MM Coatings for Monopole Bandwidth Extension
This section presents a type of novel MM coating that can extend the impedance bandwidth of a conventional quarter-wave monopole antenna to more than an octave. First, it presents the results of a parametric study of a monopole surrounded by a coating comprised of anisotropic material, which provide the guidelines for creating a practical broadband antenna design. Then it reports the MM unit cell designs targeting the 5-band. Finally, it verifies the MM coating designs by both full-wave simulations and experimental results when integrated with a monopole. A C-band design is also included to further demonstrate the versatility of this approach.
Monopole with Anisotropic Material Coating
The configuration of the monopole antenna with an anisotropic material coating is shown in Fig. 1.1a. The monopole is 28.5 mm long and is designed to be resonant at 2.5 GHz. The cylindrical coating is composed of anisotropic dielectric material with a relative permittivity tensor ?r = [ert, ?rt, ?rz], a diameter of dc, and a height of hc. The coating has a minimal impact on the monopole’s omnidirectional radiation pattern in the Я-plane. By fixing the value of ?rz to be 8 and increasing the value of dc, it can be seen that a second resonance occurs. When dc = 14 mm, a wide impedance bandwidth can be achieved. Further increasing dc will result in a single merged resonance at the center of the band. By fixing the value of dc to be 14 mm and increasing the value of ?rz, the 51i curve evolves from a single resonance at 2.3 GHz to dual resonances at 2.3 and 3.7 GHz and then to a single resonance at 3.4 GHz. The dual resonances give rise to the wideband matching. Increasing the value of ?rt degrades the impedance bandwidth, so it should be maintained at a value near unity.
Figure 1.1 (a) Schematic of the monopole antenna (ha = 28.5 mm) with an
anisotropic dielectric coating, a diameter of dc, and a height of hc. (b) Simulated 5n of the coated monopole with hc = 40 mm, ?rt = 1, ?rz = 8, and varying dc. (c) Simulated 511 of the coated monopole with hc = 40 mm, ?rt = 1, dc = 14 mm, and varying ?rz. (d) Simulated 511 of the coated monopole with hc = 40 mm, ?rz = 8, dc = 14 mm, and varying ?rt.