An Electrically Small Planar Quasi-Isotropic Antenna

A singly fed, electrically small, planar antenna that generates a quasi-isotropic radiation pattern is investigated. The antenna consists of a folded dipole, a pair of capacitively loaded loops (CLLs), and a coplanar stripline (CPS), which are printed on the top and bottom surfaces of a single-layer printed circuit board. Through near-field coupling with the driven CPS, the folded dipole and CLLs are both effectively excited and behave like an electric dipole and a magnetic dipole, respectively. A quasi-isotropic radiation pattern can therefore be obtained by combining the two orthogonal dipoles with the same radiation intensities and quadrature phases. To verify the idea, a prototype operating at 2.4 GHz is designed, fabricated, and measured. It has been shown that this electrically small antenna (0.165 × 0.164 × 0.006 λ
³
, ka = 0.73) has a -10 dB impedance bandwidth of 0.99%, a total efficiency of ~90%, and a nearly isotropic pattern with the difference between the maximum and minimum radiated power densities given by ~3 dB over the entire spherical radiating surface.