Case 1
- Frequency (MHz)
- 14.2MHz
- Velocity factor (0.5–1)
- 0.95
10.0283m
Open with these valuesAntenna Length Calculator
10.0283m
Result: 10.0283 mA half-wave dipole is one half wavelength from tip to tip, shortened a few percent because a wave travels slower along wire than through free space. Divide 299.792458 by the frequency in MHz for the wavelength in metres, multiply by the velocity factor, halve it. A quarter-wave vertical is half again.
The numbers at a glance
Held fixed: Frequency (MHz) 14.2000 MHz.
| Velocity factor (0.5–1) | Result (m) |
|---|---|
| 0.825 | 8.7088 |
| 0.850 | 8.9727 |
| 0.875 | 9.2366 |
| 0.900 | 9.5005 |
| 0.925 | 9.7644 |
| 0.950Your value | 10.0283 |
| 0.975 | 10.2922 |
| 1.000 | 10.5561 |
Worked examples
Case 2
- Frequency (MHz)
- 7.1MHz
- Velocity factor (0.5–1)
- 0.95
20.0565m
Open with these valuesCase 3
- Frequency (MHz)
- 2400MHz
- Velocity factor (0.5–1)
- 0.95
0.0593m
Open with these valuesHow it's calculated
L = (c ÷ f) × VF ÷ 2
- StepEnter the operating frequency in MHz — 14.2 is the 20-metre band.
- StepLeave the velocity factor at 0.95 for bare wire, lower it for insulated wire.
- ResultRead the total dipole length; halve it for a quarter-wave vertical.
Reference table
| f (MHz), VF | Band | Dipole length (m) |
|---|---|---|
| 2400, 0.95 | Wi-Fi 2.4 GHz | 0.0593 |
| 100, 1 | FM broadcast, free space | 1.4990 |
| 28.5, 0.95 | 10-metre band | 4.9965 |
| 14.2, 0.95 | 20-metre band | 10.0283 |
| 7.1, 0.95 | 40-metre band | 20.0565 |
Questions
How do I calculate antenna length?
First the free-space wavelength: λ = 299.792458 ÷ f, with f in MHz and λ in metres. A half-wave dipole is then λ × velocity factor ÷ 2, and a quarter-wave element is half of that. For 14.2 MHz at a velocity factor of 0.95 the dipole is about 10.03 m.
What is the velocity factor, and is 0.95 a hard number?
It is how much slower a wave travels along real wire than through free space, and no, it is not a hard number. It is a practitioner's default from amateur radio, not a physical constant; published figures for bare wire scatter between roughly 0.95 and 0.97. Insulated or coated wire sits lower still, so cut long and trim.
What is the difference between a half-wave and a quarter-wave antenna?
A half-wave dipole is fed in the middle and its total length is half a wavelength, both legs together. A quarter-wave element is half that length and is used for verticals and ground-plane antennas, where the ground or a set of radials acts as the missing half. Halve the result above to get it.
Why is the frequency entered in MHz?
Because it keeps the arithmetic in one line: light covers 299.792458 metres per microsecond, so dividing that by a frequency in MHz gives the wavelength directly in metres. Divide kilohertz by 1000 and multiply gigahertz by 1000 before entering. That figure for the speed of light is exact by definition, not a measurement.
Is the calculated length exact?
It is a precise starting point, not a final figure. Height above ground, nearby objects, wire diameter and feed-line effects all shift the resonant length. Cut the wire a little long, measure the SWR and trim a few centimetres at a time.
Sources and last check
Information, not professional advice.