RF Passives › Baluns & Transformers

Balun & RF Transformer Calculator

Design baluns and RF transformers for impedance matching and balanced-to-unbalanced conversion. Covers wound ferrite transformers, transmission-line baluns and guanella/ruthroff topologies. Computes turns ratio, winding inductance, bandwidth and core selection.

Quick impedance ratio
// Electrical Specifications
Ω
Ω
MHz
MHz
W
// Core Selection
FERRITE CORE Mix 43 / T50 Z₀ source ZL load N₁ turns N₂ turns Impedance ratio 4:1
// Winding Design Results
Turns Ratio & Windings
Impedance ratio ZS/ZL—
Voltage turns ratio N1/N2—
Primary turns N1—
Secondary turns N2—
Primary inductance Lp (calc)—
Minimum Lp required—
Bandwidth & Performance
Lower −3 dB (f_low check)—
Leakage inductance (est.)—
Estimated IL at f_low—
Max power (core limit)—
Wire & Core
Wire diameter—
Est. primary wire length—
Core AL value used—
Core circumference—
// Design Equations
Impedance ratio: n = ZS/ZL = (N1/N2)²
Turns ratio: N1/N2 = √n
Min primary inductance: Lp,min = ZS/(2π × flow × 5)
→ XLp ≥ 5×ZS at flow for <0.5 dB IL

Turns from AL: N = √(L/AL)   [N integer, L in nH, AL in nH/T²]
Leakage inductance: Llk ≈ 0.01×Lp to 0.05×Lp (coupling factor)
Upper −3 dB: fhigh ≈ ZS/(2π×Llk) — limited by leakage
Winding order matters: Wind primary and secondary simultaneously (bifilar) for minimum leakage inductance and maximum coupling. For high isolation, wind separately in opposite directions. Use enameled copper wire and keep all turns tight against the core.
// Transmission-Line Balun (Choke Balun)

A λ/4 coaxial choke balun wraps a section of coaxial cable around a ferrite core. It blocks common-mode currents from flowing on the outer shield without affecting the differential mode. No impedance transformation.

MHz
Ω
—
mm
dB
// TL Balun Results
Physical λ/4 length in cable—
Choke impedance (sheath)—
Turns on ferrite core—
Min toroid OD—
Common mode isolation—
Insertion loss (diff mode)—
Choke impedance: ZCM = j×2πf×Lchoke
Isolation: 20·log(ZCM/Z0) dB
Physical length: l = VF × c / (4 × f0)
Note: Differential mode passes through unaffected (coax shield = outer conductor). No impedance transformation — use wound transformer for that.
// Guanella Current Balun

Guanella baluns use transmission-line sections wound on ferrite cores. They force equal and opposite currents (current balun), providing high CMR and low insertion loss across wide bandwidths. Stacking N units gives N²:1 impedance transformation.

Ω
Ω
MHz
MHz
Ω
// Guanella Balun Results
Impedance ratio—
Number of units N required—
Actual transformation achieved—
Recommended Zline—
Min choke inductance per unit—
Estimated turns per unit (T50/43)—
CMR at flow (estimated)—
Units needed: N = √(ZL/ZS)   (rounded to integer)
Transformation: ZL/ZS = N²
Recommended line Z: √(ZS × ZL) / N
Min choke L: XL ≥ 5×ZS at flow
CMR: ≈ 20·log(2πf·Lchoke / ZS) dB

Balun and RF Transformer Design Guide

A balun (balanced-unbalanced) converter interfaces between balanced transmission lines (like a dipole antenna or push-pull amplifier) and unbalanced lines (coaxial cable, single-ended RF circuits). RF transformers provide both impedance transformation and galvanic isolation.

Wound Ferrite Transformer

Wound ferrite transformers use bifilar or trifilar windings on a ferrite core to achieve impedance transformation across a wide frequency range. The primary inductance must be large enough that its reactance equals several times the source impedance at the lowest operating frequency. Mix 43 material is popular for HF (1–50 MHz), Mix 61 for VHF/UHF, and Mix 31 for low-frequency applications.

Common-Mode Rejection

The common-mode rejection ratio (CMR) describes how well a balun suppresses common-mode currents. A 1:1 choke balun on a dipole prevents RF current from flowing back down the coax outer shield. Good CMR requires high choke impedance — at least 10× Z₀ at the lowest frequency.