Wireless basics: radio waves and spectrum
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Wi-Fi, Bluetooth and your phone send bits with no cable at all, on radio waves. Learn what frequency and wavelength mean (12.5 cm at 2.4 GHz), how changing a wave carries bits, why the radio spectrum is shared and regulated, and why higher bands are faster but reach less far.
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01About how long is one wave of a 2.4 GHz Wi-Fi signal?
Wavelength = speed of light ÷ frequency = 299,792,458 ÷ 2,400,000,000 ≈ 0.125 m, about 12.5 cm.
02A wave's frequency doubles. What happens to its wavelength?
λ = c ÷ f, and the speed c stays the same, so twice the frequency gives half the wavelength.
03A wave can take 2 heights and 2 phases. How many bits does each shape carry?
2 × 2 = 4 different shapes. 4 shapes can stand for 00, 01, 10 and 11, so each shape carries 2 bits (log2 of 4).
04Your laptop is two rooms away from the router. Which band will usually reach it best?
Lower frequencies lose less as they spread (5 GHz arrives about 4 times weaker in open air) and pass through walls better.
05Which statement about unlicensed bands is true?
Unlicensed bands such as 2.4, 5 and 6 GHz are open to Wi-Fi, Bluetooth and more, without a fee, so neighbours can interfere with each other.
المسرد
- Radio wave
- A ripple of electric and magnetic fields that travels at the speed of light. Wireless networks carry bits on it.
- Frequency
- How many times a wave repeats each second, measured in hertz (Hz). 1 GHz is a billion waves per second.
- Wavelength (λ)
- The distance from one wave crest to the next: the speed of light divided by the frequency.
- Modulation
- Changing a wave's amplitude (height) or phase (timing) so that it carries bits.
- Spectrum
- All the radio frequencies together. Governments divide it into bands and decide who may use each.
- Licensed band
- A band rented to one owner, such as a mobile operator. Nobody else may transmit there.
- Unlicensed band
- A band anyone may use within power rules, such as Wi-Fi's 2.4, 5 and 6 GHz bands.
- Channel
- A slice of a band that one network uses. In 2.4 GHz, channels 1, 6 and 11 do not overlap.
الملاحظات والمصادر
- Wavelength λ = c / f with c = 299,792,458 m/s: 12.5 cm at 2.4 GHz, 6.0 cm at 5 GHz, 5.0 cm at 6 GHz. Radio in air moves at very nearly c, about 3.34 µs/km.
- "About 4 times weaker" is free-space path loss between the same simple (isotropic) antennas, ITU-R P.525: 20·log10(5/2.4) = 6.4 dB. Real antennas, walls and furniture change the exact numbers.
- Band edges follow US (FCC) rules where countries differ; the mobile bands are 3GPP examples. Check your own country's regulator for the exact edges and power limits.
- The ruler is drawn to scale from 0 to 7.5 GHz. The waves on the screen and in the room are slowed down billions of times and drawn far longer than real wavelengths.
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