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.
The air choke should be placed closely to the antenna rather than near the receiver. Place it at the feed-point of the antenna.
As an example, when a balun is used with a dipole antenna the balun effectively converts the dipole (balanced) for the unbalanced coaxial cable. Place the balun at the feedpoint of the dipole antenna by creating the coil or using a ferrite choke.
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.
