[RFI] Low pass filters

Hare, Ed, W1RFI Hare, Ed, W1RFI" <ehare@arrl.org
Wed, 21 Jan 1998 10:26:00 -0500

>I find that braid breaker type HPF's at the TV aerial input to be
>very effective in stopping breakthrough from HF.  There are very easy
>to make, and a suitable design has been published by the RSGB.

Actually, the "braid-breaker" technique is not really a high-pass filter. 
 It is designed to attenuate the common-mode signal that is inevitably 
picked up on any conductors near a source of radiated RF energy (in a 
cable-TV or TV-antenna feedline, you can loosely think of this as the signal 
picked up on the OUTSIDE of a feedline, although this is technically not 
100% correct -- all the conductors in a multi-wire conductor will pick up 
radiated signals pretty much equally).  Here in the US, we generally refer 
to a braid-breaker as a common-mode choke.

The easiest way to make a common-mode choke is to take a ferrite toroid and 
wrap about 5-15 turns of the feedine onto  the toroid, forming a coil.  This 
will attenuate common-mode signals nicely, without significant effect to the 
differential-mode signals INSIDE the feedline (the desired TV signals in the 
case of TVI).  For lower HF, I prefer #73, #75, #77 or J type material, for 
upper HF and VHF, #43 is a good all around material.

I have seen some braid breaker designs that literally "break" the braid, 
with small loops formed in the coax.  These DO work, but suffer from a risk 
of increased leakage in (ingress) or out (egress) of the otherwise sealed 
coaxial system.  They also DO function to quite an extent as a high-pass 
filter.  For TV reception, even cable TV, this CAN be okay, but some cable 
systems do use HF inside the coax to control set-top converters or for other 
two-way communications between the subscriber and the cable company.  SOME 
(not all) of the designs I have seen could attenuate HF signals inside the 
coax -- undesirable in some cases.

For US sources of ferrite cores, contact:  (They may ship to Europe, but you 
can probably find local sources.)

Amidon, Inc.
3122 Alpine Ave
Santa Ana, CA 92799

Phone: 714-850-4660
Fax: 714-850-1163


alomar Engineers
PO Box 462222
Escondido, CA 92046

Phone: 619-747-3343
Fax: 619-747-3346
Email: 75353.2175@compuserve.com
Contact name: Jack Althouse, K6NY

The ARRL Web page features a number of information packages on RFI.  Go to 
http://www.arrl.org/tis/info/ and look for the RFI packages.  If you have 
email acess but not Web access, you can get them from our automated email 
server, info@arrl.org.  Send email to info@arrl.org, with HELP as the body 
of  the text.

In addition, we have a multi-page information package on cable TV 
interference that is available only on paper from the ARRL Technical 
Department Secretary, mlevesque@arrl.orgm Monique Levesque, 860-594-0278. 
 It is a reprint from a 3-part series that I wrote for Communications 
Technology, the journal for the Society of Cable Television Engineers.  We 
published it there instead of QST because I wrote it for the cable company 
people -- the ones who can acually fix a problem in the cable system, and I 
wanted them to believe what we had to say.  :-)

ARRL's book, Radio Frequency Interference, How to Find It and Fix It, covers 
a wide range of interference problems and solutions.  It is available from 
our Publications Sales Department.

All of our paper information packages are available for $2 for ARRL members, 
$4 for non-members, to cover the reproduction and postagae costs. (We can 
ship them out to members with a verbal or email request and let them send in 
the $2 later.)

73 from ARRL HQ,
Ed Hare, W1RFI
ARRL Laboratory Supervisor

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