What Is a Balun?
A balun (BALanced-UNbalanced converter) interfaces between unbalanced transmission lines (coax, microstrip) and balanced circuits (differential amplifiers, dipole antennas, push-pull PAs). An ideal balun provides equal amplitude, 180° phase split between its two balanced output ports while presenting a matched impedance to the unbalanced port.
Key Balun Parameters
| Parameter | Definition | Typical Spec |
|---|---|---|
| Amplitude balance | |S21| − |S31| (dB) | <0.5 dB |
| Phase balance | ∠S21 − ∠S31 − 180° | <5° |
| Insertion loss | −20·log|S21| − 3 dB | <1 dB excess |
| Return loss (unbal.) | −20·log|S11| | >15 dB |
| Common-mode rejection | CMRR (dB) | >20 dB |
| Impedance ratio | Z_bal / Z_unbal | 1:1 or 4:1 |
Balun Topologies
- Transmission line (Marchand): Broadband, microstrip or coax implementation, good balance
- Transformer (wound): Low frequencies, high CMRR, lossy at high frequencies
- Lattice (LC): Narrowband but compact, used in RFIC
- Rat-race (180° hybrid): Provides 0° and 180° outputs, narrowband ±20%
- MMIC active balun: Wideband, amplifying, higher power consumption
Analyzing a Balun with RF View
A 3-port balun is measured as a .s3p file. In RF View:
- Load the .s3p file — port 1 is unbalanced, ports 2 and 3 are balanced
- Plot S21 and S31 magnitude on same graph — the difference is amplitude imbalance
- Plot S21 and S31 phase — the difference should be 180°; deviation is phase imbalance
- Check S11 for input return loss
- Check S22 and S33 for balanced port match
- S23 (isolation between balanced ports) should be >15 dB
Impedance Transformation
Many baluns also perform impedance transformation. A 4:1 balun transforms 200 Ω balanced to 50 Ω unbalanced. The transformation ratio is set by the turns ratio squared (N²) for transformer-based designs, or by the line geometry in transmission-line designs.
RF View's Smith chart view shows the impedance presented at each port — useful for verifying that the transformation ratio is correct across the intended frequency range.