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 this 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 a
unbalance to balanced match.
A voltage BALUN forces voltage potentials equal in amplitude but opposite in sign with
reference to ground 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.
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.
BUXCOMM cat# B2KLISO
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 B2KLISO in the coaxial cable at the output of the transceiver or antenna tuner
often remedies this problem.
An Unun 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.
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.
BUXCOMM model B1C11
1:1 BALUN: 50 ohms to 50 ohms, or to feed dipoles and similar antennas with 40 to
75 ohm feed points. The 1:1 current BALUN is also used to feed the input to 450
ohm ladder-line of the G5RV.
BUXCOMM model B1C21
1:2 BALUN: 50 to 100 ohms. This Balun is suitable for feeding Vertical Antennas, Quads,
Loop antennas and Ladder Line antennas.
BUXCOMM model B15C41
1:4 BALUN: 50 to 200 ohms. This Balun is suitable for the coupling 50 ohm coaxial cable
to Windom’s, and off-center-fed antennas. Sometimes used with folded dipoles that are
fed with 300 ohm twin lead.
BUXCOMM model B15UN41
B1KCUN41, 52 ohm unbalanced input to 200 ohm Unbalanced output
1.5 to 55 MHz, Use to match 50 ohm coax with Vertical
50 ohm SO239 unbalanced input. (SO239) unbalanced 4:1 (200 ohm)
160 meters to 6 meters. 1.5 to 55 MHz; Multi-Core BALUN, Power Rated at
1500 watts PEP UNUN.