A real-life situation
A warehouse has good Wi-Fi in the office and almost none between the metal racks. A hospital has strong signal in the corridor but weak signal in a ward full of people. The same access points, but the materials around them change everything. Planning Wi-Fi means knowing what those materials do to a signal.
Five things that happen to a signal
Absorption
Water, people and walls turn signal into heat: weaker after.
Reflection
Metal and glass bounce the signal back.
Refraction
Passing into a denser medium bends the signal.
Scattering
Rough or small objects send it in many directions.
Diffraction
The signal bends around an obstacle, leaving an RF shadow behind it.
| Effect | What happens | Typical causes |
|---|---|---|
| Absorption | Energy is turned into heat; the signal is weaker after | Water, people, concrete, wood |
| Reflection | The signal bounces off a smooth surface | Metal racks, lift doors, coated glass |
| Refraction | The signal bends when entering a different material | Glass, layers of air with different temperature or humidity |
| Scattering | The signal splits in many directions | Rough surfaces, chain-link fences, foliage, dust |
| Diffraction | The signal bends around an edge, leaving an RF shadow | Corners, pillars, buildings |
Multipath
Because of reflections, a receiver often gets several copies of the same signal: one direct and others that bounced. They arrive a few nanoseconds apart. If they arrive out of step they can partly cancel each other (fading); in older Wi-Fi this caused errors and retries. Since 802.11n, MIMO (multiple-input, multiple-output) radios use several antennas and turn the different paths into an advantage: they combine the copies and send several spatial streams at once.
Interference
- Co-channel interference (CCI): two access points on the same channel hear each other. Wi-Fi handles it by taking turns, so each cell gets less airtime. The fix is channel planning and sensible transmit power.
- Adjacent-channel interference: overlapping channels, such as 1 and 3 on 2.4 GHz, corrupt each other's frames. Use only non-overlapping channels (1, 6, 11).
- Non-Wi-Fi interference: microwave ovens, Bluetooth, cordless phones, wireless cameras and radar (on some 5 GHz channels) share the bands. Spectrum analysis finds them.
Antennas
An antenna doesn't add energy; it shapes it. Its gain (in dBi) says how much stronger the signal is in its best direction compared with an ideal antenna radiating equally everywhere.
| Type | Coverage | Typical gain | Used for |
|---|---|---|---|
| Omnidirectional (dipole, ceiling AP) | All around, like a doughnut | 2–6 dBi | Offices, classrooms, most indoor APs |
| Patch (directional) | A wide lobe in front | 6–10 dBi | Corridors, along a wall, stadium seating |
| Yagi (directional) | A narrower, longer lobe | 10–14+ dBi | Long corridors, warehouse aisles, building-to-building |
| Parabolic dish (highly directional) | A very narrow beam | 20+ dBi | Point-to-point links over long distances |
Why it works this way
A radio wave interacts with any material it meets, and how much depends on the material and the wavelength. Water molecules absorb 2.4 GHz energy readily (the same reason microwave ovens use that band), so crowds and damp walls weaken signals. Metal reflects almost everything, which is why lifts and racks make dead spots. An antenna can only put its energy somewhere: focusing it in one direction means taking it from the others.
Common mistakes
- Turning the power to maximum to fix coverage. It increases co-channel interference and doesn't help the client's weaker reply.
- Using a directional antenna where an omnidirectional one is needed, leaving areas behind it uncovered.
- Planning Wi-Fi in an empty building. People absorb signal, so a full room behaves differently.
💡 Exam tip: match each effect to its cause: metal reflects, water absorbs, edges diffract, rough surfaces scatter, changing materials refract.
Key takeaways
- Signals are absorbed, reflected, refracted, scattered and diffracted on the way.
- Multipath delivers copies of a signal; MIMO radios use it to send several spatial streams.
- Co-channel interference shares airtime; fix it with channel planning and appropriate power.
- Antenna gain focuses energy: omnidirectional for general coverage, directional for long or narrow areas.
Check yourself
Signal is weak between tall metal shelving in a warehouse. Which effect is the main cause?
A long, narrow corridor needs coverage from one end. Which antenna fits best?
Two nearby APs both use channel 6. What is the result?