What is a balun and why do we need a balun

What is a balun

A balun is a special type of transformer that can perform functions like impedance transformation and balanced to unbalanced transformation. Balun is derived from “balanced to unbalanced”. This device is designed specifically to connect between balanced and unbalanced circuits. Balun is designed properly to interfere with unbalanced circuits to balanced circuits. Baluns are the passive component that can transfer power from balanced to unbalanced. The balun belongs to a class of devices known as transmission line transformers. These transformers are created by winding a strip-line cable on a core that has high permeability. Due to the high impedance of the winding, the balanced output terminals are isolated from the unbalanced input terminals.

Why do we need a balun

The main reason to use a balun is to reduce the feedline radiation. Feedline radiation can distort the radiation part of the antenna-the direction we want the RF to travel. This can produce nulls in the pattern directions that nothing can be transmitted or received. With the help of baluns, balanced loads can be fed through unbalanced transmission lines. Baluns are capable to provide better connection of a parallel line to a transceiver. Baluns can prevent unwanted RF currents from flowing in the ‘third’ conductor of a coaxial cable.

What is a balanced transmission line

In a balanced transmission line, the whole current flows through the transmission line and the load. The current in a balanced transmission line is symmetric to the ground so that the current won’t flow through the ground.

What is a balanced circuit

A balanced circuit is not defined according to the equality of the current or voltage on two conductors. It is defined as the circuit in which the impedances of the two conductors to the reference plane are equal in both magnitude and phase. A balanced circuit acts as a wheat-stone bridge, rejecting noise by virtue of the balance of the impedances within that system.

Why is a balun used in an antenna

A balun is used at the feed point of the dipole antenna is to prevent RF currents from flowing on the outside of the shield of the coax feeding the antenna. RF currents outside of the coax shield cause the shield to radiate.

What are the types of baluns

Baluns are classified into two types, and both of them are different from each other. Each of them is different electrically.

Voltage balun

A voltage balun is a transformer, mostly with a primary and one or more secondary windings on a ferrite core. A voltage balun is one whose output voltage is equal and opposite, balanced with respect to ground. In order to attain true balance, the baluns load must be symmetric with respect to ground. Voltage baluns are commonly used and can be easily constructed. Transmission line voltage balun is constructed solely from the transmission line and requires cores. This balun is used only over a narrow range of frequencies.

Current balun

A current balun is balanced with respect to ground, its output currents are equal and opposite. The exception of the 1:1 current balun, current baluns are more expensive to construct than the voltage baluns so they are not widely used as a voltage balun. Current baluns could be made with RF transformers on ferrite cores or with lengths of the transmission line. The common-mode field can be seen by current balun. When wound with coax, this is true because all of the magnetic field associated with common mode current is outside the line, inside the coax all of the field requires for the transmission of power from the transmitter to the antenna is present. The current baluns work by adding a high impedance in series with the common-mode impedance of the line, thus reducing the common-mode current to a small value. Current baluns are classified into three types they are solenoid balun, inductive ferrite choke balun, and lossy ferrite choke balun.

What are the advantages of baluns

Baluns are not frequency-dependent, they can be used at many different frequencies. They can be used with higher SWR if well-constructed and air core.

What are the disadvantages of baluns

A small amount of inductance or impedance is introduced in the antenna system. High SWR will destroy the poorly constructed baluns, even at low power levels. Cores will absorb the energy and saturate easily, and this may mask an antenna problem, with the core being matched as the load.