What are decibels, dBm and dBi?
A decibel expresses a power ratio as 10·log10(P2/P1); for voltages across equal impedances it is 20·log10(V2/V1). Adding a reference turns it into an absolute unit: dBm is power relative to 1 mW, so 0 dBm = 1 mW, +30 dBm = 1 W and −30 dBm = 1 µW.[1]
Why do RF engineers use decibels?
RF signal levels span many orders of magnitude, from +60 dBm transmitters to −120 dBm received signals, and cascaded gains multiply. In decibels, multiplication becomes addition, so a link budget is a column of sums.
What do the common dB units mean?
| Unit | Reference | Example |
|---|---|---|
| dBm | 1 mW | +20 dBm = 100 mW |
| dBW | 1 W | 0 dBW = +30 dBm |
| dBc | Carrier power | Harmonic at −40 dBc |
| dBi | Isotropic antenna | Half-wave dipole ≈ 2.15 dBi |
| dBd | Half-wave dipole | 0 dBd ≈ 2.15 dBi |
| dBµV | 1 µV | 0 dBm in 50 Ω ≈ 107 dBµV |
| dBµV/m | 1 µV/m field strength | Used in EMC limits |
| dBc/Hz | Carrier, per Hz | Phase noise |
The dipole value follows from its directivity of 1.64.[2]
How do you convert between dBm, watts and volts?
P(dBm) = 10·log10(P / 1 mW); P(W) = 10^((P(dBm) − 30)/10). In a 50 Ω system Vrms = √(P·50), so 0 dBm = 0.224 Vrms (0.632 V peak-to-peak for a sine) and dBµV = dBm + 107 (more precisely 106.99). Useful anchors: 3 dB ≈ 2×, 10 dB = 10×, 6 dB ≈ 2× in voltage.
What mistakes are common?
- Adding dBm values directly: two −10 dBm signals sum to −7 dBm, not −20 dBm.
- Using 20·log for power ratios.
- Forgetting that voltage dB conversions assume equal impedances.
- Confusing dBi and dBd (2.15 dB difference).
Frequently asked questions
What is 0 dBm in watts?
1 milliwatt.
How many dBm is 1 W?
+30 dBm.
What is a 3 dB change?
A factor of about 2 in power (1.995).
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.