On Fri, 06 Aug 2010 11:48:22 -0500, Dr. Gerald N. Johnson wrote:
The purpose of these "baluns" is to attenuate any RF current on the
outside of the shield. That helps cut feed line radiation when the
coax
is hooked to an antenna. Sometimes that helps cut TVI and received
interference from local sources. Sometimes that hurts working
stations
that need the vertically polarized component from feedline radiation.
First, let's call these things by their real name so that we
understand what they do. They are COMMON MODE CHOKES, NOT BALUNS.
As Jerry notes, the function of a common mode choke is to kill current
on the feedline -- that is, to prevent the feedline from acting as
part of the antenna. That's good for at least four reasons. First, it
preserves the directional properties of the antenna. Second, it
prevents the feedline from receiving noise and coupling it to the
antenna. Third, it prevents the feedline from radiating RF into your
living room (and your neighbor's living room). Fourth, it prevents the
feedline from coupling RF into your shack.
Also as Jerry notes, there is no benefit from using a choke on jumpers
in the shack. The ideal place for a common mode choke is at the
feedpoint of the antenna (that is, up in the air where the coax
connects to the antenna). Chokes are also useful as "egg insulators"
to break up feedlines into non-resonant lengths (like an egg insulator
in a guy wire) so that they can not act as parasitic elements to other
antennas (like a vertical nearby).
Strings of beads are next to useless at HF frequencies, both because
their impedance is far too low to be effective, and because they are
inductive, not resistive at HF. A common mode choke must be resistive
so that it cannot resonate with the line. If it resonates, it is
useless. A string of beads IS quite useful at VHF and UHF.
There is a lot of detail and a tutorial discussion about this in
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim Brown K9YC
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