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A 1.2-GHz magnetically insulated transmission line oscillator (MILO) is investigated numerically and experimentally in this paper. Simulation optimization is performed with the particle-in-cell code KARAT. When the diode voltage and current are 680 kV and 53 kA, the output microwave power is 4.15 GW, the microwave frequency is 1.169 GHz, and the power efficiency is 11.5%. In order to radiate a boresight peak pattern, a TEM-TE11 mode-converting antenna is introduced into the MILO device. In the TEM-TE11 mode-converting antenna, four metal plates are inserted into the coaxial waveguide to convert the TEM into TE11 mode, and then, the coaxial TE11 mode is converted into circular TE11-like mode and radiated by a conical horn antenna, which generates a boresight peak pattern in a far-field region. The gain of the mode-converting antenna is 16.3 dBi, and the aperture efficiency of the conical horn antenna is 79% at 1.169 GHz. The 3-dB beamwidths are 24° in E-plane and 32° in H-plane, and the sidelobes of the radiation patterns are both less than -21 dB. In the experiments, the MILO device is driven by a 590 kV 49 kA electron beam. The measured results show that the peak microwave power is about 2.9 ± 0.3 GW, the pulse duration is above 20 ns, the microwave frequency is about 1.20 GHz, and the power conversion efficiency is about 10%. The experimental results validate the simulation prediction.