[Top] [All Lists]

Re: [TowerTalk] ferrite beads was...Re: [RFI] HIGH POWER and RFI...

To: "Jim Brown" <>,"" <>
Subject: Re: [TowerTalk] ferrite beads was...Re: [RFI] HIGH POWER and RFI...
From: Jim Lux <>
Date: Fri, 09 Mar 2007 15:56:08 -0800
List-post: <>
At 08:03 AM 3/9/2007, Jim Brown wrote:
>On Thu, 8 Mar 2007 23:40:40 -0800, Marlon K. Schafer wrote:
> >Out of my 25ish tower sites I have a couple that are a LOT of trouble.
> >Can't find any other cause so I'm hoping that it's just a simple 
> harmonic or
> >induction on the cat5 cables.
>That's pretty vague. What sort of trouble? What sort of system? What is the
>ethernet doing?
>Ferrite chokes can effectively kill common-mode antenna behavior of ethernet
>lines at HF if you use the right materials and wind them appropriately. BUT:
>I've never seen an RFI problem with ethernet systems RECEIVING interference,
>only generating it, and I've run 1 kW in VERY close proximity to my ethernet
>wiring. Perhaps there's enough error correction to prevent data errors when
>RFI occurs.

Ethernet (at least 10bT and 100bT, as well as the coax versions) 
doesn't have error correction (in the sense of ECC memory or FEC 
codes).  But it will do retries (at least with TCP.. not with 
UDP).  It's possible that you could lose 10% of the packets and not 
really notice much.  Say you're running 100 Mbps and sending 1500 
byte packets.  The packet only lasts about a couple hundred 
microseconds.  Unless you were blasting away with RTTY or PSK31, you 
might not notice.  If you're running CW, and the cable were 
completely blocked during key down time, you'd still be good to go 
during the remaining 50% of the time between dits and dahs and characters.

Given that relatively few applications actually pump anywhere near 
100 Mbps (a few Mbps is really cranking for most applications), you 
could have significant packet loss on your own network and still not 
even notice.

Now... blocking a 2Mbps 802.11b connection is another story 
entirely.  Then, the packets last 50 times longer (call it 10 
milliseconds), and being half duplex as well, buzzing along at 20 
WPM, your dits and dahs are in the 10s of milliseconds, and you could 
completely shut down the link, if your RF causes the 802.11 
receiver's front end to saturate or AGC down.



TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>