[RFI] ethernet EMI revisited - questions for the pros

Mike Tindor AA8IA aa8ia at aa8ia.org
Fri Nov 28 20:22:47 EST 2008

Thanks to all for the comments/suggestions.  I'm not opposed to buying new 
hardware - But I hate to buy and find out that doesn't sufficiently remedy 
the problem.    So I'll start off by recabling and seeing if this reduces 
the interference.   I do know that simply swapping out the current cable 
between the two routers does not lessen RFI, so I imagine I'll have to 
simply replace the Zoom - I really don't need A DSL router anyway.   A 
simple DSL modem stuffed into the WAN of the Linksys will be fine since all 
of my wired devices plug into the Linksys anyway.   And I do plan on getting 
a PCI wireless card for at least one of the PCs, leaving only one plugged 

I'll let you all know what the outcome is when I'm finished.   I'll also 
purchase some type 31 snapons to see how those help - they seem to be quite 
useful and something we all ought to have a few of lying around.


----- Original Message ----- 
From: "Leigh L. Klotz, Jr. WA5ZNU" <Leigh at WA5ZNU.org>
To: <aa8ia at aa8ia.org>
Cc: "Jim Brown" <jim at audiosystemsgroup.com>; <rfi at contesting.com>
Sent: Tuesday, November 25, 2008 5:29 PM
Subject: Re: [RFI] ethernet EMI revisited - questions for the pros

aa8ia wrote:
> 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.

I've found the same, that 100BaseT is noisy and 10BaseT is quieter.
Jim K9YC reported the opposite at some point in the past, but I don't know
what his current thinking on that issue is.

Here's what I did: I went to fiber.  It's not that expensive, especially
surplus or on eBay.  Fiber with ST connectors (they look like tiny BNC
connectors) is fairly cheap.  I've found ten-dollar 10 MBbit PCI cards
that look exactly like copper Ethernet cards, only with a fiber transducer
instead of ethernet magnetics.  I've found $40 transition boxes that go
between 10MBit or 100MBit and fiber.   Stick a 10MBit one of these on your
router and the PCI Card in your computer and you have only eliminated
100MBit from your router, and achieved total galvanic isolation from your
computer to the ethernet box (note: don't do this if you are that one guy
in DC who has *more* than 10MBit from your cable provider).

You can do it with 100Mbit as well for lines between computers, or just
move to 802.11g or 802.11 "pre n" throughout the house.

Finally, if you don't want to go the fiber route but do want to force your
router or computer to negotiate 10Mbit and can't get at the software, buy
a 10 MBit hub at a garage sale and put it in between them. The devices on
either side will have no choice.


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