A balun puzzle asked several questions of this VK5AJL balun which was cited online.
- Under what circumstances is the current into one terminal of the secondary significantly different to the current out of the other terminal of the secondary?
- Is this in fact a better current balun than voltage balun under some conditions? What?
- Is it a good voltage balun?
Under what circumstances is the current into one terminal of the secondary significantly different to the current out of the other terminal of the secondary?
At low frequencies, the current into one terminal of the secondary is identical to the current out of the other terminal of the secondary.
Factors that contribute to departure from that ideal are phase change through a secondary winding that is electrically long (ie more than say 5°), and lower capacitive reactance coupling of secondary to primary winding. Both of these are more likely as frequency is increased.
It is possible to build a ferrite cored transformer of this type that has very high common mode impedance, with almost perfect current balance… but it is challenging to achieve that objective over a very wide frequency range (say 10:1).
The fact is that the winding configuration does not exclude the device being used as an effective current balun, ie to drive equal currents of opposite phase into the load.
Is it a good voltage balun?
Lets deal with this question out of turn.
An ideal voltage balun delivers voltages that are equal in magnitude and opposite in phase.
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).
Consider the transformer driving a load comprising a 100Ω and 200Ω resistor in series, and the junction between them grounded.
A first approximation at low frequencies is that the balun will drive twice as much voltage in magnitude on one terminal as the other, and they will be of opposite phase.
The load voltages are not equal in magnitude and opposite in phase. The subject balun is an abject failure as a voltage balun.
Is this in fact a better current balun than voltage balun under some conditions? What?
The subject balun is likely to be a good current balun at frequencies where the length of the secondary winding is less than say 5° electrically, and where capacitance from secondary to primary is kept very small (say Xc>2000Ω at the operating frequency).
Since the subject balun is an abject failure as a voltage balun under any circumstance, and it can be a good current balun (albeit over a limited frequency range), it can be a better current balun than voltage balun under the conditions given above. (Note that the characteristics of a good current balun and good voltage balun are mutually exclusive.)
In fact A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use describes such a transformer with high common mode impedance from 1.8 to 18MHz (albeit 1:4 transformation).
(ARRL 2011) gives guidance on the function of current and voltage baluns:
A current balun forces symmetrical current at the balanced terminals. This is of particular importance in feeding antennas, since antenna currents determine the antenna’s radiation pattern.
A voltage balun forces symmetrical voltages at the balanced terminals. Voltage baluns are less effective in causing equal currents at
their balanced terminals, such as at an antenna’s feed point.
A (far) more complete development and definition IMHO is at Definition: Current Balun, Voltage Balun.
- Silver, H Ward ed. 2011. The ARRL handbook for radio communications. 2011 ed. Newington: ARRL.