Wavelength and Frequency Calculator

λ = c / f, where c is 299,792,458 m/s. With frequency in MHz that comes out as λ = 299.792458 / f, and almost every handbook rounds it to 300 / f.

That rounding is worth a moment, because this page used to do it and it is why the answer here now differs slightly from older printouts. Using 300 makes every wavelength 0.07% long. On 40 m that is about 3 cm on a 42 m span, which is nothing when you are cutting wire and are going to trim for lowest SWR anyway. It stops being nothing when you print the result to three decimal places, as this page does, because then you are showing five significant figures of a number that only has three. So the exact constant is used throughout and 300 / f is left where it belongs, in your head at the operating position.

Going backwards

Bands are named by wavelength and tuned by frequency, which is the small daily annoyance this page exists for. The 20 m band is not at 20 MHz, it is at 14 MHz. Switch the direction above to go from metres to megahertz and the arithmetic is the same relationship read the other way: f = 299.792458 / λ.

Velocity factor, and why a quarter wave of coax is not a quarter wave

A radio wave travels slower through a dielectric than through free space, and the ratio is the velocity factor. Solid polyethylene coax such as RG-58 or RG-213 runs at 0.66, so a quarter-wave matching section for 14.2 MHz is 5.28 m of physical cable rather than the 5.28 / 0.66 you might expect from the free-space figure alone. Get this backwards and a phasing line, a matching stub or a delay line lands nowhere near where it should.

Typical figures: bare wire in air behaves at about 0.95, window ladder line around 0.91, foam coax like LMR-400 at 0.85, solid PE coax at 0.66, PTFE at 0.70. Manufacturers vary, so use the figure on the datasheet for the reel in your hand if you have it.

Cutting an actual antenna

This page deliberately stops at the wavelength and the electrical length. For dimensions you are going to cut, the dipole antenna length calculator applies the 468/f rule and accounts for wire gauge, and the quarter-wave vertical calculator handles radials and element diameter. Both do the end-effect correction properly, which a bare λ/2 figure does not.

Band edges shown here are the United States allocations, which is IARU Region 2. Region 1 and Region 3 differ, most noticeably on 40 m and 80 m, so check your own licence conditions before transmitting anywhere near an edge.