Calculator Inputs
Formula Used
This calculator uses a standard lossless parallel-wire approximation.
Let D be center spacing, d be conductor diameter,
and εr be relative permittivity.
Z0 = (120 / √εr) × arcosh(D / d)L′ = (μ0 / π) × arcosh(D / d)C′ = (π × ε0 × εr) / arcosh(D / d)VF = 1 / √εrv = c × VFλ = v / fElectrical Length = 360 × Length / λ|Γ| = |(ZL - Z0) / (ZL + Z0)|VSWR = (1 + |Γ|) / (1 - |Γ|)
These equations are suitable for educational design estimates and planning checks. Real cable construction, moisture, nearby metal, and frequency-dependent losses can shift practical results.
How to Use This Calculator
- Enter conductor diameter in millimeters.
- Enter center-to-center spacing between the two conductors.
- Set relative permittivity for the insulating environment.
- Provide the full electrical line length.
- Enter the operating frequency in MHz.
- Enter load impedance to evaluate matching quality.
- Optionally enter forward power to estimate reflected power.
- Click the calculate button.
- Review impedance, delay, wavelength, and matching outputs.
- Use CSV or PDF export for reporting or documentation.
Example Data Table
| Example | Diameter (mm) | Spacing (mm) | εr | Length (m) | Frequency (MHz) | Approx. Z0 (Ω) |
|---|---|---|---|---|---|---|
| Ribbon TV Feed | 1.02 | 7.50 | 1.10 | 10.00 | 100.00 | 306.83 |
| Compact Balanced Pair | 1.20 | 6.20 | 1.20 | 8.00 | 145.00 | 254.61 |
| Low-Er Open Style | 1.50 | 12.00 | 1.03 | 15.00 | 28.00 | 327.14 |
| High-Er Plastic Spacer | 0.80 | 6.00 | 1.60 | 6.00 | 450.00 | 256.31 |
These sample rows help compare geometry and dielectric changes quickly.
FAQs
1. What does this calculator estimate?
It estimates characteristic impedance, capacitance, inductance, delay, wavelength, electrical length, VSWR, return loss, and reflected power for a balanced twin lead line.
2. Why does spacing affect impedance so much?
Greater conductor spacing weakens electric coupling between the wires. That increases characteristic impedance for the same wire diameter and dielectric environment.
3. Why does dielectric constant reduce impedance?
A higher relative permittivity increases capacitance between the conductors. Higher capacitance lowers characteristic impedance and slows wave propagation.
4. Is the result exact for every cable?
No. It is a design estimate. Actual products can differ because of insulation shape, conductor finish, nearby objects, moisture, manufacturing tolerances, and frequency-dependent losses.
5. What load impedance should I enter?
Enter the impedance seen at the line end, such as an antenna feedpoint or balanced network input. Matching quality is based on that value versus calculated line impedance.
6. What does electrical length tell me?
Electrical length shows phase rotation across the line at the chosen frequency. It is useful when matching stubs, phasing sections, and resonance-sensitive designs.
7. Why is VSWR important?
VSWR summarizes mismatch severity. Lower VSWR means less reflected energy, better power transfer, and generally more predictable line behavior.
8. Can I use this for window line too?
Yes, as a first-pass estimate. The same parallel-wire geometry principles apply, but real window line may require measured data for best accuracy.