[RFI] ethernet EMI revisited - questions for the pros

Roger (K8RI) k8ri at rogerhalstead.com
Sun Nov 16 21:27:50 EST 2008

aa8ia at aa8ia.org wrote:
>> From: "Martin Ewing" <martin.s.ewing at gmail.com>
>> Subject: Re: [RFI] ethernet EMI revisited - questions for the pros
>> This is an interesting discussion.  I would add a few points/questions:
>> -Does anyone know of published "typical" spectra of 1000baseT vs 100 vs 10?
>> I agree that 10bT is much quieter on HF than 100bT, but it is possible that
>> 1000bT pushes the spectrum up enough to be quieter on HF. I did some work on
I doubt that is what is happening.  You have the basic clock frequencies 
in the chips generating or regenerating the basic data frame. Typical 
systems that run Gigabit also run 10 and 100 base-T. Whether the clock 
actually changes I don't remember.  The data stream as you have surmised 
generates a noise that is pretty much random. That noise not only 
depends on the basic speed but character string. Typically the highest 
frequency would be generated by a stream of alternating 1's and 0's. 
Unfortunately life is not simple in the network world and much depends 
on the chip set and the implementation.

Wired networks normally use twisted pair which is very good at 
discriminating against common mode ingress, but as they are wound around 
each other it can make a good antenna for radiating common mode. However 
the inductance and capacitance from this method also limits the distance 
and speed.  Theoretically the 1's and 0's should be "square, but in real 
life they are rounded off and  the waveform slopes across the 
transition. The longer the run of CAT5e the more they are rounded off 
and attenuated until you reach a point where the amplitude and 
transition have degraded to the point where the receiver can no longer 
recognize the transitions reliably.  This is *about* 300' in a straight 
line with no bumps in the impedance.  Then the chip sets do not generate 
a perfect squared off transition. Some of the older chip sets and 
implementations were terrible, not only with the wave form but they 
generated lots of noise.

I once made the mistake of stapling  a run of CAT5e to the garage 
ceiling. To say its range was limited would be an understatement and no 
I didn't stick any through it. <:-))

Two things I remember about Networking.  We had to figure the data rate 
for a given string of characters using different base speeds such as 10 
and 100 base-T. That's a lot of Calculus that I sorta vaguely remember.  
The second was that class was without doubt the toughest one I had in 
college and that includes writing a full blown C compiler in "Compiler 
Design", that worked using an input scanner..
>> this some years back: http://aa6e.net/rfi/ether_details.html .
> After hearing what Roger had to say regarding his all-gigabit network,  
It's not exactly the typical home installation.<:-))
> I was wondering the same thing.  I'll check out your link.
It's quite interesting.
>> -Do you know if your RFI is coming in through your antenna?  It may be
>> coupling in to your feedlines locally in the shack.  If this is so, you need
>> to look into balancing your antenna system.  In-shack RFI sources should not
>> matter very much if you're only listening to what's coming from your
>> (distant?) antenna.
> I probably should have mentioned that my antenna is only up about 20  
> feet right now - It is outside and the closest point is about 10 feet  
> from the house.   The RFI can be heard, but to a lesser extent, from  
> inside the house using a portable antenna.   When I hook up my  
> receivers to the antenna outside, via the feedline running next to the  
> routers, the signals are definitely stronger.   My feedline is LMR400  
> and runs into the basement to within a foot of the DSL router. 
I had 5, 200 foot runs of LMR-400 plus 28' pigtails at the top. It's 
about 74' from the basement to the base of the tower.  From the rigs to 
where the lines ran through the conduit to the tower they all ran in one 
bundle for about 6 to 8 feet. This is the equipment box and run to the 
tower. http://www.rogerhalstead.com/ham_files/cablebox.htm  
Unfortunately I have no photos of the "rats nest" prior to leaving the 
basement.  You'll note two conduit runs to the base of the tower.  The 
equipment at the ends has two different power feeds, but a common ground 
system. So essentially the grounds for the shop and the house are 
eventually tied into one large network of 32 or 33 8' ground rods 
cadwelded to many, many feet of bare cable.
>   I  
> wouldn't have expected the intereference to be picked up by the  
> LMR400, but I'm guessing that is a possibilty?  If I hook the receiver  
> up to the antenna outside, bypassing the feedline that runs into the  
> basement here, the signal levels of the interference are much less.   
> So yes it does sound like it is getting in through the feedline.
The shield might possibly carry the signal out to where the antenna 
hears it and at that point it is no longer common mode due to frequency 
and phase changes.
>> -I have found that much of the Ether RFI is generated by switches/routers
>> and passed on CAT 5 as common mode signals.
> I think that is my experience as well - specifically from the ethernet  
> transceivers.   If I have nothing plugged into the ethernet of either  
> router, there is no interference from those devices.
I'm wondering what generation of chips and equipment you might be 
running?  The newer equipment is much cheaper (or less expensive) than 
the older when it was on the market and the signals are *IN GENERAL* 
much cleaner.  My router suffered a catastrophic  accident and is 
currently pretty much PC board hanging on a bunch of leads.
>> -The "right" solution for wiring around the house is WiFi in my case.
>> Security is a minor issue in my situation, and all the high-speed traffic is
>> between my systems in the shack.  Long runs of any copper cabling are an
>> invitation to HF troubles, esp. if you're QRO.
I have KW stations on each end, plus the CAT5e runs to the shop pass 
within 10 feet of the base of the 100' 45G where the antennas top out at 
130 feet. For 6 to 8' it runs in a bundle with all of the cables. That 
tower takes on average 3 *verified* direct strikes a year although there 
were none seen this past summer.  I have seriously considered optical 
cable, but  the pieces that go at the ends are just a tad pricey for me 
as yet.
> I agree with you, as long as you don't have a need such as Rogers' for  
> high speed backup.
Guess I haven't changed much then: <:-))
Reminds me of when I was working.  I had a developmental Laboratory 
Information Management System, better known as LIMS. The data from the 
running system was backed up to my system daily and tested programs were 
uploaded/transfered to the running system when I approved them. Thing 
is, this was at a very large chemical corporation with many labs.  There 
were many millions of records that ended up being transferred at each 
daily update. The system architecture was changed and all of my traffic 
ended up going across  the bridge that carried all of the office and 
records traffic.  It almost brought things to a stand still. 
>   The wireless adapters I currently have in use are  
> running at 54 mbit and are "quiet."
I believe the newer wireless will run almost twice that fast, but not 
quite the equivalent of 100 Base-T.
>    I'm going to get another USB  
> wireless adapter and a PCI wireless card (for an older machine) and do  
> away with ethernet for normal operation.  But there will always be  
> times when I have to plug some ethernet device in for a day or two in  
> order to work on it, and so I want to clean things up.
The one router here has 5 RJ45 ports and can still handle quite a few 
Wi-Fi connections when turned on. Of course with multiple computers it 
really slows down. That's one of the things I like about the switch.  I 
can have two computers exchanging files without having much effect on 
the rest of the network including connections to the Internet.


Roger (K8RI)
> Mike
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