Definition of small antennas

Here in this book, small antennas are treated with a concept that embraces not only electrically small antennas, but also other types of small antennas. Categories used to classify small antennas include functions as well as dimensions, because small antennas being used practically are not only what we call “Electrically Small Antennas,” but also simply physically small antennas – antennas of partly electrically small dimensions and antennas equivalently small in terms of functions. Conventionally, ESA has been the main subject when small antennas are discussed; however, other types of small antenna have comparable significance with the ESA, depending on the situation of the practical applications. The categories used here are Electrically Small Antenna (ESA), Physically Constrained Small Antenna (PCSA), Functionally Small Antenna (FSA), and Physically Small Antenna (PSA) [1].

An ESA is an antenna conventionally defined as an electrically small-sized antenna; i.e., one having dimensions much smaller than the wavelength. However, this definition is unclear, since the dimensions are not described precisely. Wheeler defined the ESA as an antenna having the maximum size that can be circumscribed by a radian sphere, with a radius of one radian in length (= λ∕2π) [2]. However, an antenna having the maximum dimension of a radian length may not necessarily be categorized as an ESA, because taking a dipole antenna as an example – which has the length of a radian length, 2 × λ/2π (= 0.32λ) – it can no longer be called electrically small, as the size becomes no longer a small fraction of the wavelength. Hence classifying the radian-length dipole antenna as an ESA is not reasonable.