[RFI] Got RFI ? - Some Good Solutions Material

Kimo Chun kimo at lava.net
Mon Dec 4 04:29:50 EST 2006

I recently purchased 40 each of Mix 31 EMI Suppressor cores: Fair-rite P/N

They are 2.4 in OD and 1.4 in ID.


You might think it is a Toroid, I did. However, the manufacturer says, "
No". I presume it

has to do with the application of the product. I believe the 31 mix is not
suitable for use

as a toroidal transformer.  When I could not locate it in their on-line
catalog I called them

on the phone. I was redirected to the EMI Suppressor section of their
catalog where I

finally found it.


Given the work that Jim Brown, K9YC, and an unnamed friend with the lab

did it seems to me that the logical choices for HF common mode chokes
depends on

a few factors.


For use on HF transmitters, especially at the lower frequencies and covering
the whole

amateur HF spectrum this 2.4 inch core gives much more performance per
dollar spent

provided you need or want to maximize the impedance of the choke and can
meet the

physical requirements of 10 to 14 turns of coax through the core, satisfy
the minimum

bend radius of the coax used (some foam dielectrics may not keep the center

correctly coaxially spaced or protected from shorts) and survive the heating
(if any

significant amount) from the power being transmitted and losses in the


Only around 5 or 6 turns are possible with RG-213 sized cable. For (near?)
full legal

limit you may substitute RG-142 or perhaps RG-303 / 304. The first two have
a 1900V

rating while 304 has a 3000V rating. They have Teflon dielectrics and
smaller OD.

FSJ1-50A Heliax from Andrew has a 1 inch minimum bend radius so it may also

be a candidate. There may be others that I am unfamiliar with. I am due to

some of these combinations soon.


The bead type suppressor cores are less effective due to the one turn you
can put

through it with RG-213. You must use many cores to compensate and equal the

performance of one of the 2.4 inch cores. Thus the cost goes up. I
understand that

there is a positive side in that the capacitance between the input and
output of the

choke is minimized using beads as the leads essentially are as far apart as
they can

be, unless you bend the bead string ends together for some reason.


However, I wonder if using more of the 2.4 inch cores - "overkill" according
to Jim,

in a single choke, with the fewer turns of RG-213 (due to the size limit) is
still more

cost effective than a string of bead cores. I suppose you should instead

several single 2.4 inch core multi-turn chokes in series to also compensate
for the

lower turns. I am still learning.


For receive side or consumer RFI suppression I think this part IS the one to

in most cases for suppression of frequencies in the MF/HF spectrum. Type 43

wins for upper HF and VHF/UHF applications. I have no personal experience

with Type 77.


I may well be trying to see what it takes to quiet a Plasma TV as the next

Seeing very recent posts by Jim and others about this is timely information

That 200V plus I heard that they use (true?) (let alone the heat they
generate) makes

me want to RUN to LCDs presuming their power supply is quiet.


The point, here, is that I recently purchased these from Lodestone Pacific
at under

$3 each. Kreger Components had better stock availability of the several
types I was

considering at the time but their prices were generally higher and Lodestone

the item I needed. They were backordered on the others 1 to 2 weeks.


Lodestone does have a $25 minimum per line item (P/N) and an overall $50

order. So long as you need enough to meet these minimums I think they should

considered one of the better sources. I did not ask Kreger what their
minimums were.

However, it appears the may give good service as they followed up on my

inquiry right away.


Read Jim Browns articles on the subject. The lab results are reassuring that

can obtain measured results by following them. Then it is a matter of
applying the

right tool and amount to your situation.




If you want to print it out for offline reading try the pdf version he has
listed as

a paper he presented to the 119th AES Convention in October 2005.




Kimo Chun, KH7U

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