Electronics components, Magnetism, RF circuits, Telecomunications

What is balun?

The balun is used in telecommunication circuits, usually, it’s connected to an antenna. The operation and some types are today’s subjects.

What the balun do?

Balun is the abbreviation of balanced-unbalanced, this passive component’s functions are:

  • Make the connection between balanced and unbalanced. For example, between a balanced antenna and an unbalanced transmission line.
  • Impedance transformation. 

Balanced and unbalanced lines

When a line or circuit is balanced, it means that input and output currents are equal and in opposite directions. It’s told that the currents are symmetric in relation to ground.

balanced circuit
In this line, both currents Pr1 and Pr2 have the same value of 0.2 A.
unbalanced circuit
This line is unbalanced, because Pr1 is different from Pr2. The values are 0.25 A and -0.1 A respectively.

Transmission lines in radiofrequency (RF) circuits must be balanced to protect themselves against external electromagnetic waves and to not distort signals.

noise in balanced and unbalanced lines
In unbalanced circuits, signal is distorted on output. While in balanced circuits, noises are canceled and the signal isn’t distorted. Source: Musician’s friend.

Without balun, the antenna’s radiation pattern is distorted, harming directivity

The importance of impedance matching (or transformation) in RF circuits

Physically, impedance is the measure of opposition to electric current in a determined voltage. The impedance matching is important to have maximum power transfer from the circuit to antenna and vice-versa. Therefore, higher efficiency as possible.

Voltage and current baluns

In a voltage balun, potential differences are equal and symmetrical in relation to the ground. While in a current balun, currents are equal and in the opposite direction.

Some types

LC

It’s a low cost solution, made of 2 capacitors and 2 coils. Used in narrowband applications, because this circuit has a resonance frequency f_{0}, whose formula is:

f_{0}=\frac{1}{2\pi \sqrt{LC}}

L and C are respectively inductance and capacitance.

discrete-lc-balun-1
The LC balun can have a single input and differential output or vice-versa, depending on the grounding spot. If Y is grounded, single input and differential output. If Z is grounded, it’s the opposite. Source: Analog RF/IntgCkts.

Transformer

transformer balun
Source: VK6YSF.

It’s the most known type, used in impedance matching between the TV antenna and unbalanced cable. The impedance rate depends on the number of turns in the primary and secondary windings. If secondary has the double of turns than the primary, input impedance becomes:

Z_{I}=Z_{L}(\frac{N}{2N})^2=\frac{Z_{L}}{4}

N is the number of turns and Z_{L} is the load impedance in output. This type only works for frequencies below 1 GHz, above this value, parasite capacitances between the wires and Foucault currents become higher.

Microstrip

microstrip balun
A single microstrip balun. Source: SIM University.

Made in a printed circuit board, the strip’s length must have a quarter of wavelength of RF signal. Exist many balun microstrip structures.

Exist many types of balun that will be shown in other posts.

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About Pedro Ney Stroski

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