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The terms Current Balun and Voltage Balun are often used. This
article explains the key characteristics that determine which best
describes a particular balun. The approach take is to charaterise a
balun because of how it behaves externally rather than the internal
implementation.
The discussion in this article is for signal flow from the unbalanced side to the balanced side.
Balun loads are often though of as a simple two terminal device. That is actually quite inadequate, they should be thought of as a three terminal load which can be simplified to a delta or wye equivalent circuit and one terminal of that network is connected to the unbalanced input 'ground' terminal.
Baluns may appear ideal if they are driving an isolated two
terminal load, and there would be no need for a balun if that was the
case.
Common mode current is the difference between the output terminal
currents, it flows to the unbalanced input 'ground' terminal.
An ideal current balun
delivers currents that are equal in magnitude and opposite in phase.
A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance (including symmetry).
Common mode current will be small.
If the load impedance is not symmetric, then the voltages at
each output terminal will not be equal in magnitude and opposite in
phase. (Note that for a truly 'isolated' load, one well represented as a two
terminal load, the currents MUST be equal in magnitude and opposite in phase,
but the voltages may not be equal in magnitude and opposite in phase.)
A parameter often used to quantify the effect of a current balun is its
common mode impedance or choking impedance. An ideal current balun has
infinite common mode impedance, a good one has very high common mode
impedance (typically thousands of ohms for an effective general purpose
balun in an antenna system).
A good voltage balun will approach the ideal condition. It will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).
Common mode voltage
((V1+V2)/2) will be
small.
If the load impedance is not symmetric, then the currents flowing in each output terminal will not be equal in magnitude and opposite in phase.
An ideal voltage balun has zero common mode impedance, a good one has very low common mode impedance (ohms).
A balun cannot be a good Voltage Balun and a good Current Balun at the same time, it is one or the other, or neither.
An ideal balun performs an ideal impedance transformation, nominally 1:1 unless specified otherwise. Practical baluns depart from the ideal, and the departure is often specified as Insertion VSWR.
It is possible to design a balun to not only facilitate the unbalanced to balanced transition, but to perform a nominal impedance transformation (eg 4:1 is common). Voltage Baluns and Current Baluns are both capable of impedance transformation other than nominally 1:1.
If the application is one where current balance is important then a current balun is the better choice. For example:
If the application is one where voltage balance is important then a voltage balun is the better choice. For example:
Above are two baluns from the ARRL Handbook, neither uses a magnetic core. The second also performs 4:1 impedance transformation. Classify each as a Voltage Balun, Current Balun, or neither. Why?
Version  Date  Description 
1.01  21/03/2011  Initial. 
1.02  
1.03  
1.04  
1.05 
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.