[RFI] ethernet EMI revisited - questions for the pros

Christopher E. Brown cbrown at woods.net
Sat Nov 22 02:35:16 EST 2008

I just got done taking care of my in home ethernet, a number of runs at 
10, 100 and 1000 made for alot of noise.

Some of the noise can be from poorly made cables.  Many cheap jumpers that 
come with gear are not correctly wired.  They work ok for data, but the 
pair splits make things worse.

I reduced my ethernet signals by 3 - 6 db on 20M by replacing all of the 
cables with correctly wired CAT5  (some of the "came with the cear" 
jumpers were flat (not twisted), or while wiremapping correctly had pair 

Further testing (disconnect all and reconnect one at a time) showed a 
large difference in signal level between devices even with the same cable 
length.  Some of the higher end gear had simple pow less common mode RF on 
the wire.  All of the cheaper gear was very noisy, only the higher end PoE 
(802.1af Power over Ethernet) gear was fairly decent (I assume the power 
splitter hardware provides more RF isolation).

I made all new jumpers using stranded CAT5 with a fairly thin jacket. 
This allowed 3 turns through a 1.5 inch long .4 ID FairRite type 31 snap 
on.  About half the gear is < 5 foot jumpers, the longer runs have 2 
snap-ons  (gear - jumper - in wall run - jumper - gear), one on each 

My 20M birdies went from S7 - S9 to no meter reading, only detectable when 
20M is quiet and only if I tune to exactly the right spot.

40M is slightly louder, but still low enough to miss.

I need to order some FT-240 size type 31 toroids though.  I have 2 long 
runs that still radiate on 80M.  I think about 12 turns through a FT-240 
core on all 4 jumpers should do it.

My antenna is a 35ft vertical up the back side af a tree about 40 feet 
from the house.

On Sat, 15 Nov 2008, aa8ia at aa8ia.org wrote:

> Hi Folks,
> I've been subscribed to this list for about a year.  I've read through
> much of the archives in the past.   I'll admit the answers to the
> question[s] I pose are probably found somewhere in the archives.
> Forgive me for asking them again.
> I've got a Zoom X5 DSL router, wired via cat5 of questionable origin
> to a 4-port Linksys WRT54G wireless router.   I've got three computers
> cabled via cat5 to the linksys as well as a couple of computers
> connected over the wireless.
> I have noticed significant interference across HF and up to at least
> 50 Mhz that I have tracked down specifically to the 100 mbit ethernet
> connections I have spread throughout my office.   I know this because
> when I unplug all of these, the specific interference goes away.
> Looking to put as little effort into this as possible, the first thing
> I did was decrease the ethernet speeds of the hardwired computers to
> 10 mbit/FD.   This has signfiicantly lessened the interference but
> obviously has some undesired consequences.   We all know faster is
> better.
> Unfortunately, I'm now at the point that the ethernet link between my
> DSL router and my Linksys wireless router is autonegotiated with no
> way on either device, via web config or command line configuration, to
> actually set the speed specifically to 10 mbit / FD in order to lessen
> the interference there.
> So now I'm moving on to doing things over.   I'll still have a
> combination of wired and wireless devices and will still have to have
> the cat5 cabling between the two routers.   I want to be able to run
> 100 mbit across the board on the cat5 without the interference issues
> to my HF receivers and low band VHF.
> I want to incorporate ferrites into the mix from the beginning.   My
> understanding is that I could buy premade cables and trust that they
> are wired using the recommended T568B standard.   I could then get
> some torroidal ferrites to wind the cat5 around.   I'm guessing it
> would be preferable to buy the cable, connectors, installation tool to
> put the connectors on, and then do the needed winding before I install
> the connectors (allowing possible more turns on a torroidal ferrite
> than I might get if I have to deal with the connectors).
> I don't know what type/size of torroid is recommended for my specific
> application and would like some recommendations.   I only want to do
> this once, and thus I want to do it right the first time.
> 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.
> Thanks for your assistance.
> Respectfully,
> Mike
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