Understanding RF Antenna Baluns

February 22, 2024

Cadence PCB Solutions

Key Takeaways

Baluns are used in RF PCB design for matching impedances and converting signals from balanced to unbalanced forms, particularly in antenna systems.
Balum types include: Classical Transformer, Auto-transformer, RF Choke, and more, each selected based on specific frequency and power requirements.
Baluns prevent radiation or pickup from coaxial feeder's outer conductor, ensuring minimal interference and optimal antenna performance.

A common simple balun configurations using an RF-compatible transformer. The center tap on the balanced side is usually optional and not needed.

In RF PCB design, there are instances where it is necessary to match impedances and simultaneously convert signals from balanced to unbalanced forms. RF antenna baluns are particularly useful in these scenarios: converting signals from balanced feeds or lines of antennas to unbalanced lines. By facilitating the conversion between a single-ended RF input signal and a differential signal, baluns enable the input of signals into differential receivers, dipole antennas, or other components that operate differentially.

Types of RF Antenna Baluns

Type	Description
Classical Transformer Balun	Utilizes a transformer with two separate windings, often on a toroidal ferrite core. This design electrically isolates the two circuits.
Other Transformer Antenna Balun	This can be either an air core with a former (possibly porcelain) or a magnetic conductor like ferrite, chosen based on frequency and power requirements. The turn ratio between primary and secondary windings adjusts input and output impedance, allowing it to also function as a matching transformer.
Auto-transformer Balun	Utilizes a single-winding auto-transformer for the balanced-to-unbalanced transition. Provides inherent DC grounding path, useful for discharging static.
RF Choke Balun	Increases inductance on the outer conductor of an unbalanced feeder to prevent signal travel along it. Implemented by coiling feeder turns at the antenna feed-point or winding coax around a toroidal former, considering coax bend radius and toroid properties.
Center-tapped Transformer Balun	A more refined version of the transformer balun with a center tap providing a shared reference for the balanced signal. Impedance varies based on the turns ratio.
Coupling in LC Circuits	Involves creating a circuit that mimics a center-tapped transformer's impedance transformation using reactive elements in an LC circuit. This complex approach often requires simulations.
Distributed Balun Designs	Advanced designs using coupling impedance between PCB traces or within integrated circuits for more complex applications

Why is an Unbalanced Feed Line Important?

When such antennas are fed directly with an unbalanced feeder, such as coaxial cable, issues arise. Coaxial feeders offer notable advantages for use in VHF and higher frequencies and are more manageable when routed through buildings or near other objects. For optimal performance and ease of use, it's necessary to use a balun to convert a balanced feeder or antenna feed point to an unbalanced system.

Why Baluns are Needed for Antenna Systems

The need for baluns in antenna systems stems from several factors.

Firstly, they prevent radiation or pickup from the outer conductor of a coaxial feeder. If a balanced antenna is fed directly with a coaxial feeder, the outer shield of the coax can radiate and pick up radio frequency signals. This can result in interference from nearby electrical devices or feeder loss during transmission.
Using a balun maximizes the antenna system's performance. In some cases, a system may seemingly function without a balun, but the only definitive way to determine the necessity of a balun is by measuring the common mode currents on the feed-line.
Minimizing radiation near the transmitter is also important. If the feeder radiates part of the RF signal meant for the antenna, it can disrupt transmissions in various ways, such as causing feedback in audio lines or failures in data mode transmissions. Using an RF antenna balun can mitigate these issues even if it doesn't completely resolve them.
Impedance matching between the balanced and unbalanced ends. In transformer baluns, for example, this would be done by selecting the appropriate turns ratio, or rather the ratio of primary and secondary coil inductances.
Baluns transfer power through (electric or magnetic) coupling, and so they provide some natural isolation between balanced and unbalanced signals. As long as the balun is designed correctly, this nicely helps isolate radiated EMI from passing between each side of the balun.
The balanced end of the balun also has high common-mode noise immunity if the signal is fed into a differential receiver.

RF Antenna Balun Applications

Baluns, commonly associated with RF antennas, serve diverse roles in several other areas, such as power line communications, data communications, and more, discussed below.

Applications	Role/Usage
RF Antennas	Crucial for balanced antennas like a dipole; preferred for coaxial cables feeding due to flexibility in installation. Easy routing along antenna towers, poles, across the ground, and through buildings while maintaining performance.
Power Line Communications	Serve diverse roles in power line communications.
Data Communications	Involved in converting signals between different types like Cat 5, Cat 6, and Twinax.
Video Applications	Facilitate the conversion between twisted pair and coaxial cables.
Passive amplifiers, frequency multipliers, phase shifters, modulators, and feeding mechanisms of dipole antennas	Used in single-ended power amplifiers to drive a balanced load such as a dipole-antenna element, or when a single-ended whip antenna is connected to a differential front-end amplifier.

RF Antenna Balum Architecture

The primary purpose of a balun is to separate the direct connection between a balanced antenna or feeder and an unbalanced coaxial feeder. This separation ensures that the earth connection of the unbalanced cable does not directly link to the balanced system, maintaining its balanced state.

The most common approach to constructing a balun involves creating a transformer with two windings: one side grounded and the other providing differential outputs. Although it's possible to ground a center tap on the balun, this practice is generally not employed in antenna systems.

A balun operates bidirectionally, meaning it doesn't have distinct inputs or outputs, but rather features balanced and unbalanced sets of terminations. Baluns are defined by several key RF parameters, such as

Frequency range
Bandwidth
Insertion loss
Magnitude imbalance
Phase imbalance
Linearity
Distortion
Power rating
Size, and cost

Current vs Voltage Baluns

Current balun forces equal current through the two sides of a balanced load. The 1:1 current balun is effectively a simple transmission line such as a bifilar winding or coaxial cable wound on a ferrite core.
Voltage balun forces equal voltage across the two sides of a balanced load. Voltage baluns are a little more complicated than current baluns and prove to be very inefficient in preventing the coaxial cable shield as well as open ladder line to carry common mode currents and radiate energy. Accordingly, voltage baluns are not used for most antenna applications.

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