For dBm, dB, dBi, which of the following is correct? (Multiple choice)

Question

Options

• AdBm: Characterizes the absolute value of power
• BdBi and dBd: Relative Values Characterizing Gain
• CdB: Characterizes the relative value of power
• DIf antenna A is 12dBi and antenna B is 14dBi, it can be said that A is 2dBi smaller than B

• A
  82% (18)
• D
  18% (4)

Explanation

A, B, and C are correct because each statement accurately describes how these logarithmic units are defined and used in RF engineering.

• A is correct: dBm expresses power relative to 1 milliwatt (0 dBm = 1 mW), making it an absolute power reference - you know the actual power level without needing another reference signal.
• B is correct: dBi (gain over an isotropic radiator) and dBd (gain over a half-wave dipole) both express antenna gain as a ratio, not an absolute quantity - they tell you how much better the antenna performs compared to a reference antenna.
• C is correct: Plain dB is always a dimensionless ratio between two power (or voltage/current) levels - it has no meaning without context, confirming it characterizes a relative value.

D is wrong because when comparing two antenna gains, the difference between them is expressed in dB, not dBi. Saying antenna A is "2 dBi smaller" is incorrect terminology; the correct phrasing is "2 dB smaller." dBi describes an antenna's absolute gain figure, while dB describes the difference between two such figures.

Memory tip: Think of the suffix after "dB" as the reference. dBm → referenced to a milliwatt (absolute). dBi/dBd → referenced to an antenna (relative gain). dB alone → just a ratio, no reference baked in. If there's a letter, ask "relative to what?"

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