Small Spherical Antennas Research Articles

Tuning Method for a New Electrically Small Antenna With Low $Q$

We theoretically describe and experimentally demonstrate a new class of electrically small spherical antennas with near-minimum radiation <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> . The antenna operates by supporting the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> spherical mode and minimizing excitation of higher order modes. The antenna compares favorably to existing electrically small antennas and can be easily designed for any frequency or with any electrical size. The antenna can be matched without an external network using methods described in this work. Measurements of two antennas of different electrical sizes are presented to demonstrate the tunability of the structure.

A Novel Electrically Small Spherical Wire Antenna With Almost Isotropic Radiation Pattern

Two new electrically small spherical antennas are presented in this letter. The antennas have perpendicular arms to reduce radiation cancellation. While having low resonant frequency and high radiation efficiency, the radiation patterns of the antennas are almost isotropic. The proposed folded spherical wire antenna (SWA) shows approximately 50-Omega radiation resistance which eliminates the need for using a matching circuit for antenna feeding. Moreover, the proposed folded SWA is fabricated to verify simulation results. An attempt has been made to approach the fundamental limitations of small antennas for better characteristics while keeping the size as small as possible.

Small Spherical Antennas Using Arrays of Electromagnetically Coupled Planar Elements

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter presents the design, fabrication, and experimental characterization of small spherical antennas fabricated using arrays of non-interconnected planar conductor elements. The antennas are based upon spherical resonator structures with radiation Q-factors approaching $1.5\times$ the fundamental lower limit. The antennas are formed by coupling these resonators to an impedance-matched coplanar strip transmission line. Direct electrical connection between the feed and the antenna are made only to conductor elements coplanar with the transmission line, simplifying the fabrication process. The incident energy excites a collective resonant mode of the entire sphere (an electric dipole resonance), thereby inducing currents in each of the rings of the structure. The presence of the conductor elements outside of the feed plane is critical towards achieving the excellent bandwidth behavior observed here. The fabricated antennas have a normalized size $ka=0.54$ (where $k$ is the wavenumber and $a$ is the radius of the sphere) and exhibit high radiative efficiencies ($&gt;$ 90%) and bandwidth performance near the fundamental limit for their size. </para>

Electrically Small Antenna Elements Using Negative Permittivity Resonators

We show how resonators composed of negative permittivity materials can form the basis of effective small antenna elements. A quasi-static analysis of the resonant properties of a sub-wavelength negative permittivity sphere predicts that such a resonator will have a Q-factor that is only 1.5 times the Chu limit, matching the performance of other known electrically small spherical antenna designs, such as the folded spherical helix and the spherical capped dipole. Finite element simulation is used to demonstrate an impedance-matched radiating structure formed by coupling the resonator (a half-sphere above a ground plane) to a 50 ohm coaxial transmission line, where the coupling is mediated by a small conducting stub extending partially into the half-sphere. The resulting antenna has a ka<0.5, and its bandwidth and efficiency performance corresponds well to that predicted by the quasi-static analysis of the resonator.