English is currently available.
Telecommunications Engineering DISCUSSION
I am working through RF exercises and keep mixing up the decibel units. A transmitter is given as 20 dBm, a cable as 2 dB loss, an antenna as 8 dBi and a receiver sensitivity as -95 dBm, and I am not sure which of these I may add and what the result means.
What is the difference between a plain dB and the units with a suffix, and what are the rules for combining them? Is 20 dBm plus 20 dBm equal to 40 dBm?
A plain dB is a ratio: 10 × log10(P2 / P1). It has no absolute size. The suffixed units are ratios to a fixed reference, so they are absolute levels. dBm is referenced to 1 mW and dBW to 1 W, so 0 dBm = 1 mW, 30 dBm = 1 W = 0 dBW, and -95 dBm is about 0.3 pW. dBi is antenna gain relative to an isotropic radiator.
Useful anchors: +3 dB is double the power, +10 dB is ten times, -3 dB is half. So 20 dBm is 100 mW, 23 dBm is 200 mW and 17 dBm is 50 mW.
The rules follow from that. A level plus or minus a ratio is a level: 20 dBm - 2 dB + 8 dBi = 26 dBm, which here is the EIRP. A level minus a level is a ratio: a received -70 dBm against a sensitivity of -95 dBm is a 25 dB margin. Two levels cannot be added in dB form.
20 dBm plus 20 dBm is not 40 dBm. Convert to milliwatts: 100 mW + 100 mW = 200 mW = 23 dBm. 40 dBm would be 10 W. Combining two equal, uncorrelated powers always gives +3 dB.
For voltage or current ratios the factor is 20, not 10, because power goes with the square of voltage: dB = 20 × log10(V2 / V1), valid when both are measured across the same impedance. Doubling the voltage is +6 dB, and that is the same +6 dB in power terms, a factor of four. It is one decibel scale, not two different kinds of dB.
To turn dBm into volts you need the impedance. In a 50 Ω system 0 dBm is sqrt(0.001 × 50) = 0.224 V rms, and 13 dBm, which is 20 mW, is 1 V rms.
Watch the antenna units. dBi is gain over an isotropic source, dBd is gain over a half-wave dipole, and dBi = dBd + 2.15. A supplier quoting 6 dBd and another quoting 8.15 dBi are describing the same gain, so convert before comparing, and use dBi in a link budget built on the free-space formula.
Similar care applies to dBc (relative to the carrier) and dBm/Hz (power density). The suffix always tells you the reference, and a number read with the wrong suffix is off by a fixed but large amount.
