Calculator Inputs
Formula Used
| Free-space wavelength | λ₀ = c / f |
|---|---|
| Line wavelength | λline = λ₀ × VF |
| Target length | L = λline × electrical degrees / 360 |
| Two-wire line impedance estimate | Z₀ ≈ 276 × VF × log₁₀(2S / d) |
| Input impedance | Zin = Z₀ × (ZL + jZ₀ tan βl) / (Z₀ + jZL tan βl) |
| Reflection coefficient | Γ = (ZL - Z₀) / (ZL + Z₀) |
| Standing wave ratio | SWR = (1 + |Γ|) / (1 - |Γ|) |
How to Use This Calculator
Enter the operating frequency in MHz. Add the velocity factor for your ladder line. Use 0.95 for many air-spaced lines when no better value is known.
Enter the desired electrical length in degrees. Use 90 for a quarter wave, 180 for a half wave, or any custom value.
Leave actual physical length blank when you want the tool to calculate a target cut length. Enter a measured length when you want input impedance and phase for an installed line.
Add spacing, wire diameter, antenna load resistance, and reactance. Then enter power, line loss, system impedance, and tuner SWR limit. Press calculate to view the result above the form.
Example Data Table
| Band | Frequency | VF | Electrical Length | Spacing | Diameter | Load |
|---|---|---|---|---|---|---|
| 40 m | 7.15 MHz | 0.95 | 90° | 50 mm | 2 mm | 300 + j100 Ω |
| 20 m | 14.2 MHz | 0.95 | 180° | 50 mm | 2 mm | 200 + j150 Ω |
| 10 m | 28.4 MHz | 0.90 | 45° | 38 mm | 1.6 mm | 450 - j75 Ω |
Balanced Feeder Planning
Ladder line is a balanced feed line. It often has lower loss than coax. It can also tolerate high standing waves. The calculator helps estimate those effects before cutting wire. You enter frequency, velocity factor, spacing, conductor size, load data, and loss. The tool then converts those values into length, impedance, phase, and match figures.
Why Length Matters
A feed line is not only a wire path. It is also an electrical transformer. A short physical section can become a large electrical angle on higher bands. Quarter wave and half wave sections behave very differently. A half wave repeats the antenna impedance at the shack end. A quarter wave can transform resistance strongly. Custom degree entry helps compare both cases.
Impedance and Spacing
Open wire line impedance depends mainly on conductor spacing and wire diameter. Wide spacing raises impedance. Thicker wire lowers impedance. The formula used here is a practical two wire estimate. Real line may differ because of insulation, nearby metal, rain, bends, and construction accuracy. Use the result as a planning value, not as a final lab measurement.
Using Match Results
The input impedance result shows what the tuner may see. Resistance, reactance, SWR, and reflection data describe the match. A very high SWR is not always a failure with ladder line. Loss can still be low. Yet tuner voltage, current, and arcing limits matter. Check the warning line before applying high power.
Good Field Practice
Measure the line after installation. Keep it away from gutters, masts, walls, and grounded metal. Maintain spacing through turns and entry points. Use a balanced tuner or a proper current balun when needed. Export the CSV or PDF report for station notes. Those records help when you retune antennas, change bands, or compare seasonal performance.
Math Value
The maths view makes repeated experiments simple. Try several frequencies and watch the electrical angle change. Then change velocity factor and spacing. Small inputs can shift the tuner load by a large amount. This is useful for multiband doublets, loops, and temporary portable antennas. The numbers also help choose safe line routes and practical tuner settings before you climb, solder, or drill. Save each trial so future antenna changes remain easy later.
FAQs
What is ladder line?
Ladder line is a balanced two-conductor feed line. It is often used with doublets, loops, and multiband antennas because it can have low loss, even when SWR is high.
What velocity factor should I enter?
Use the maker’s published value when available. Air-spaced ladder line is often near 0.95. Window line or insulated line may be lower, depending on materials and construction.
Can this calculator replace antenna testing?
No. It gives planning estimates. Real feeders are affected by nearby metal, bends, rain, insulation, and installation quality. Measure the final station setup when possible.
Why does physical length change impedance?
A feed line acts as an impedance transformer. The same antenna load can look very different at the shack when line length changes by a quarter wave or more.
Should I always avoid high SWR?
Not always. Ladder line can work well with high SWR because its loss is often low. Still, tuner voltage, current, heating, and arcing limits must be respected.
What does load reactance mean?
Reactance is the imaginary part of antenna impedance. Positive values are inductive. Negative values are capacitive. Enter zero when the antenna load is purely resistive.
Why enter conductor spacing and diameter?
Those values estimate the characteristic impedance of the two-wire line. Wider spacing increases impedance. Larger wire diameter lowers impedance for the same spacing.
What does delivered power estimate mean?
It combines entered power, line attenuation, and mismatch effect. Treat it as an estimate for planning. Actual delivered power depends on tuner behavior and real line losses.