Radio Frequency (RF) Fundamentals
This domain tests the behavior of radio waves and the math used to compare power, loss, and sensitivity. Expect wavelength, dB, dBm, EIRP, free-space path loss, signal-to-noise ratio, and multipath questions. You must be able to convert between mW and dBm and to combine gains and losses correctly.
Core RF rules
- Frequency and wavelength are inversely related: wavelength = speed of light / frequency. A 2.4 GHz wave is about 12.5 cm; a 5 GHz wave is about 6 cm.
- Decibels compare ratios. +3 dB doubles power, -3 dB halves it, +10 dB multiplies by 10, and -10 dB divides by 10. dBm is an absolute power: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W.
- EIRP = transmit power (dBm) + antenna gain (dBi) - cable/connector loss (dB). Regulatory limits apply to EIRP, not just radio output.
- Propagation includes reflection, refraction, diffraction, scattering, and absorption. Multipath causes fading and intersymbol interference; MIMO can exploit it. Free-space path loss rises about 6 dB each time distance doubles and is higher at higher frequency.
- SNR is signal minus noise in dB. A noisy floor, co-channel interference, and adjacent-channel overlap reduce usable range more than raw signal strength alone.
Worked example
An AP transmits at 20 dBm into a 6 dBi antenna through 2 dB of cable loss. EIRP = 20 + 6 - 2 = 24 dBm. Since 20 dBm is 100 mW and +4 dB is about 2.5x, 24 dBm is about 251 mW.
Traps
Never add dBm to dBm; add dB to dBm. Do not treat RSSI as a standardized unit. Do not assume a stronger signal always means better throughput if noise is also high. Remember that 5 GHz has more path loss and less penetration than 2.4 GHz.