Radio waves
A radio link needs a transmitter, an antenna and a receiver. The transmitter makes a current that swings back and forth. The antenna turns it into a radio wave that spreads out at the speed of light. The receiver's antenna catches a little of it.
Frequency is how many times the wave swings each second, measured in hertz (Hz). Wi-Fi and Bluetooth use about 2.4 GHz: 2.4 billion swings every second. The screen shows the wave slowed down billions of times.
Wavelength (λ, the Greek letter lambda) is the distance from one crest to the next. It is the speed of light divided by the frequency: λ = c ÷ f. At 2.4 GHz that is about 12.5 cm. Higher frequency means shorter waves.
Antennas and modulation
An antenna is a conductor sized to the wave. This one is a quarter-wave monopole: a rod λ/4 long, about 3.1 cm at 2.4 GHz, standing on a metal ground plane. Sized like this, it resonates and sends the power out well.
A steady wave carries no news. To send a message, the transmitter changes (modulates) the wave. AM, amplitude modulation, makes the height of the wave follow the message. AM radio stations broadcast at about 0.5 to 1.7 MHz.
FM, frequency modulation, keeps the height steady and instead makes the waves slightly tighter or looser to follow the message. FM radio uses about 88 to 108 MHz. Most noise changes a wave's height, so FM sounds cleaner.
Digital radios change height and timing (phase) together. In 16-QAM each short burst, called a symbol, takes one of 16 shapes, so it carries 4 bits. Wi-Fi 7 goes up to 4096-QAM: 12 bits per symbol.
Signal strength
Radio power spans huge ranges, so engineers use decibels (dB): +10 dB is ten times the power, +3 dB about double. dBm counts from 1 milliwatt. Our transmitter sends 20 dBm (100 mW); 1 m away the meter reads −20.1 dBm.
The wave spreads over a bigger and bigger sphere, so less of it reaches each spot. As the receiver moves away, each doubling of distance drops the signal by 6 dB, a quarter of the power. At 8 m the meter reads about −38 dBm.
A modulated wave is not one frequency but a slice of them; the width of the slice is its bandwidth. A Wi-Fi channel is 20 MHz wide: channel 1 covers 2402 to 2422 MHz. More bandwidth carries more bits per second.
Wi-Fi, Bluetooth and mobile phones all work this way. Each picks a frequency, sizes its antenna to the wavelength, modulates bits onto the wave and must fight path loss. The networking lessons on wireless and Wi-Fi build on exactly these ideas.
In detail
;; step 1/11 · radio transmitter makes an alternating current at the radio frequency antenna turns that current into an electromagnetic wave (and back) speed c = 299,792,458 m/s (exact), radio in air is very nearly c receiver its antenna turns a tiny part of the wave back into a current
The 20 dBm (100 mW) transmitter and the simple 0 dBi antennas are a teaching model with round numbers, not one product's datasheet. A real quarter-wave monopole has some gain (a few dBi), which would raise every meter reading by the same amount. Wavelength λ = c / f with c = 299,792,458 m/s (exact): 12.5 cm at 2.4 GHz, so a quarter-wave rod is about 3.1 cm. The waves on the screen and on the table are slowed down billions of times; the rings are not drawn to scale. Path loss uses the free-space formula (Friis 1946; ITU-R P.525): 40.1 dB at 1 m and 2.4 GHz, plus 6.02 dB for every doubling of distance. Walls, furniture and people add more. The distance marks on the table are spaced by doubling, not to scale.