Ka-band Axisymmetric Mode Antenna Design And Simulation

Although this is the era of solid state semiconductor devices, vacuum devices are still extensively employed in certain areas for applications while their solid state counterparts are not able to compete as far as delivering the required power and frequency at microwave to millimeter wavelengths. Many microwave generators, especially high-power microwave sources, produce an azimuthally symmetric output mode such as the circular waveguide TE01 mode or the coaxial TEM mode. If such a mode was projected into an antenna aperture and radiated immediately, then a doughnut-shaped radiation pattern with a boresignt null would result.Antenna designs to directly accommodate an azimuthally symmetric output mode, but they tend to be low gain and the pattern peak direction changes with frequency. Mode-transformation technology to alter the aperture field distribution (for example, TE01 to TE11 in circular waveguide) has also been explored, but efficiency, size and cost additions of the transformation system impact negatively on the total antenna system design. This paper mainly introduces two kinds of antennas: Vlasov antenna and COBRA antenna, which directly use azimuthally symmetric mode as feed and produce high-gain radiation patterns. The main work of this paper shows as follows:1. According to geometrical optics theory, working mechanism of Vlasov antenna was analyzed. By utilizing vector diffraction theory, the radiation formula of Vlasov antenna was derived. Structural parameters had been optimized and software simulations in resulted parameters were carried out. Meanwhile, Vlasov horn antenna was designed simply. Vlasov single reflector antenna provided a gain of 23.5dBi, while Vlasov horn antenna was 21.3dBi. Both of main directions were not in the axial direction of antenna systems and off feed axis 64°.2. The operation principle of COBRA antenna was described in detail. Structural formulas of reflector surface were deduced. The radiation formulas of COBRA antenna were derived from aperture field method and surface current method. Numerical calculation and software simulation were carried out. The radiation pattern and polarization of COBRA antenna were analyzed and compared. The results between numerical calculation and software simulation were almost same. COBRA antenna radiated a gain of 27.5dBi with a boresight peak and had a narrow main lobe and a low side-lobe level. In addition, COBRA antenna could produce a pencil like beam in axial direction.3. Based on geometrical optics theory, the operation principle of COBRA lens antenna was described and a conventional lens horn was designed. However, the horn size was very long, if cut down the size of antenna horn directly, it should increase the phase error at the aperture because of large flare angle which caused gain decrease. For overlooking the problem, carried out a novel miniaturization method. The improved COBRA lens horn compensated phase error with a modified COBRA lens. Meanwhile, added a 1/4 wavelength matched layer inside the aperture and reduced the reflected wave from lens. Radiation characteristics such as gain and radiation pattern of the miniaturized COBRA lens horn are similar to the conventional one while the length of the horn is reduced to 1/3 of conventional one.