WLAN RF Fundamentals and Propagation
Signals, units and the link budget
Wi-Fi is radio, so every design and troubleshooting decision reduces to signal versus noise. Frequency and wavelength are inversely related (λ = c/f): 2.4 GHz is roughly 12.5 cm, 5 GHz roughly 6 cm, 6 GHz roughly 5 cm. Longer wavelengths diffract and penetrate better but need larger Fresnel zones and offer fewer channels.
Know the decibel cold. dBm is absolute power referenced to 1 mW (0 dBm = 1 mW, 20 dBm = 100 mW, 30 dBm = 1 W). dB is a relative ratio: +3 dB doubles, −3 dB halves, +10 dB is ten times, +20 dB is one hundred times. EIRP = radio transmit power (dBm) + antenna gain (dBi) − cable and connector loss (dB).
Propagation and noise
Reflection, refraction, diffraction, scattering and absorption all shape a real signal; multipath produces fading, intersymbol interference and nulls. Free-space path loss increases about 20 dB per decade of distance, i.e. 6 dB per doubling. The thermal noise floor is about −174 dBm/Hz, so a 20 MHz channel sits near −101 dBm, 40 MHz near −98 dBm and 80 MHz near −95 dBm: wider channels raise the noise floor and require more SNR. Higher-order modulation also costs SNR - 256-QAM needs roughly 30 dB or better.
Traps to expect
Questions love to mix dB and dBm, to ignore the 3 dB noise penalty of doubling channel width, and to assume more antenna gain is always better (it narrows beamwidth and can push EIRP past a regulatory limit). Do not assume lower frequency always wins - 5 GHz often performs better because of higher permitted power and cleaner spectrum.
Worked example
A 20 dBm radio feeding a 5 dBi antenna through 2 dB of cable: 20 + 5 − 2 = 23 dBm EIRP (about 200 mW). If the regulatory ceiling is 20 dBm EIRP, the radio must drop to 17 dBm to stay compliant.