What does "dBi" measure in relation to antennas?

"dBi" measures the gain of an antenna compared to an isotropic radiator, which is an idealized point source that radiates equally in all directions. This measurement allows for a standardized way to express how much more power an antenna transmits in a particular direction than would be transmitted by the isotropic radiator.

When we say an antenna has a gain of, for example, 5 dBi, it indicates that the antenna radiates 5 decibels more power in its favored direction than an isotropic radiator would, under the same input power conditions. This concept is crucial in understanding antenna performance, as it helps in evaluating how effective an antenna is at directing radio waves to achieve communication over greater distances or increased clarity in transmission and reception.

The other options do not accurately characterize what "dBi" represents. The measurement does not pertain to resistance, signal directivity alone, or the physical size of the antenna—these factors may influence performance but are not defined by the dBi term itself.