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Re: [RFI] Surge Suppressors and EMI

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Surge Suppressors and EMI
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Thu, 18 Mar 2010 10:44:52 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Thu, 18 Mar 2010 07:23:27 -0400, Pete Smith wrote:

>Has anyone explored the EMI suppression capabilities of the Staples 
>Model 16949 Surge Protector?  The spec sheet promises "150 KHz -100 MHz, 
>up to 58 dB."  Of course the "up to" renders the spec meaningless, but 
>the strip otherwise seems like a good value.

>Is there another surge protector strip that is known to be particularly 
>effective in suppressing EMI at HF?

Virtually all consumer surge suppressors use MOVs to short out the surge. 
An MOV across the line is OK (that is, from Line to Neutral), but an MOV 
from Line to Ground (the green wire) has the potential to cause destructive 
failure of equipment because it raises the potential at the outlet where it 
is connected by virtue of IR drop (and IZ drop due to inductance) of the 
green wire. If there is a signal line connecting equipment plugged into 
different outlets, one or both protected by MOVs to the green wire, a high 
voltage exists between those two pieces of gear and is likely blow up 
circuitry in one or both units. 

If you want EMI suppression on a power line, a FAR safer (and likely more 
effective) way to do it is with a dedicated power line EMI filter. Corcom 
and Delta are two respected brands. Both can be found at "closeout" prices 
with a google search, and their published data sheets are easily available 
on line. The link below shows part numbers and my plots of manufacturer's 
specs (NOT my measurements) for a variety of such filters that I found at a 
Silicon Valley surplus house for under $10 (some as cheap as a buck!). Put 
one of these in a standard electrical backbox with outlets and you've got a 
FAR better line filter than almost anything you're likely to find at 
consumer retail. There are photos in the pdf to give you ideas. I've built 
up a bunch of these, and plug all my consumer gear into them (hi-fi, TV, 
wall warts, etc.)


Since I wrote this up, I put one of the largest of these (the one on the 
top right in the photo) in a six-gang box with connectors to use it at the 
output of our Honda Field Day generators. 

Another VERY important point. The place for a power line filter is at the 
SOURCE of the noise -- that is, where those noise-generating power supplies 
are plugged into the power system. That's because the trash they generate 
is RADIATED by power wiring directly to your antennas. It is NOT usually 
conducted via the power line to your equipment. The only thing that a power 
line filter in your shack does is keep YOUR RF trash (including that 
generated by noisy power supplies) off of the power line.  


Jim Brown K9YC

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