Antenna length calculator
A resonant antenna is a fraction of a wavelength, shortened slightly because current travels a little slower in a wire than in free space.
MHz
%
Quarter-wave element
0.4910m
- Half-wave dipole, total length
- 0.9821
- Each dipole leg
- 0.4910
- Five-eighths wave element
- 1.2276
- Free-space wavelength
- 2.0675
Show the working
- Frequency = 145
- Velocity factor = 95
- f_hz = 145 * 1e6 = 145 000 000
- Free-space wavelength = c / 145 000 000 = 2.067 53
- effective = 2.067 53 * 95/100 = 1.964 16
- Quarter-wave element = 1.964 16 / 4 = 0.491 039
- Half-wave dipole, total length = 1.964 16 / 2 = 0.982 079
- Five-eighths wave element = 1.964 16 * 5/8 = 1.2276
- Each dipole leg = 1.964 16 / 4 = 0.491 039
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- wavelength = c / f
- element length = wavelength x velocity factor / 4 for a quarter wave
The velocity factor for a thin wire in air is around 0.95; thicker elements and insulation lower it further.
The speed of light is exact by definition, so the free-space wavelength here is an exact rational number.