Differentiate between dBm, dBW, and dBi; which statement is correct?

• A. dBm is a measure of power relative to 1 W.
• B. dBW is a measure of gain relative to isotropic radiator.
• C. dBi is gain relative to an isotropic radiator.
• D. 0 dBW equals 0 dBm.

Answer: (C)

Power and gain use different reference points measured in decibels. dBm and dBW express absolute power levels, while dBi expresses directional gain relative to an ideal isotropic radiator.

dBm is decibels with 1 milliwatt as the reference, so it measures actual power in milliwatts on a logarithmic scale. For example, 0 dBm equals 1 mW. dBW uses 1 watt as the reference, so 0 dBW equals 1 W (which is 30 dB higher than 0 dBm). dBi, on the other hand, is decibels relative to an isotropic radiator and indicates how much the antenna concentrates power in a given direction compared to that ideal, non-directional radiator. If an antenna has 6 dBi gain, it radiates 6 dB more power in its main direction than an isotropic radiator would.

The statement about dBi is the correct one because it accurately describes dBi as a gain metric relative to an isotropic radiator. The other statements are off: dBm references 1 milliwatt, not 1 watt; dBW measures power relative to 1 watt, not gain; and 0 dBW does not equal 0 dBm (0 dBW = 1 W, which is 30 dBm).