Here's the "balun" solution. These work great, cost little and can
handle
more power than you can generate. Do note, RG-213 is the coax to use.
Don't use the foam stuff. Thanks to Rich Measures, AG6K for the
write-up.
73
Bob, K4TAX
Building a no-grief 1.8MHz to 30MHz 50ohm-balun is easy. No costly
ferrite-cores are needed, just a short length of 3 to 5 inch size plastic
pipe, about 25 feet of 50ohm coax plus some nylon cable ties.
Solid-dielectric coax is best for this application because
foam-dielectric
has a tendency to allow a change in the conductor to conductor spacing
over a period of time if it is bent into a tight circle. This can
eventually result in voltage breakdown of the internal insulation. The
required length of the plastic pipe depends on the diameter and length of
the coax used and the diameter of the pipe. For RG-213/U coax, about one
foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For
3.5MHz
to 30MHz coverage, about 18 to 20 feet of coax is needed. This length of
coax is also adequate for most applications on 1.8MHz. The number of
turns
is not critical because the inductance depends more on the length of the
wire (coax) than on the number of turns, which will vary depe
nding on the diameter of the plastic pipe that is used. The coax is
single-layer close-wound on the plastic pipe. The first and last turns of
the coax are secured to the plastic pipe with nylon cable ties passed
through small holes drilled in the plastic pipe. The coil winding must
not be placed against a conductor. The name of this simple but effective
device is a choke-balun.
Some people build choke-baluns, without a plastic coil-form, by
scramble-winding the coax into a coil and taping it together. The problem
with scramble-winding is that the first and last turns of the coax may
touch each other. This creates two complications. The
distributed-capacitance of the balun is increased and the RF-lossy vinyl
jacket of the coax is subjected to a high RF-voltage. The single-layer
winding on the plastic coil-form construction method solves these
problems
since it divides the RF-voltage and capacitance evenly across each turn
of
the balun.
A more compact, less ugly, 1 to 1 impedance-ratio, 50ohm trifilar-wound
(with wire) ferrite-core balun could also be used but there would be some
tradeoffs. Ferrite cores are not cheap. Also, the air-core of the
coax-balun can't saturate like the ferrite-core and, unlike ferrite-core
wire-wound baluns, single-layer wound coax-baluns almost never have an
insulation breakdown problem. Also, a trifilar-wound balun does not like
to work into anything but a perfectly balanced load. With an imperfectly
balanced load, the coax-balun will not, as does the trifilar balun,
generate a differential, third RF-current on the outside of the coax that
brings the RF to the input of the tuner. The choke-balun is not fussy. It
will work as well into a less than perfectly balanced load as it will
into
a perfectly balanced load, and do so without the possibility of creating
a
differential RF-current on the station ground and fricasseeing the
operator's fingers.
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, November 23, 2007 2:44 PM
Subject: Re: [TenTec] balun noise?
On Fri, 23 Nov 2007 09:46:03 -1000, Ken Brown wrote:
Usually a balun is used to connect an unbalanced feed line, such as
coax, to a balanced antenna such as a dipole. The balun should improve
the current distribution on the dipole and reduce the current on the
outside of the coax.
Most so-called "baluns" of the type Ken describes are really common mode
chokes, and they do exactly what he says.
See http://audiosystemsgroup.com/RFI-Ham.pdf for a tutorial on many
things,
including these chokes ("baluns".
73,
Jim K9YC
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