[RFI] AC Line filters
jim at audiosystemsgroup.com
Thu Apr 3 13:06:54 EDT 2008
On Thu, 03 Apr 2008 16:25:11 +0000, w70m at comcast.net wrote:
> Is there another source for these relative inexpensive AC line filters.
You can, indeed, roll your own, following guidelines in my RFI tutorial.
You can also buy pretty good line filters from a surplus house called
Halted Specialties (also known as HSC). On the link below, search on "line
filters" and then google the part numbers to see what they are. I've never
dealt with them on the internet, but am in their warehouse-sized store
about once a month. The filters you see pictures of and data for in the
tutorial came from them, all for $10 or so.
It is VERY important that the filter case be bonded with a VERY short lead
to the chassis of the noise source, and that the line cord between the unit
and the filter is VERY short. In other words, you probably need to cut the
line cord and insert the filter.
In addition to this filter, I would also wrap some turns of the line cord
after it leaves the filter through a #31 ferrite toroid of the big clamp-on
shown in the tutorial. How many turns will depend on what bands you operate
and where the noise is the worst. See the graphs of choke impedance vs
As noted, both filters will knock down the RF coupled to the AC power line.
It is NOT correct, however, to only call that RF "conducted" even though
some test standards may do so -- the power line is acting as an antenna
that RADIATES that RF as a common mode signal. It isn't the differential
signal tha bites you, it's the common mode, because the power line is a
wonderful long wire antenna at ham frequencies.
These filters wil NOT affect RF radiated from the unit iself due to
inadequate shielding. Nothing will fix that other than adequate shielding
and improved circuit design that reduces the size of current loops carrying
the noise, and lengthening the rise time of the square waves generating the
Jim Brown K9YC
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