Calculate a 5/8-wave antenna length or reverse frequency with kHz-to-GHz and metric-imperial conversions, velocity factor, and trim allowance.

Find the free-space 5/8 wavelength, with optional physical shortening and extra cutting length.

Optional settings

Default: 1 for free space. Enter a known construction-specific factor; a coax cable’s velocity factor is not automatically the radiator factor.

Blank means zero. Adds extra length after shortening; this is not a tuning prediction.

5/8 Wave Antenna Length Formula

The calculator uses c = 299,792,458 m/s and the free-space 5/8 wavelength relationship. The default shortening factor is 1 and the cut-long allowance is zero. Optional settings apply a supplied construction factor and extra cutting length separately.

L = 187.37028625 / f
f = 187.37028625 / L
  • L = 5/8-wave antenna length in meters
  • f = frequency in MHz
  • 187.37028625 = 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 6.940 m 22.77 ft CB radio
50 MHz 3.747 m 12.30 ft 6 meter band
146 MHz 1.283 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 = 187.37028625 / 146

The result is:

L = 1.2834 m

So a free-space 5/8-wave antenna for 146 MHz is about 1.283 meters, or about 4.210 feet.

Example 2: Find the frequency for a 0.5 meter 5/8-wave antenna

Use the frequency formula:

f = 187.37028625 / 0.5

The result is:

f = 374.7406 MHz

A 0.5 meter free-space 5/8-wave antenna corresponds to 374.7406 MHz.

FAQ

Is this the exact physical length I should cut?

Not always. With the default factor of 1 and no allowance, the calculated value is the free-space 5/8 wavelength. With optional settings, it is the adjusted cut-long 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 187.37028625?

The exact vacuum speed of light gives 299.792458 × 0.625 = 187.37028625 when frequency is in MHz and length is in meters. The common 187.5 shortcut uses 300 instead of 299.792458 and is approximate. For an adjusted physical length, multiply by the supplied shortening factor; add any cut-long allowance separately.

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.