RF basics: frequency, wavelength and bands

A real-life situation

A new office has Wi-Fi on 2.4 and 5 GHz. Users near the access points get fast connections on 5 GHz; users two rooms away keep dropping back to 2.4 GHz, which is slower but still works. Nothing is broken. The two bands are different kinds of radio wave, and they behave differently in the same building. To design or troubleshoot Wi-Fi you need a few basic ideas about radio waves.

What a radio wave is

A radio transmitter makes an electric current flow back and forth in its antenna very quickly. That creates an electromagnetic wave that travels away at the speed of light. Three properties describe it:

• Frequency: how many cycles per second, in hertz. Wi-Fi uses about 2.4, 5 and 6 GHz (billions of cycles per second).
• Wavelength: the distance one cycle covers. Wavelength = speed of light ÷ frequency, so higher frequencies have shorter waves.
• Amplitude: how strong the wave is. It falls as the wave spreads out and passes through things.

Why it works this way

A wave spreads its energy over a growing sphere as it travels, so the signal weakens with distance in any band (free-space path loss). Higher-frequency signals also lose more on the way: their small antennas capture less energy, and walls, water and people absorb more of them. That is why 2.4 GHz reaches further and 5 or 6 GHz drops sooner.

On the other hand, there is far more spectrum at 5 and 6 GHz. More spectrum means more channels side by side and wider channels, which means more speed and less interference between neighbours.

How bits ride on a wave

A plain wave carries no information. The transmitter modulates it: it changes the wave's amplitude, phase, or both, in a way the receiver can measure. Each distinct state is a symbol that stands for a group of bits. Wi-Fi uses QAM (quadrature amplitude modulation):

• 16-QAM: 16 states, 4 bits per symbol.
• 64-QAM: 64 states, 6 bits per symbol.
• 256-QAM (Wi-Fi 5), 1024-QAM (Wi-Fi 6), 4096-QAM (Wi-Fi 7): 8, 10 and 12 bits per symbol.

More states per symbol means more speed, but the states are closer together and easier to confuse. So dense modulations only work with a strong, clean signal. As a client moves away, it steps down to simpler modulations: slower, but still working. That is the 5 GHz user two rooms away in the situation above.

Wi-Fi also uses OFDM: it splits each channel into many narrow subcarriers, each carrying its own symbols in parallel. Wi-Fi 6 adds OFDMA, which gives different groups of subcarriers to different clients at the same time.

Common mistakes

• Thinking higher frequency means a stronger signal. It means shorter waves and shorter reach.
• Confusing bandwidth in hertz (channel width, e.g. 20 MHz) with bandwidth in bits per second.
• Expecting maximum data rates far from the access point. The modulation, and the speed, drop with signal quality.

💡 Exam tip: know the three Wi-Fi bands, that 2.4 GHz travels further and has three non-overlapping channels (1, 6, 11), and that 5 GHz offers more non-overlapping channels with less range.

Key takeaways

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