Design and fabrication of a modified COBRA mode converter with continuous lens structure

Most of high power microwave generators have a TM01 mode in their circular waveguide output which makes them improper due to its doughnut shaped pattern. To overcome this problem, Coaxially Beam Rotating Antenna (COBRA) is used which transforms TM01 mode to TE11. The conventional structure of COBRA, depending on the required polarization type, consists of 1-4 discrete steps. Minimum number of steps to create circular polarization is 3. In this paper, a modified COBRA including a continuous tapering which mimics an infinite number of steps, has been proposed. This structure is implemented using CNC technology. A convex lens is used to increase the gain up to 1dB. Compared to conventional circular polarized COBRA with 3 or 4 steps, this structure has improved side lobe level, significantly. In addition, impedance and axial ratio bandwidths of this antenna are more than 5.5 and 4GHz, respectively. The experimental test results are in good agreement with the computer simulations.