RF Field Guide · reviewed reference
Decibels
Decibels turn multiplication of power ratios into addition, making a link budget inspectable term by term.
Question
Why do RF planners use dB, dBm, and dBi?
Inputs
- A reference: one milliwatt for dBm, isotropic radiator for dBi
- Power ratios or gains and losses
Answer
Working
Power ratio in dB = 10 log10(P2/P1). A 3 dB change is approximately a factor of two in power; 10 dB is a factor of ten. Link budgets use signed dB terms so the final received level remains in dBm.
Visual explanation
- TX dBm
- + antenna dBi
- − cable dB
- − path dB
- + RX dBi
- = received dBm
Interpretation
Write every term with its sign and unit. That makes omitted cable loss, a reversed antenna gain, or a doubled loss visible before it becomes a field failure.