PDF《resonator antenna》

3 Pa. Taking into account our experimental results for cylindrical plasma antenna [4], we estimate that a microwave signal with about

10 W of power could sustain plasma with electron density of 7.5*1017 m-3 inside the cavity. In this case, the complex permittivity of plasma will be ε = -4 + j0.004. Hemispherical DRA offer analytical solution [2] for resonant frequencies and antenna radiation pattern. However, formulas for calculating resonant frequencies of hemispherical DRA are inapplicable for the plasma antenna because of the negative permittivity. For this reason, the plasma cavity is computer-simulated as a hemispherical resonator antenna with unknown properties. The software used is Ansoft HFSS, which allows simulation of materials with negative permittivity. The plasma antenna design is described in figure 1. The exciting element consists of a microstrip line (1) fed by a lumped port (2). The dielectric substrate is sandwiched between the microstrip and the ground plane (3). Substrate thickness is 0.6 mm with εr = 3. The plasma antenna (4) with radius of

13 mm is placed above the circular slot (5) in the ground plane, and is excited by the loop element embedded in the microstrip (6). The loop element has inner radius of 5.0 mm. The radiation domain is a sphere with radius of ~5λ. Figure 1. 3D model of hemispherical resonator plasma antenna. During investigations, several methods for slot excitation were examined with results showing that circular slot above a loop element is the most efficient. The microstrip line length is designed for optimal VSWR. 3. Results To better clarify the obtained results, the plasma resonator antenna is compared to a dielectric resonator antenna. A simulation is performed with a regular dielectric material with εr =

9 inside the hemispherical cavity. As expected, HEM11 mode (figure 2) is excited in the dielectric antenna at 3.5 GHz, radiating in vertical direction. In this case, results for electromagnetic field configuration and radiation pattern confirm known theoretical models on hemispherical DRA [1]. Substituting the dielectric with plasma with permittivity of ε = -4 + j0.004, the S11 parameter remains the same, but different distribution of electromagnetic fields inside the cavity is obtained. Results (figure 3) show that TM01 mode is excited with vertical and radial components of electric field while magnetic field has azimuthal dominant component. Radiation of the plasma antenna is similar to monopole antenna over ground plate. The resonant frequency is 3.5 GHz, which is a common band for the WiMAX and LTE standards in wireless communications. Operating frequency and antenna bandwidth depend on the value of negative permittivity (figure 4), and, consequently, on plasma density inside the cavity. Figure 2. HEM11 mode at 3.5 GHz resonance frequency inside ordinary dielectric hemispherical resonator antenna. Figure 3. TM01 mode inside plasma hemispherical resonator antenna at 3.5 GHz resonance frequency. Figure 4. Dependence of S11 parameter on operating frequency of plasma antenna at various values of plasma permittivity. Operating mode is TM01. As shown in figure 4, small variance in permittivity of plasma results significant deviation of resonant frequency, which allows dynamic frequency tuning. The best result at 3.5 GHz is obtained with permittivity εr = -4.05 with VSWR of 1.02. The bandwidth of about

150 MHz is sufficient for WiMAX and LTE operation. Radiation pattern of hemispherical plasma antenna (figure 5) shows relatively high gain of

5 dBi and omnidirectional profile. Wave attenuation in the plasma resonator antenna is very low, which leads to high efficiency, close to 100%, like in dielectric resonator antennas. The radiation pattern shape of hemispherical plasma antenna is similar to the radiation pattern of a wire monopole antenna over finite (with radius ~5λ) and over infinite ground plane. Figure 5. Plasma antenna 3D radiation model over finite (a) and over infinite ground plane (b). 4. Conclusion Plasma resonator antenna with hemispherical shape is investigated with an EM simulator. Obtained radiation pattern is similar to radiation pattern of a monopole antenna. Plasma antenna is characterized by high efficiency and configurability. Plasma density can be used to dynamically tune resonant frequency and antenna bandwidth. Plasma density needs high precision control to achieve optimal operating characteristics. The plasma resonator antenna is a possible candidate for demanding 5G modern communications. It is compact, efficient, with low noise, and quick response, and within the reach of current technology. Compared to equivalent DRA, plasma antennas possess the unique properties as instant turn-off and frequency tuning. Further development of the research is aimed towards experimental verification of the simulation results. Acknowledgements This work is financially supported by Science Fund of Sofia University "St. Kliment Ohridski", project N114/2014. References [1] Leung K W and Luk K M