A compact radiation system for L band magnetically insulated transmission line oscillator (MILO) is present. It consists of a coaxial TEM-TE11 mode converter and a coaxial multimode horn antenna. Some metal plates are inserted into the coaxial waveguide to divide it into several sectoral waveguides with different phase propagation constants to compose a coaxial TE11 mode. Then some of the sectoral waveguides are folded up to reduce the length of the mode converter. Multimode horn antenna is adopted to enhance aperture efficiency and power handling capability of the radiation antenna. A radiation system for a 1.58 GHz MILO device is designed with a length of 33 cm and an aperture radius of 20 cm. The gain is 15.8 dBi and aperture efficiency is 86.8 %. Both low power and high power microwave experiment is carried out to test the radiation character. The experiment test results agree well with computer simulation.
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