the equipment pretty much stops when nothing is
> hooked to it and although others will probably disagree, it's like
> voodoo.

I believe the worst of the unwanted interference is generated from  
within the Zoom DSL router and then radiated from the CAT5 cable that  
is in between the Zoom DSL router and my Linksys wireless router.    
And that interference is only generated when the ethernet transceivers  
are active.   If nothing is plugged into them, the noise is not  

I'm sure many including yourself would agree that switching to 10 mbit  
isn't a 'cure' - It's more like a poor / lazy man's solution to the  
problem.   But it's a good temporary fix.  Unfortunately, it is not  
possible for me to force 10 mbit between the two routers, as neither  
device allows me to set the ethernet ports to 10 mbit either via web  
interface config or CLI config.

If I disconnect power from the DSL router, the noise goes away.   If I  
unplug the ethernet from either the DSL router or the Linksys router,  
the noise goes away.   If any of the ethernet ports on the DSL router  
or the wireless router are active, the noise is there - but the worst  
noise is coming from devices that get plugged directly into the Zoom  
DSL router - It is the biggest offender.  So all of my machines are  
plugged in at 10 mbit speeds to the Linksys wireless router or are  
using wireless themselves.  The exception is the link between the two  
routers, which is 100 mbit/FD and can't be forced otherwise.

I don't doubt that the linksys itself generates unwanted RF because I  
do notice an improvement when forcing 10 mbit on all of my machines  
connected to it, or when disconnecting all ethernet from it.  But the  
Zoom DSL router is the big offender.   Sure, I could buy another DSL  
router (perhaps an all in one Actiontec DSL router/4 port hub/wireless  
all in one) but that won't guarantee that my problems will go away.

So I would like to address my issue by making my own cables to spec  
and adding some ferrite right from the start.

My LMR400 is "temporarily" run into the house via a path that just  
happens to be within one foot of both routers.   I'm sure this doesn't  
help the issue at all.

My place will always have a combination of wireless and wired devices  
simply because I bring a lot of servers from work in here to work on  
before sending them back to the datacenter.  Otherwise, I'd get an  
all-in-one DSL/wired/wireless combination and then just get wireless  
PCI cards or USB wireless adapters for my machines and be done with it.

> I have two computers here in the den with the VHF duobander control

I'm thinking you're very fortunate that all of your computer equipment  
doesn't cause interference to your receivers, especially since you've  
said you don't use any specific technique for suppression.   I imagine  
the quality of all of your install is much better than mine.

> I run Gigabit as 100baseT is far too slow for the system backups across
> the network.  My router has wireless capability but it is normally
> disabled for security.

I have to wonder if ethernet transceivers running at 1 gigabit are  
less prone to creating noise than those running at 100/10.   Logically  
that wouldn't sound right, but then again these types of problems seem  
to defy logic.  As for the speed, sure faster is always better, and  
gigabit interfaces will pass even 100 mbit much more efficiently than  
a 100 mbit adapter will.   But none of my equipment currently has  
gigabit interfaces.  I don't run any video / media servers on my  
network, so I'm not starving for bandwidth internally.

> and the newer equipment is probably quieter.

I won't count on that.

> Why run two routers?  Why not run one multipurpose with the CAT5e
> through a switch? IOW I have one port from the router running to a
> switch. There the computers can talk via CAT5e without any wireless
> activity or even bothering the router unless connecting to the Internet.

Well for me, it is that one port from the [DSL] router that is causing  
the majority of my problem.... although all ethernet connections on  
both devices do provide interference somewhere on HF / lowband VHF.

I am considering buying an Actiontec wireless DSL router, which would  
give me four ethernet ports, wireless, and my DSL all in one.  I'm not  
confident that it would eliminate all problems though since I still  
need to connect wired devices to its ethernet ports.

I have a Cisco 2924 and 2948 as well as an HP Procurve 4000 that I  
could use for a switch.   All of those devices are bulky and are  
overkill for my application though.

I'm wanting my cake and wanting to eat it too.  One DSL router with  
four ethernet ports and wireless - with everything connected at 100  
mbit without interference.  And I'm thinking the best way to do that  
is to make sure the wiring is of the highest quality and to include  
ferrites into the mix.

> My guess is the RF is related to one or both of the routers, possibly
> talking to each other.
> Have you tried running just one router as a test?  If not, completely
> disconnect one including the power and all connections. Operate what
> you can on the other and check for interference. Then do the same with
> the other router.

You're right... See my comments above for the outcome of my testing.

> Typically I see the clamp on ferrites used although I used none.

My problem is that I don't know the type/mix/size that is best, or the  
# of turns to make around them.  I've read Jim Brown's writeup and  
have gotten some clues from that.  But I'll admit that I'm lost when  
reading most of it.

>> While I'm at it, does it make a difference if I'm buying cat5 or   
>> cat6?    I won't have any gigabit ethernet adaptors in play.
> Only in expense or you want longer runs. Both are groups of unshielded,
> twisted pair. OTOH shielded is available.
> One of my longer runs is CAT6

Ok.   I won't be picky about whether it is CAT5 or CAT6 then.

I appreciate your input.  Youv'e been very helpful.


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