Planar Single-Layer Wideband Electrically Small Quasi-Isotropic Antenna Characterized by Filtering Response

A single-layer, electrically small, quasi-isotropic antenna with filtering response is developed and experimentally investigated in this article. This electrically small antenna (ESA) incorporates a pair of orthogonally arranged crossed electric dipoles and the C-shaped elements serving as the near-field resonant parasitic (NFRP) structures and the driven elements. The driven elements are utilized to motivate a 90° phase difference, and then, with almost the same amplitudes and 90° phase difference between the crossed NFRP electric dipoles stimulated by the driven elements, a quasi-isotropic coverage can be realized. Owing to the C-shaped NFRP elements and the Egyptian axe structures of the crossed dipoles, the electrical size of the antenna can be efficiently decreased. Additionally, these C-shaped NFRP elements can introduce two radiation nulls in the lower and higher stopbands to enhance the frequency selectivity, respectively. The prototype of this quasi-isotropic ESA with filtering response was fabricated and measured, and the tested results exhibit good consistency with the simulated ones. This developed quasi-isotropic ESA demonstrates an electrically small size [
$\pi \times $
(
$0.145\,\,\lambda)^{2} \times 0.004\,\,\lambda $
and ka = 0.91]. The measured −10-dB impedance bandwidth is 14.2%. Furthermore, the bandwidth of 6-dB realized gain variation is 7%, which demonstrates that it possesses good isotropic coverage within the entire bandwidth. Specifically, the minimum realized gain difference of the quasi-isotropic ESA is 4.6 dB (4.2 dB in simulation). Moreover, due to the introduction of the two radiation nulls, this quasi-isotropic ESA exhibits a good frequency selectivity.