Calculator Inputs
Example Data Table
This example uses the common practical relation: total length (meters) = 143 ÷ frequency in MHz.
| Frequency (MHz) | Total Length (m) | Length per Arm (m) |
|---|---|---|
| 7.10 | 20.1408 | 10.0704 |
| 14.20 | 10.0704 | 5.0352 |
| 27.20 | 5.2574 | 2.6287 |
| 100.00 | 1.4300 | 0.7150 |
| 433.00 | 0.3303 | 0.1651 |
Formula Used
λ = c / f
Lideal = (λ / 2) × velocity factor
Lpractical = Lideal × shortening factor
Larm = Lpractical / 2
Γ = |(ZL - Z0) / (ZL + Z0)|
SWR = (1 + Γ) / (1 - Γ)
The calculator combines electrical wavelength, physical shortening, and impedance mismatch checks. This gives a practical first-cut dipole length plus basic feedline performance estimates.
How to Use This Calculator
- Choose whether you want to calculate from frequency or wavelength.
- Enter the value and select the matching unit.
- Adjust the velocity factor if insulation, loading, or nearby materials affect the wire.
- Leave the shortening factor at 0.953 for a common practical starting point.
- Enter feed line impedance, antenna impedance, power, efficiency, and gain.
- Click Calculate Dipole to show the results above the form.
- Review total length, arm length, SWR, return loss, and chart trend.
- Use the CSV or PDF buttons to export the calculated values.
Frequently Asked Questions
1. Why is the practical dipole length shorter than λ/2?
Real dipoles usually resonate slightly shorter than the pure half-wave value because of end effects, conductor diameter, insulation, and nearby objects. That is why a shortening factor is commonly applied.
2. Should I cut the wire exactly to the calculator result?
Use the result as a strong starting point, but leave a little trimming margin. Real installations shift with height, wire thickness, insulation, and surroundings.
3. Can I use this for 50-ohm and 75-ohm feed lines?
Yes. Enter the feed line impedance you plan to use. The mismatch section then estimates reflection coefficient, SWR, return loss, and mismatch loss.
4. What does velocity factor mean here?
Velocity factor helps model physical length changes caused by insulation, loading, or nearby dielectric effects. For a simple bare-wire starting estimate, many users begin near 1.00.
5. Does antenna height affect resonance?
Yes. Height above ground changes current distribution, feedpoint impedance, and radiation pattern. A dipole in free space behaves differently from one mounted close to earth or metal structures.
6. Is 2.15 dBi the normal gain for a half-wave dipole?
That is a common free-space reference value for an ideal half-wave dipole. Real gain changes with height, environment, conductor loss, and measurement conditions.
7. Can I use metric and imperial units together?
Yes. The calculator accepts multiple input units and reports both meters and feet for the most important length outputs.
8. What should I do if the final SWR is still high?
Check feedpoint balance, wire length symmetry, nearby conductors, height, choke or balun use, and connector quality. Then trim carefully and retest in small steps.