>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
USA
Phone: 714-850-4660
Fax: 714-850-1163
and
alomar Engineers
PO Box 462222
Escondido, CA 92046
USA
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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