Which of the following descriptions is correct about the relationship between power measurement units? (Multiple choice)

Question

Options

• AIf the power of A is 26dBm and the power of B is 20dBm, then it can be said that A is 6dBm
• BdB represents the relative value of power, 0dB represents the original power
• C-3dBm power is equivalent to one fifth of the original power
• D10dBm is equivalent to ten times the original power

• A
  12% (5)
• B
  84% (36)
• C
  5% (2)

Explanation

B is correct because dB is a dimensionless ratio, not an absolute unit. 0 dB means the ratio P₁/P₂ = 1 (i.e., no change from the reference), which is exactly "the original power." D is correct because 10 dBm = 10^(10/10) × 1 mW = 10 × 1 mW = 10 mW, which is ten times the 0 dBm reference power of 1 mW.

A is wrong because subtracting two dBm values (26 dBm − 20 dBm) yields a relative difference of 6 dB, not "6 dBm." dBm is absolute (referenced to 1 mW); saying the result is "6 dBm" is a unit error.

C is wrong because −3 dB corresponds to approximately half the power (10^(−3/10) ≈ 0.5), not one-fifth. One-fifth would be roughly −7 dB (10 × log₁₀(0.2) ≈ −7 dB).

Memory tip: Use the "3-10 rule" - 3 dB ≈ ×2 power (or ÷2 for negative), 10 dB = ×10 power. And remember: dB is always a ratio (relative), while dBm is absolute (referenced to 1 mW); you can only subtract dBm values to get dB, never dBm.

No community discussion yet for this question.