Narrowband LC Balun Calculator

A balun converts a single-ended (unbalanced) signal into two outputs of equal amplitude and 180° phase difference (balanced). It is used to connect a differential transceiver output to a single-ended antenna, or a dipole to coaxial cable. This tool sizes a lumped LC lattice balun that also transforms the impedance.

Single-ended ↔ differentialImpedance transform Z1 → Z2Only 2 L and 2 CInductors in nH · capacitors in pF

① Input parameters

Unbalanced impedance Z1Ω Balanced impedance Z2Ω Center frequency foMHz

Z1 is usually a 50 Ω coax or single-ended microstrip. Z2 is the impedance between the two balanced terminals (for example a differential RF port or a folded dipole).

② Results

Part	Description	Value
L	Inductor (series in upper path, shunt in lower path)	nH
C	Capacitor (shunt in upper path, series in lower path)	pF

Schematic

Blue = inductor, green = capacitor, gray = ground, orange = differential port. The upper path is a low-pass L-section, the lower path a high-pass L-section.

Output phase

How to use

Enter the unbalanced impedance Z1 (normally 50 Ω).
Enter the balanced impedance Z2 (between the two differential terminals) and the center frequency fo in MHz.
Press Calculate to get L and C; both inductors share one value and both capacitors share the other.
Build the circuit as shown, then measure amplitude and phase balance of the two outputs and fine-tune.

Theory and formulas

The LC lattice balun combines a low-pass L-section (series L, shunt C) and a high-pass L-section (series C, shunt L) in parallel. At the center frequency they shift the phase by −90° and +90°, so the outputs are 180° apart. Every element has a reactance equal to the geometric mean of the two impedances, which also transforms Z1 to Z2:

Z = √(Z1 · Z2)
ω = 2π · f_o
L = Z / ω
C = 1 / (ω · Z)

In other words, at fo: X_L = X_C = √(Z1·Z2).

Design tips

An LC balun is narrowband, typically about 10–20% of the center frequency. For wide bandwidth use a transmission-line or transformer balun.
Symmetry sets the phase balance: use high-Q inductors and C0G/NP0 capacitors from the same series and keep both paths similar in length.
DC bias for the differential side can be fed through the grounded shunt inductor, as long as its current rating is adequate.
Before connecting an antenna, check any remaining mismatch with the antenna impedance matching calculator.

FAQ

Is Z2 the differential or the single-pin impedance?: Use the differential impedance, i.e. the total impedance between the two balanced terminals. If a port is specified as 100 Ω differential, enter 100.
Do I still need a balun when Z1 equals Z2?: Yes. Even with equal impedances, the single-ended to differential phase conversion is still required; each reactance then equals Z1.
Why is the measured phase difference not exactly 180°?: Component tolerance, parasitic capacitance and asymmetric routing unbalance phase and amplitude. Trim the capacitor or inductor in one path to improve it.

Related design tools

Lumped directional coupler Unequal-split Wilkinson divider Antenna impedance matching PCB loop antenna calculator Microstrip line impedance All 28 design tools ›

Designing an RF product? WENSHING supplies RF transceiver modules, LoRa modules, SMD ceramic antennas, directional antennas and UHF RFID readers, and supports custom RF circuit design.

Contact an engineer