Refer to figure.
Radio waves are electromagnetic waves that travel outwards from a transmitter at the speed of light. They are transverse waves, meaning that the electric and magnetic fields involved travel from side to side (or up and down) whilst the wave goes "forwards".

There are a few components of these waves that you are required to know. Two of the most important are wavelength and frequency.

We have already mentioned that the wave travels at the speed of light, but that does not tell us how long the wave is. The wavelength is the distance from one part of a wave cycle to the same part of the next wave cycle. For instance, from one trough to the next, or from one peak to the next.

We can use this information to tell us how many times a wave peak (or trough) will pass us every second, which is the frequency of the wave. This can also be described as the number of wave cycles per second, measured in Hertz (Hz)

The relationship between these values is by the formula:

c = f x λ

c = the speed of light (m/s)
f = frequency (Hz)
λ (Lambda) = wavelength (m)

VHF direction finding uses standard VHF two-way communication frequencies, which range from 118 MHz to 137 MHz. These are very close together, so we can do the wavelength calculation with any, and will therefore choose a well known one, 121.5 MHz (121.5x10⁶ or 121 500 000 Hz)

Rearranging the above formula to get:

λ = c / f

λ = 3x10⁸ / 121.5x10⁶ = 2.47 m

As this wavelength is in metres (and not more than 10 metres), then we would class it as metric.

A good one to remember is that standard VHF frequencies (30-300MHz) all have "metric" wavelengths, so from 1m to 10m.