What unit is commonly used to express the gain of an antenna?

The gain of an antenna is commonly expressed in decibels isotropic (dBi), which provides a standardized measure of an antenna's performance relative to an ideal isotropic radiator. An isotropic radiator is a theoretical antenna that radiates power uniformly in all directions. The dBi measurement indicates how much better an antenna is at directing radio waves compared to this idealized antenna.

Using decibels as a unit allows for a logarithmic scale, which is more convenient for handling the wide range of values that can occur in antenna gain. This metric is particularly useful in telecommunications, as it helps professionals quickly compare the efficiency and effectiveness of different antennas.

The other options, while related to the field of telecommunications and radio frequency, do not appropriately represent antenna gain. Watts and milliwatts refer to power levels, while hertz is a measurement of frequency, rather than gain. Thus, the use of decibels isotropic is the most accurate and relevant measure for antenna gain in practical applications.