Download Differences between a voltage balun and a current balun

BALUNS
By Glynn E. “Buck” Rogers, Sr., K4ABT
Definitions: BALUN = Symmetrical to Asymmetrical; (Balanced to Unbalanced)
UNUN = Asymmetrical to Asymmetrical; (Unbalanced to Unbalanced)
A lot of antenna designs and open wire, ladder-lines, require symmetrical or balanced
feeding; The voltage potentials and antenna currents need to be symmetrical with
respect to ground, or 180° out of phase.
The advantage of using coaxial cable is best seen when it is routed near tower legs,
masts and conductive objects. Local metal does not influence the RF energy inside
the shielded coaxial cable. The property of having an outer shield at reference ground
is possible. Coaxial cables are referred to as asymmetrical or unbalanced.
As we note, a balanced antenna cannot be connected to an unbalanced transmission
line without experiencing large losses, therefore a BALUN is required to provide an
unbalanced to a balanced match.
Voltage BALUNS:
A voltage BALUN forces voltage potentials equal in amplitude but opposite in sign with
reference to ground that is present at its output terminals. A voltage BALUN may
simultaneously act as an impedance transformer, changing the voltage-to-current ratio
of the output with reference to the input.
Current BALUNS:
A current BALUN forces currents equal in amplitude but opposite in sign (180°out of phase)
to flow through its output terminals. A current BALUN may simultaneously act as an
impedance transformer, changing the voltage-to-current ratio of the output with respect
to its input. The Guanella BALUNS are examples of current BALUNS.
If an antenna is fed at a low-impedance point or current maximum, which most often
happens to be the case, current BALUNS are more effective than voltage BALUNS in
forcing the right currents on the antenna and hence maintaining the desired radiation
pattern. This is especially important when nearby metallic objects are interfering with
coupling to the antenna.
A current BALUN protects against RF currents flowing on the outer shield of a coaxial
cable or transmission line.
When feeding a a Windom or off-center-fed dipole, it may be necessary to use a line-isolator,
to prevent the antenna RF from inducing currents onto the shield of the coax cable.
Adding a line isolator in the coaxial cable at the output of the transceiver or antenna tuner
often remedies this problem.
UNUNS:
An Unun is a type of transmission line transformer that has a common line between the
earth side of the input and the ground side of the output terminal. An Unun is never
a substitute for a BALUN.
IMPEDANCE:
For a transformer or BALUN to perform efficiently, the windings should have an
inductive reactance that is 5 to 10 times the impedance of the input, with reference
to the output. A BALUN will not perform at impedance levels greater than its designed
impedance levels. This is the reason a balanced antenna tuner which uses a BALUN
inside the tuner between the transceiver and the tuner input is preferred. An antenna
tuner with a BALUN between the balanced transmission line inside the tuner, is desired.