[RFI] Low pass filters
Hare, Ed, W1RFI
Hare, Ed, W1RFI" <firstname.lastname@example.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.)
3122 Alpine Ave
Santa Ana, CA 92799
PO Box 462222
Escondido, CA 92046
Contact name: Jack Althouse, K6NY
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73 from ARRL HQ,
Ed Hare, W1RFI
ARRL Laboratory Supervisor
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