Discussion of Balun vs. Unun

Key Takeaways

• Baluns convert between balanced and unbalanced circuits, ensuring a balanced pair of terminals on the antenna side. Ununs, on the other hand, are used for impedance matching between two unbalanced circuits, connecting the antenna side directly to the ground.

• A balun, the pin and ground pass equally through the transformer, maintaining balance. In contrast, an unun features a direct connection between the antenna and ground, creating an imbalance

• Baluns also serve to reject common-mode signals and can be made using various technologies like coaxial and stripline

Schematic of Balun (left) and Unun (right)

Baluns are primarily designed to convert between balanced and unbalanced circuits, while ununs are used for impedance matching between two unbalanced circuits. This distinction becomes evident when comparing their schematics, particularly in the way the feed line center and ground relate to the antenna's leads. In the design of an unun, the antenna side is directly connected to the ground on the feed line side. The central pin of the unun extends through the transformer, leading to an imbalance in the two pins on the antenna side. In contrast, a balun's design is different. Both the pin and ground pass equally through the transformer, resulting in a balanced pair of terminals on the antenna side.

Balun vs. Unun: A Comparison

Baluns vs. Ununs

The internal circuitry of a balun vs unun differs: In an unun, there are connections to earth on both the input and output sides. In contrast, a balun features an earth-side connection only on its unbalanced side. Notably, in a balun, the coaxial cable's outer layer, which is at earth potential, connects to the transformer's center point. This leaves the ends of the autotransformer balanced without an earth connection for radio frequency (RF) signals.

• A balun not only matches a balanced load to an unbalanced line but also serves additional functions. For instance, a current balun can create a high impedance to common-mode signals, aiding in noise rejection. These common-mode signals, being identical on both conductors, are not balanced or differential.

• An unun, acts as an impedance transformer, typically with ratios of 4:1 or 9:1. It is used to match an unbalanced antenna to a feedline. For example, a 9:1 transformer is commonly utilized with an end-fed half-wave antenna.

Notes on Baluns

Baluns, commonly known for their core-and-wire transformer designs, can also be constructed using coaxial and coupled stripline technologies. Its primary function is to match the impedance between an aerial (antenna) and a transmission line. The essential role of an HF balun is to transform an unbalanced signal coming from a coaxial cable into one that is compatible with a balanced antenna, like a dipole, or to perform the reverse process. This transformation is achieved by employing a transformer with a differing number of turns on each side. The ratio of these turns is carefully chosen based on the specific requirements for achieving the desired impedance match and balance, which is the fundamental principle of a balun.

Using Baluns

Baluns are most frequently used when a single-ended power amplifier drives a balanced load. This scenario is common with various antenna types, such as dipole antennas or single-ended antennas like whip antennas. In these cases, baluns facilitate the connection to an additional front-end amplifier, ensuring proper signal transmission and impedance matching.

With the advent of RF Integrated Circuits (RFICs), the application of baluns has expanded significantly. They are now integral in enhancing noise immunity and improving common mode rejection in complex electronic systems. This advancement is particularly important in the context of the burgeoning 5G technology, where there is a significant demand for small, wideband baluns. These baluns are crucial for interfacing with highly integrated radio transceivers that employ differential inputs and outputs. The growth of 5G and its applications has thus spurred a need for more sophisticated balun designs, capable of operating effectively across a broad range of frequencies while maintaining compactness and efficiency.

Notes on Ununs

Ununs are useful in scenarios where an unbalanced feedline is used to drive an unbalanced antenna, and there's an impedance mismatch between the feedline and the antenna. For example, a whip antenna, which typically has a low input impedance, would greatly benefit from an impedance-transforming unun. This device efficiently couples a 50Ω feedline with the antenna, ensuring better signal transmission and reduced losses.

The advantage of using an HF unun lies in its ability to provide a more effective impedance match for certain antenna types, such as end-fed wires. This efficiency stems from the unun's capacity to offer a more suitable transformation ratio between the high impedance of the antenna and the low impedance of the feed line. This results in less signal loss and improved antenna performance. For instance, in an unun where the output circuit has twice as many turns as the input, the impedance is transformed by a factor of four. This transformation ensures that the impedance levels between the antenna and the feedline are more closely aligned, enhancing overall system efficiency. Unun impedances can be quantified by the following formula:

Input vs output impedance as a function of number of turns on the transformer.

Choosing Between Baluns and Ununs

When deciding between a balun and an unun, consider whether your setup requires a feed line choke or an impedance transformer. This decision hinges on whether you need the output of this device to be configured in a balanced manner, which would call for a balun, or in an unbalanced way, for which an unun would be appropriate.

Explaining Balanced and Unbalanced Sources and Loads

Unbalanced circuit: Voltage is established between a single line and the ground. Current flowing between the ground and the source is identical to the current within the circuit. Balanced circuit: Demonstrates a differential signal flow

In a balanced circuit, signals move along two paths, with each path having an equal impedance relative to the ground. The overall impedance is determined by the impedance existing between these two paths. On the other hand, an unbalanced system is characterized by one of its terminals being directly connected to the ground.

The accompanying illustration contrasts the signal responses in balanced and unbalanced circuits. For the unbalanced circuit, it depicts how the voltage is established between a single line and the ground. Here, the current flowing between the ground and the source is identical to the current within the circuit. In contrast, the balanced circuit demonstrates a differential signal flow, where the voltage is the difference in potential between the two lines. In this scenario, the amount of current flowing to the ground on one line is exactly matched by the current flowing from the ground on the other line.

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