Balun Types and Their Applications in RF Systems

This article explores various types of Baluns and their specific applications. We will cover L-C baluns, transformer baluns, folded baluns, coaxial baluns, microstrip baluns, and more.

What is a Balun?

Introduction:

A Balun is a device that connects a balanced transmission line to an unbalanced transmission line. The term “Balun” is a portmanteau of “Balanced-Unbalanced.” Essentially, it’s a device that matches two different impedances. For example, a balun transformer can be used to connect a transmission feed line to an antenna.

In this scenario, it might convert impedance from 300 ohms to 75 ohms, or vice versa. Here, 300 Ohms represents free space impedance, while 75 ohms is the characteristic impedance of a coaxial cable.

In summary, a balun serves two primary functions:

• Impedance Transformation
• Balanced to Unbalanced Transmission

Balun Types and Their Applications

Let’s delve into different types of baluns and their respective applications.

LC Balun

Description: Figure 1 shows an LC balun. It consists of two inductors and two capacitors and introduces a phase shift of +/-90 degrees. In this circuit, the operating frequency is kept below the self-resonance frequency.

Application: It’s commonly used at the output of a push-pull amplifier to convert a balanced signal to an unbalanced signal output.

Transformer Balun

Description: Figure 2 illustrates a transformer balun. This type of balun does not have a transmission line equivalent circuit. Its balanced mode output impedance is four times that of its input unbalanced mode.

Application: It’s often used in television antennas for impedance matching.

Folded Balun

Description: Figure 3 shows a folded balun. A dummy coaxial outer conductor is connected to the pole fed from the center conductor. It runs alongside the feeder coax for λ/4 and connects to ground. The other pole connects directly to the shield of the feeder coaxial. The outer conductor of the coaxial cable and the extra line form two lines in a three-wire line with ground.

Application: It enables the direct connection of a coax line to a dipole antenna.

Coaxial Balun

Description: This type of balun is designed using a λ/4 length of transmission line or coax cable. There are two main types: 1:1 coaxial baluns and 1:4 coaxial baluns. Figure 4 depicts these designs. The 1:1 impedance transformation is achieved using a quarter-wavelength of coax cable. The 1:4 impedance transformation is achieved using the alternative balun design as shown in the figure.

Application: Used in transmission line or coax cable-based systems to achieve 1:1 and 1:4 impedance transformations.

Microstrip Balun

Description: These balun types are realized on PCBs or specially manufactured microstrip dielectric substrates. Figure 5 shows a simple coupled-line microstrip balun. The structure has a λ/4 wavelength at the center frequency and is also known as a parallel line balun.

Application: Suitable for bandwidths over an octave, provided the coupling between the lines is very high. Microstrip balun types include rat race couplers, multiple coupled lines, printed Marchand baluns, and microstrip-based LC baluns.

Other Balun Types

Here are some other notable types of baluns:

• Sleeve balun (or Bazooka balun)
• Split coax balun
• Half-wavelength balun
• Candelabra balun
• Ferrite core balun
• Ferrite Candelabra balun
• Split Tapered coax balun
• Natural balun