What is a current balun ?
18.4.2005
A balun is a device, which allows to feed a
symmetrical
antenna (e.g. dipole ) with an asymmetrical line
(e.g. coax
cable) and vice versa.
It is balancing an
unbalanced input.
With the expression balun one means mostly a
voltage
balun, which is a transformer with input and output coils.
The
relation of the active number of turns determines
the voltage transformation.
If the same number of turns
is active on input and output, we have a 1:1
balun,
having the same output voltage as input voltage
and the same output
impedance as input impedance.
The impedance transformation is the square of
the
voltage transformation.
A voltage balun performs mostly a rigid
symmetry,
as the center of the output coil is mostly connected
to the
ground connection of the input.
A current
balun is a device, which is made by coiled coax
cable, wound over
ferrite core (or ferrite over coax cable),
or a coil, wound of an rf
cable.
The effect of this device is, that a high resistance
is formed
along the coil on the operating frequency.
This resistance isolates the input
from the output.
As the same cable is at the input as on the output,
no
transformation takes place, therefore we have
a 1:1 balun.
If this
device is inserted between a symmetrical antenna and
an asymmetrical coax,
automatic balancing occurs, as
through the balun the differential input
voltage is forwarded
to the output, while the ground reference is
eliminated.
Its output is "floating" from the input.
If this device is
inserted in a coax where shield current flows,
the isolation resistance
blocks the shield current and
the device acts as a shield current eliminator.
This is very
important to prevent RFI and TVI.
If a symmetrical
antenna is capacitively unbalanced by
a nearby building or tree, a current
balun has to be used
insted of a 1:1 voltage balun, as the current balun
adapts
automatically to the asymmetric situation.
A voltage balun
would create shield currents in this
situation, which is very
undesired.
Felix HB9ABX