Balanced two-wire line calculator

Why ladder line is still with us

An open-wire line has almost no dielectric loss and is happy at a high SWR, which is why it feeds multiband wire antennas through a tuner. The price is symmetry: you cannot run it along metal, and you cannot connect it to a rig without a balun.

g — gap between the wires, d — wire diameter, D — spacing between the axes

Gap between wires g (mm)
Wire diameter d (mm)
Result
Characteristic impedance Z₀
Centre-to-centre wire spacing D
Propagation delay
Inductance per metre
Capacitance per metre

📐 The formulas

Characteristic impedance (wires in air):

D  = g + d
Z₀ = 119.917 × ln(D/d + √((D/d)² − 1))

For D ≫ d the formula simplifies to the familiar form:

Z₀ ≈ 276 × lg(2D / d)

Per-metre parameters are worked out as for coax, with ε = 1.000586 (air).

Symbols:

• g — the clear gap between the wires [mm]
• d — wire diameter [mm]
• D — centre-to-centre spacing [mm]

Where this calculator stops:

• The calculation is for bare wires in air; insulation lowers the impedance by 5–10 %
• The wires must be the same diameter and parallel along their whole length
• Spacers, fixings and nearby metal change the result
• Commercial 300/450 Ω ladder line has a rated impedance of its own, and there is no need to recalculate it

🎯 In practice

• Feeding a dipole through a tuner. A 450–600 Ω line lets one wire work on several bands.
• A home-made feeder. Two 2 mm wires spaced 20 mm apart give about 500 Ω.
• Quarter-wave transformers to high impedances, where coax is awkward.

Practical advice:

1. The impedance rises slowly: to add 100 Ω you have to roughly double the spacing.
2. Fit spacers every 30–50 cm, or the line breathes in the wind and the impedance wanders.
3. Always put a 1:1 or 1:4 current balun at the transition to coax.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «Two-Wire Line Impedance Calculator (Ladder Line)». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/balanced_line/ (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.