Radio frequency (RF) power is the amount of energy radiated by an antenna or a transmitter in the form of electromagnetic waves. RF power can be measured in different units, such as watts (W), milliwatts (mW), or decibels (dB). However, some units are more convenient than others for expressing RF power due to its logarithmic nature1

Logarithmic units, such as dB, are useful for comparing power levels that span several orders of magnitude, such as the power output of a radio station versus the power received by a mobile device. Logarithmic units also simplify the calculation of power ratios, gains, and losses, as they can be added or subtracted instead of multiplied or divided12

One common logarithmic unit for RF power is the decibel relative to milliwatt (dBm), which is defined as the power level in dB with reference to 1 mW. For example, 0 dBm means 1 mW, 10 dBm means 10 mW, 20 dBm means 100 mW, and so on. Conversely, -10 dBm means 0.1 mW, -20 dBm means 0.01 mW, and so on. The dBm unit is convenient because many RF signals, especially in wireless communication, are at fairly low power levels, and the dBm unit can express them in a short form23

Another logarithmic unit for RF power is the decibel relative to isotrope (dBi), which is defined as the power level in dB with reference to an isotropic radiator, which is a hypothetical antenna that radiates equally in all directions. The dBi unit is used to measure the gain of an antenna, which is the ratio of the power radiated by the antenna in a specific direction to the power radiated by an isotropic radiator. For example, a 3 dBi antenna means that it radiates 3 dB more power than an isotropic radiator in its main direction. The dBi unit is not suitable for expressing absolute power levels, as it depends on the reference antenna45

Therefore, the recommended unit of measurement for expressing RF power due to its logarithmic nature is the dBm, as it is an absolute unit that can compare power levels across a wide range and simplify power calculations. The dBi unit is only used for expressing antenna gain, which is a relative unit that depends on the reference antenna. The other units, such as lumens and milliwatts, are not logarithmic and are not commonly used for RF power measurement12345 References: 1: Radio frequency - Wikipedia 2: dBm - Wikipedia 3: RF power measurement, Part 1: Why and where - Analog IC Tips 4: Radiant intensity - Wikipedia 5: Fundamentals of RF and Microwave Power Measurements - UC Davis