TABLE OF CONTENTS Steven R. Best
The MITRE Corporation
The definition of an electrically small antenna can be somewhat ambiguous. Often the definition of a small antenna is one having overall dimensions (including any ground plane image) less than one-quarter wavelength ( ?/4), one-eighth wavelength ( ?/8), or one-tenth wavelength ( ?/10). In previous editions of this Handbook and in his work on small antennas,1 Wheeler defined the small antenna as one whose volume occupies a small fraction of one radiansphere. The radiansphere is a spherical volume having a radius of ?/2 ?.2 The significance of the radiansphere dimension is that it is approximately the region surrounding the small antenna primarily occupied by the stored or reactive energy of its near electric and magnetic fields.
The electrical size or volume of the small antenna is defined by the value of ka, where k is the free-space wavenumber 2 ?/ ?, and a is the radius of an imaginary sphere circumscribing the maximum dimension of the antenna, as illustrated in Figure 6-1. Figure 6-1 illustrates the definition of a for both a free-space dipole-like antenna and a monopole-like antenna that requires a ground plane for operation. If the finite ground plane is sufficiently large so the monopole's impedance is similar to that exhibited by the monopole on an infinite ground plane, the dimensions of the ground plane do not need to be included in the...
