Baluns

A balun is short for "balanced to unbalanced" and refers to a device that converts a balanced line into an unbalanced one, and vice versa.

In amateur radio, it is used to connect an antenna, which is usually balanced (tuned), to an unbalanced transmission line, such as a coaxial cable.

A balanced line is a two-wire line with no ground connection, while an unbalanced line, like a coaxial cable, has one of its conductors connected to ground.

It also blocks sheath currents.

The balun must be protected against weather, humidity and water ingress.

It also blocks sheath currents, which can cause interference in radio communication systems.

Duct currents can sometimes be strong enough to damage radio equipment.

Balun types

Each type has its own advantages and disadvantages, and the choice of the appropriate balun will depend on the specific application.

Numbers such as 1/1, 1/4, 4/1, 1/9, 9/1 refer to the balun's transformation ratio. This ratio indicates the relationship between antenna impedance and transmission line impedance.

•     1:1: A transformation ratio of 1:1 means that there is no change in impedance between the antenna and the transmission line. It is generally used to connect a balanced antenna (such as a dipole antenna) to an unbalanced transmission line (such as a coaxial cable) without changing the impedance.
  
  
•     1:4 and 4:1: Transformation ratios of 1/4 and 4/1, respectively. The 1:4 is used to match a 50-ohm antenna impedance to a 200-ohm transmission line, while a 4:1 balun does the opposite, matching a 200-ohm antenna impedance to a 50-ohm transmission line. Often used with long-wire or V-shaped antennas.
  
  
•     9:1 and 1:9: Transformation ratios of 9/1 and 1/9, respectively. The 9:1 is used to adapt a 50-ohm antenna impedance to a 450-ohm transmission line, while a 1:9 balun does the opposite. Often used with non-resonant antennas, such as long non-resonant wire or T-shaped antennas.