Electrically small antenna

An electrically short antenna is an antenna of length 2h, such that ${\displaystyle 2\pi h \over \lambda }$${\displaystyle \scriptstyle \ll 1}$, where λ is the free space wavelength.

The far-field radiation pattern of an antenna is the sum of its near-field spherical modes, expressed using Legendre functions and spherical Bessel functions. In its simplest form, it is an omnidirectional radiation pattern with no variation in the azimuth plane. When the antenna becomes electrically small, the propagating modes are replaced by evanescent modes with high Q factor, where

In short, the maximum bandwidth of an electrically small antenna is regulated by its maximum dimension enclosed within a sphere of radius ${\displaystyle r}$.

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