[RFI] EMI/RFI AC line filter design info?

Christopher Brown cbrown at woods.net
Tue Sep 24 04:16:14 EDT 2013

There was a fairly simple filter in the older handbooks, but it was not
really targeted at switched mode noise.

There is also the issue of building something that is functionally
included in your house AC system.

As far as snap-ons, unless it is big enough to take multiple turns of
the power cord it is not going to do anything useful on HF.

You might try visiting Corcoms site (cor.com).

First off, they have 100+ filters designed for various things, and are
generally considered to be good quality.

Second, datasheets w/ schematics and performance charts against a STD
LISN network test setup.

They can be had from mouser, or often surplus and are sealed metal case
smaller and better built they most can do themselves.

I used to build my own, but these days I use the filters I have colleced
over the years as building blocks (generally combined with a nice
enclosure and addition ferrite toroids).

If you really want to build...

Review the Corcom datasheets, look at the schematic and pay attention to
the atten curves for the filter design at each power rating.

The schematic will not generally have component values as they are diff
for each power rating.

Find the filter model that best matches you need.

Now, google the P/N or search fleabay...  Fun fact, while the data
sheets do not have component values, the schematic usually printed on
the case generally does.

You can often get the inductor and cap values just by finding a picture.

Of course you then need to determine the correct core materials and
figure out the winding style, Z wound combined common/diff mode cores
are common in the higher amperage filters.

I run a 3kva rackmount UPS here, not a full time just a heavy duty true
sine w/ external battery capability for long run times.

The charger is a bit noisy, the ethernet card is noise and when
producing power, goodby HF.

I ended up removing the outlet bank and placing a pair of Corcom 30EMC6
filters inside.  One on the UPS input and one in the out...With a single
large cable for the feed out to a L5-30 socket.

At 30amps, the EMC and S series looked best...with a smaller load
available attenuation is much higher, just remember to account for full
UPS load and full charge current.

On 9/23/13 6:39 AM, Ray, W4BYG wrote:
> Did a search online for info on how to build some AC line filters with
> little luck.  They are fairly simple to construct, but I need the correct
> values for the spectrum of interest.  
> Also not  sure what input/output impedance to use for the filter calcs.
> Does anyone have any guidance on where to go for good info on building some
> dual section AC line filters for 1.8 to 30 MHz?
> Looking to minimize conducted and radiated noise from switching supply power
> cords and my double conversion (full time) UPS in the shack.  Snap on
> ferrites aren't helping enough.
> Your assistance would be appreciated.
> Ray, W4BYG
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