CURRENT BALUNS

ARTICAL ON CURRENT BALUNS BY RAY G8AWB

08 Jan 2011

CURRENT BALUNS
Voltage-type baluns produce equal and opposite voltages at the balun's balanced port. Since low impedance antennas are current fed, a balun that produces equal and opposite currents at its output over a wide range of load impedances is desirable. There is little to be gained by forcing the voltages of the two antenna halves, whether the antenna is balanced or not, to be equal and opposite compared with the cold side of the balun input. The antenna field is proportional to the currents in the elements, not the voltages at the feed point.
Current-type baluns are not a new idea. They have been used in TV receivers for many, many years. TV tuners require a very wide bandwidth balun that will work with a severely mismatched antenna, like a TV's so-called 'rabbit ears' antenna. The Current-type balun was the best choice for that application.
Unfortunately, when baluns were first popularized for use with wire antennas, a voltage-type design was chosen. Other balun makers just followed along. It was years before the first true, Current-type baluns appeared on the market.

Misconceptions
Baluns will not improve SWR (the exception is where a balun used as part of a matching network, i.e., 4:1 baluns used in loops)

They are not Lightning arresters. The winding inductance in most baluns is far too low.

Also, built-in Spark gaps don't work. The radio equipment is destroyed long before the 'gap' arcs over.

Baluns do not allow multiband operation of single band, coax fed, antenna: they do not make antennas more broadbanded.
These are all generalizations and, of course, there may be specific exceptions to any of them.
A balun really has only two jobs.
Isolate transmission line

Provide balanced output current

Proper Balun Design:
A properly engineered balun will include these design points:
1. High winding inductance (reactance)
2. Low stray capacitance
3. Very short internal transmission lines-