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HB9ABX > TECHNIK 14.06.20 11:05l 47 Lines 2194 Bytes #999 (999) @ WW
BID : E6UDB0FHN03W
Read: GUEST
Subj: What is a current balun ?
Path: IW8PGT<IZ3LSV<DB0ERF<DB0RBS<OE5XBL<OE2XZR<DB0FHN
Sent: 200614/0848z @:DB0FHN.#BAY.DEU.EU [JN59NK Nuernberg] obcm1.07b12 LT:999
From: HB9ABX @ DB0FHN.#BAY.DEU.EU (Felix)
To: TECHNIK @ WW
Reply-To: HB9ABX @ HB9EAS.CHE.EU
X-Info: Sent with login password
What is a 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.
73 de Felix, HB9ABX ( felix-abx at gmx.ch )
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