Calculate 5/8 wave antenna length from frequency or find the needed frequency from antenna length in MHz, GHz, kHz, meters, feet, or inches.
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5/8 Wave Antenna Length Formula
The calculator uses the free-space 5/8 wavelength relationship. Frequency is converted to MHz and length is converted to meters before the formula is applied.
L = 187.5 / f
f = 187.5 / L
- L = 5/8-wave antenna length in meters
- f = frequency in MHz
- 187.5 = free-space constant for 5/8 wavelength when frequency is in MHz and length is in meters
If you enter frequency, the calculator finds the 5/8-wave length. If you enter length, it finds the frequency where that length is 5/8 of a wavelength. The calculator also converts between MHz, GHz, and kHz for frequency, and meters, feet, and inches for length.
Common 5/8-Wave Antenna Lengths by Frequency
These values are free-space lengths. A real antenna may need trimming or adjustment depending on conductor size, loading coils, ground plane, mounting height, and nearby objects.
| Frequency | 5/8-Wave Length | Length in Feet | Typical Band Area |
|---|---|---|---|
| 27 MHz | 6.944 m | 22.78 ft | CB radio |
| 50 MHz | 3.750 m | 12.30 ft | 6 meter band |
| 146 MHz | 1.284 m | 4.21 ft | 2 meter band |
| 440 MHz | 0.426 m | 1.40 ft | 70 cm band |
| 915 MHz | 0.205 m | 0.67 ft | ISM band |
Frequency and Length Unit Conversions
| Quantity | Conversion Used |
|---|---|
| 1 GHz | 1000 MHz |
| 1 kHz | 0.001 MHz |
| 1 ft | 0.3048 m |
| 1 in | 0.0254 m |
Examples
Example 1: Find the 5/8-wave length for 146 MHz
Use the length formula:
L = 187.5 / 146
The result is:
L = 1.2842 m
So a free-space 5/8-wave antenna for 146 MHz is about 1.284 meters, or about 4.213 feet.
Example 2: Find the frequency for a 0.5 meter 5/8-wave antenna
Use the frequency formula:
f = 187.5 / 0.5
The result is:
f = 375 MHz
A 0.5 meter free-space 5/8-wave antenna corresponds to 375 MHz.
FAQ
Is this the exact physical length I should cut?
Not always. The calculated value is the free-space electrical length. A real antenna can be affected by the material, conductor diameter, insulation, end effects, matching network, loading coil, and nearby objects. In practice, you usually cut slightly long, measure SWR or resonance, then trim in small steps.
Why does the calculator use 187.5?
A full wavelength in meters is approximately 300 divided by frequency in MHz. A 5/8 wave is 0.625 of that full wavelength, so 300 × 0.625 = 187.5. That gives the formula L = 187.5 / f when length is in meters and frequency is in MHz.
Does a 5/8-wave vertical need a ground plane?
Usually, yes. A 5/8-wave vertical commonly needs a ground plane, radials, vehicle body, or other counterpoise system to work properly. It also typically needs matching because its feedpoint impedance is not usually a direct 50-ohm match.
