[RFI] Wired vs Wireless Routers for RFI

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Mon Jan 14 12:31:39 EST 2008

Jim and All,

For what little it may be worth, I'll toss in a few comments "from the 
field" on this topic:

1.  I have had two different types of experience with CAT 5 cabling and 
the associated 10Base and 100Base-T Ethernet signals.  The first was here 
at ye olde salt mines where I, along with several colleagues, have had to 
measure and evaluate emissions from Ethernet interfaces for the past 
several years, relative to aerospace standards.  The other was at home, 
where although my Internet access there is via satellite dish, the 
resulting Ethernet for connection is routed via unshielded CAT 5e cabling 
that I installed when the house was built in 2000. 

2.  Aerospace emissions requirements are generally to either MIL-STD-461E 
(soon to be "F", as 461F was just released) requirement RE102 or RTCA 
DO-160E, Section 20 (for commercial).  There are varying levels of 
allowable emissions in both documents, with variances due to type of 
application, placement on a given platform, and (for MIL-STD-461) branch 
of the military.  However, all of these requirements are more stringent 
than FCC Part 15 requires, in some cases being more than 40 dB more 

3.  As most of you probably know, FCC Part 15, Class A devices are NOT 
intended for home use.  Unshielded CAT 5 with standard RJ-45 connectors 
will usually meet Class A requirements, and with care in design, can also 
meet Class B requirements (Class B being approved for use both at home or 

4.  As was stated in another posting, shielded CAT 5 cables are NOT 
connected to a chassis ground via one of the 8 conductors.  The contact to 
chassis ground is via the metal shrouds of the plug and mating shielded 
RJ-45 socket.  This configuration does not meet the requirement for a low 
impedance connection at all frequencies of interest, but it can help 
reduce emissions from the cable if there is a limited frequency range of 
interest (such as one or a few ham bands).

5.  Emissions from 100 Base-T are much more difficult to control than 
those from 10 Base-T.

6.  Believe it or not, there is a fair amount of aerospace and military 
equipment on the market, or on the way to market, that has 100 Base-T 
Ethernet interfaces.  The fact is, though, that those interfaces do not 
and can not use RJ-45 connectors.  The usual connection is via either a 
sub-D connector or aerospace circular connector, both of which must have 
EMI backshells and very short, low impedance terminating means for the 
cable shields.  What I am saying here is that if your equipment meets FCC 
Part 15, Class A and/or B requirements, and you are receiving RFI from 
that equipment or the interconnecting wiring, then the emission level is 
obviously greater than what you can tolerate.  Jim has presented many ways 
to reduce emissions on the cables.  There is another alternative: greater 
separation distance between your router or hub, the CAT 5 cabling, and 
your antennas.  Keep in mind that Part 15 measurements are made at a 
distance of 3 meters for Class B and 10 meters for Class A.  For those 
having indoor antennas or antennas located adjacent or over their houses, 
the separation distance is not enough to prevent RFI to your station.

7.  At home, I am fortunate enough to have all my HF antennas, except a 
10m vertical, not closer than 50 or 60 feet to the house.  (That vertical 
is at the far end of the house and is at least 20 feet away from the 
nearest CAT 5 cabling.)  Some of the CAT 5 passes through or near the 
shack, but no problems.  The only RFI issue I have been able to identify 
and confirm as being due to the Ethernet signals is a birdie I hear on 
145.39 MHz, which just happens to be the output frequency of our local 
repeater.  I do not hear that birdie on my base 2m radio (with its outside 
antennas), but if I have my 2m HT on me and pass near either the hub, 
satellite terminal, or any of the CAT 5 wiring, then the HT hears the 
birdie.  I have to be within about 3 to 5 feet of any of those items in 
order for the birdie to be a problem.

Bottom line:  If you can not get your antennas further away from the noise 
source, then you should apply any of the various fixes that have been 
discussed by Jim in which the problem is either eliminated or reduced.  In 
severe cases, you may have to go to the point of either disabling Ethernet 
when you want to operate or perform "major surgery" by burying the hub, 
router, or terminal in a fully shielded and grounded metal box, using 
nothing but shielded CAT 5 for all cable runs, and replacing RJ-45 
connectors with sub-D types having EMI backshells and decent shield 
terminations.  In can be done, but may not be easy.   As usual, YMMV.

73, Dale


"Jim Brown" <jim at audiosystemsgroup.com> 
Sent by: rfi-bounces at contesting.com
01/12/2008 10:18 AM

"gd0tep at safe-mail.net" <gd0tep at safe-mail.net>, "rfi at contesting.com" 
<rfi at contesting.com>

Re: [RFI] Wired vs Wireless Routers for RFI

On Fri, 11 Jan 2008 16:27:45 -0000, gd0tep at safe-mail.net wrote:

>The screened CAT5 I use here is fitted with suitable plugs, complete with
>screen. The plug connects to the screen of the router or what ever it is
>you're plugging into. Most modern items using CAT5 sockets are fitted 
with a
>screen connection.

In general, shielded ("screened" in British English) Ethernet cable is 
standard in Europe, whereas unshielded CAT5 is standard in North America. 
Thus, the Ethernet equipment and connectors you describe are the standard 
Europe, but not in North America. Indeed, shielded CAT5 is not easy to 
and I've never seen an Ethernet connector with a shield connection. I'm 
saying they don't exist, they're just not widely available to the general 


Jim Brown K9YC

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