ELECTROMAGNETIC SCATTERING FROM AND NATURAL RESONANCES OF THE CIRCULAR STRIP.

The formulation and calculation of the electromagnetic fields scattered from, and the natural resonances of, a thin, perfectly conducting circular strip mounted on a perfectly conducting ground plane is presented. The fields are excited by an axial electric dipole, oriented normal to the surface of the ground plane. An electric field integral equation is formulated in terms of the induced surface current and solved in closed form in the quasi-static limit as an expansion of Chebyshev polynomials. In addition, the integral equation is solved for the general case by the method of moments (MOM). Numerical results are given which show the excellent agreement between the quasi-static and method of moments solutions. The MOM currents are used to determine the field scattered from the circular strip, in both the frequency and time domains. The frequency domain analysis is applied to the problem of fencing small vertical broadcast antennas. We find that appropriately designed fencing can enhance the ground wave of the antenna, while decreasing the sky wave. In addition, we determine that the radiation efficiency of the dipole is sensitive to the size of the circular strip, and that the radiation efficiency displays resonant behavior when the frequency of operation is near a natural resonance of the circular strip. We, also, calculate the time domain transient scattered fields for different circular strip geometries, using the MOM currents and GTD. By deemphasizing the low frequencies in the transient pulse spectrum, we obtain good agreement between the GTD early time scattering and the transient scattering determined from the MOM formulation. Using the MOM formulation, we determine the natural resonances of the circular strip. These resonances are divided into two classes: the exterior resonances and interior resonances. We show the pole trajectories for the first layer of exterior resonances for a wide range of strip height-to-radius ratios. In addition, we locate strong interior resonances which correspond to TM₀(pq) circular cavity modes. Included in these interior resonances are weakly damped resonances which correspond to the TM₀(p)₀ cavity modes. This is the first known report of these TM₀(p)₀ interior resonances for the circular strip geometry. We find that these resonances dominate the scattering in our problem. Using the transient scattered fields calculated from the MOM formulation as input and output data for a single input, single output identification algorithm, we identify the dominant poles in the scattered fields. We show that these dominant poles are those associated with the TM₀(p)₀ interior resonances of the circular strip. In addition, we show that by using intelligent filtering and source selection, a few resonances with higher damping can also be identified