Basic RF Concepts
What you must understand
RF is about signal, noise, and how they relate. Signal strength is measured in dBm (a logarithmic absolute unit, negative for typical Wi-Fi), and every 3 dB is a doubling or halving of power. Every 10 dB is a tenfold change. Gain and loss in antennas and cables are expressed in dBi and dB. SNR is signal minus noise in dB; higher is better, and roughly 25 dB or more supports high data rates while below about 10 dB is unusable.
Know the 2.4 GHz and 5 GHz bands. In 2.4 GHz, only channels 1, 6, and 11 are non-overlapping in North America; each channel is 20 MHz wide but spaced 5 MHz apart, causing overlap. In 5 GHz, channels are spaced 20 MHz apart, so adjacent 20 MHz channels do not overlap. Wider channels (40, 80 MHz) increase throughput but consume spectrum and raise the noise floor.
Free-space path loss increases with frequency and distance, so 5 GHz attenuates faster than 2.4 GHz. Higher frequency means shorter range but more available spectrum. Multipath causes fading and is partly mitigated by MIMO. Fresnel zone clearance matters for long links.
Traps to expect
- Thinking a higher dBm number is always better without sign: -50 dBm is stronger than -70 dBm.
- Assuming 2.4 GHz channels 1-11 are all non-overlapping.
- Confusing dB (relative) with dBm (absolute).
- Believing 5 GHz has better range than 2.4 GHz.
- Forgetting that doubling channel width does not double real throughput.
Rule of thumb
Use the 3/10 rule: +3 dB doubles power, +10 dB multiplies by ten; subtract for loss. Estimate SNR as RSSI minus noise floor, with noise floor near -90 to -95 dBm in a quiet environment.
Worked example
A client sees -67 dBm signal and a -92 dBm noise floor. SNR is 25 dB, which is good and supports high MCS rates. If noise rises to -82 dBm, SNR falls to 15 dB and the client should rate-shift down or roam.