Calculator Inputs
Example Data Table
| Frequency (MHz) | Wavelength (m) | Corrected Total Length (m) | Each Leg (m) | Classic Rule (ft) |
|---|---|---|---|---|
| 7.1 | 42.224 | 20.057 | 10.028 | 65.915 |
| 14.2 | 21.112 | 10.028 | 5.014 | 32.958 |
| 21.2 | 14.141 | 6.717 | 3.359 | 22.075 |
These sample rows use a velocity factor of 0.95, a straight-wire profile, and zero trim allowance.
Formula Used
1. Wavelength: λ = c / f
2. Ideal half-wave dipole length: Lideal = λ / 2
3. Practical corrected length: Lpractical = Lideal × VF × CF × (1 - Trim/100)
4. Each leg length: Lleg = Lpractical / 2
5. Classic comparison rule: Lclassic, ft = 468 / fMHz
Here, c is the speed of light, f is frequency, VF is velocity factor, and CF is the selected correction factor.
The classic 468 rule is included for comparison because many field builders still start from that approximation before trimming the antenna to resonance.
How to Use This Calculator
- Enter the target operating frequency and select its unit.
- Choose a build profile that matches the planned installation.
- Set velocity factor and correction factor, or keep the recommended defaults.
- Add any trim allowance if you want a shorter starting cut.
- Pick the preferred output unit and decimal precision.
- Press calculate to show the result block, export options, and the Plotly graph above the form.
FAQs
1. What frequency should I enter?
Enter the intended resonant frequency for the band segment you want to use most. For voice, digital, and CW sections, different center frequencies can give slightly different cut lengths.
2. Why is each leg half the total length?
A center-fed dipole has two conductive halves extending from the feedpoint. The calculator finds the full element length first, then divides it evenly to give the cut length for each side.
3. What does velocity factor change?
Velocity factor adjusts the ideal free-space length to better match practical conductor behavior. It helps account for real electrical propagation effects and gives a more buildable starting length.
4. Why compare with the 468 rule?
The 468 rule is a familiar field shortcut in feet for half-wave dipoles. Comparing both methods helps you see whether your selected practical factors create a longer or shorter starting length.
5. Does an inverted-V need a shorter wire?
Often yes. An inverted-V arrangement can resonate slightly differently than a straight dipole, so builders commonly shorten the total element a little. The preset correction profile helps reflect that behavior.
6. Why does trim allowance alter resonance?
Shortening the wire raises the resonant frequency. A trim allowance is useful when you intentionally start below the final physical length and then fine-tune after measurement or on-air testing.
7. Are these lengths final build lengths?
They are strong starting values, not guaranteed final dimensions. Height above ground, nearby metal, insulation, conductor diameter, and feed arrangement can all shift the actual resonant point.
8. Can I use this for receiving antennas?
Yes. The same geometry is useful for receive-only work. However, receive antennas may tolerate larger frequency offsets because efficiency and matching demands are often less strict than transmit designs.