Size effect of the ground plane and axial coupling aperture on the monopole antenna performance

Our systematic and detailed investigations of the X-band cylindrical monopole antennas allowed us to clarify the physical nature of oscillating dependencies of antenna characteristics such as the resonance frequency, elevation angle, and gain for the quarter-wavelength monopole. For the first time we have demonstrated the role of the finite ground plane and the axial coupling aperture on the cylindrical monopole antennas performance. In contrast to the quarter-wavelength monopole the radiation of the grounded three quarter-wavelength monopole is formed at the angle near zenith thereby diminishing the edge effects. The optimal radii of the axial coupling aperture providing the maximum antenna efficiency have been determined for all the AUT's. Furthermore, it has been shown that the axial coupling aperture radius variation is accompanied by the resonance frequency shift of the antenna. The quarter-wavelength monopole antenna with the best characteristics and permissible minimal ground plane radius R=λ/4 demonstrating the mono-beam radiation pattern, -10dB impedance bandwidth of 1.6GHz, and the gain G=4dBi has been presented.