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HB9ABX > TECH 26.08.06 10:07l 57 Lines 2101 Bytes #999 (365) @ WW
BID : J4LI93HB9EAS
Read: DL1LCA GUEST DL1RX OE7FMI
Subj: What is: Current Balun
Path: DB0FHN<DB0RGB<DB0MRW<DB0SON<DB0SIF<HB9EAS
Sent: 060826/0803z @:HB9EAS.CHE.EU [Basel JN37TM] DP6.00 $:J4LI93HB9EAS
From: HB9ABX @ HB9EAS.CHE.EU (Felix)
To: TECH @ WW
What is: Current Balun
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 Meyer HB9ABX
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