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.
| 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:
- The impedance rises slowly: to add 100 Ω you have to roughly double the spacing.
- Fit spacers every 30–50 cm, or the line breathes in the wind and the impedance wanders.
- Always put a 1:1 or 1:4 current balun at the transition to coax.
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UR3PKI. «Balanced two-wire line calculator». 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.