wavelength
/ WAYV-length /
Drop a stone in a pond and ripples spread out; the distance from one ripple crest to the next is the wavelength. An electrical signal travelling down a wire is the same kind of wave, and its wavelength is the physical distance the wave covers during one full cycle. This single number decides the most important question in RF: is your wire short enough to treat as an ordinary node, or is it long enough that the voltage is genuinely different at its two ends at the same instant?
The rule is wavelength = speed / frequency, written lambda = v / f. In free space the wave travels at the speed of light, c = 3 times 10^8 metres per second. So a 1 GHz signal has lambda = 3 times 10^8 / 10^9 = 0.3 metres, about 30 cm. At 100 MHz it is 3 metres; at 2.4 GHz (WiFi) it is about 12.5 cm. Inside a cable or PCB the wave moves slower, typically 0.6 to 0.7 times the speed of light, so the wavelength on the board is shorter by that same factor. A common rule of thumb: once a connection is longer than about one tenth of a wavelength, you can no longer treat it as a simple wire.
Wavelength is the yardstick that tells you when ordinary circuit thinking stops working. A 5 cm trace is nothing at 1 kHz (wavelength 300 km) but is a meaningful fraction of a wavelength at 2 GHz and must be treated as a transmission line. Wavelength also sets the size of antennas, why a dipole is about half a wavelength long, and why filters and matching networks at microwave frequencies can be built from carefully sized strips of copper. Honest note: the geometry that matters is electrical length in wavelengths, not centimetres, so the same trace is harmless at one frequency and critical at another.
A WiFi signal at 2.4 GHz has a free-space wavelength of about 12.5 cm, so a half-wave dipole antenna for it is about 6.25 cm long, and a quarter-wave whip about 3.1 cm. That is why the antenna inside a router is so small.
Higher frequency means shorter wavelength and smaller antennas.
On a cable or PCB the signal is slower than light, so the on-board wavelength is shorter than the free-space value, often by a factor of 0.6 to 0.7. Always use the in-medium wavelength when judging a trace length.