Wireless Communication Basics
A radio transmits 20 dBm. Which is this power expressed in mW?
- A20 mW
- B100 mW
- C200 mW
Show answer and explanation
Answer: B. 100 mW
dBm expresses power relative to 1 mW as P[dBm] = 10·log₁₀(P/1 mW). Conversely, P = 10^(20/10) mW = 10² mW = 100 mW. Power rises tenfold every 10 dB and roughly doubles every 3 dB, so 0 dBm is 1 mW, 10 dBm is 10 mW and 23 dBm is about 200 mW. Working in dBm lets gains and losses (dB) be handled by addition and subtraction.
- A. This reads the dBm value directly as mW. 20 mW is about 13 dBm.
- B. Correct. 10^(20/10) = 100, so it is 100 mW.
- C. This is 20 multiplied by 10, a power of about 23 dBm. dBm is logarithmic, so 20 dBm is 10² times 1 mW.