Choke Balun
A balun is used to connect a balanced line to an unbalanced line. In a balanced line you have two signals working against each other where ground is irrelevant and in an unbalanced line you have just one signal working against ground. A balun will help reduce or eliminate RF current on the outside of a coaxial cable. A balun is also known as an air choke.
The word balun comes from the words "balanced" and "unbalanced." With RF a balun deals with impedance matching between antenna and line. Coaxial cable is a type of unbalanced feedline. Coaxial cable is the most common feedline in use today for RF. RF cable tends to travel both on the inside and the outside of a coax shield, which can give the coaxial cable characteristics of three independent wires.
Types of balun
Generally a balun consists of two wires (primary and secondary) and a toroid core: it converts the electrical energy of the primary wire into a magnetic field. Depending on how the secondary wire is done, the magnetic field is converted back to an electric field.
- Autotransformer balun: two coils on a ferrite rod
- Classical transformer: isolated transformers
- Transmission-line transformer: transformer type magnetic coupling combined with the transmission line type electro-magnetic coupling
- Delay line: connected transmission lines of specific lengths w/o transformer
- RF choke: coaxial cable near to the feed point of a balanced antenna
RF Balun Examples
Coiled-Coax Balun
To alleviate RF on the outside of a coaxial cable shield a simple three loop air choke can be introduced. You can make a simple current balun by coiling coaxial feedline in a particular way. The coil inductance and distributed capacitance resonate as a parallel trap whose high impedance inhibits unwanted shield current.
The coaxial cable can be wrapped three times around a piece of PVC that is 2.5" to 3". Basically, not so tight as to damage the cable, but no large a loop as to nullify the effective choking of the unwanted RF.
Ferrite Chokes
Rather than constructing an air coil you can place a ferrite choke on the coaxial cable. The ferrite will increase the cable common-mode impedance without affecting its differential impedance.
