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The magnetically insulated line oscillator (MILO) is an attractive high-power microwave source. It is a compact lightweight gigawatt-class coaxial crossed field device that needs no externally applied magnetic field to insulate electron flow in a slow-wave structure. An improved MILO model has been presented by Fan, Yuan and Zhong. A novel beam dump, a one-cavity RF choke section, and a novel mode-transducing antenna are introduced into the improved MILO. In simulation, high-power microwave of TEM mode is generated with peak power of 4.2 GW, frequency of 1.76 GHz, and peak power conversion efficiency of 12% when the voltage is 600 kV and the current is 52 kA. The TEM mode from the extractor gap is converted into a coaxial TE11 mode and radiated directly by the mode-transducing antenna. The direction of the radiated microwave agrees with the axis of the MILO. The antenna gain is 17.6 dBi at 1.76 GHz in simulation. The experiments have been carried out on the improved MILO device, which had been fabricated in accordance with the optimized configuration. The detailed experimental results are discussed in this paper. The improved MILO is driven by a self-built 600-kV, 10-Omega, 50-ns pulser: SPARK-04, a capacitor- and transformer-driven coaxial-water-line machine in our laboratory. The radiated microwave was detected with crystal detectors in the far-field region. The improved MILO has been extensively investigated by experiments. In the experiments, the measured microwave frequency ranges from 1.74 to 1.78 GHz, with a peak power level of above 2.4 GW, when the diode voltage is 550 kV and the current is 57 kA. The pulse duration (full-width at half-maximum) of the radiated microwave is 22 ns. The cold test and hot test results of the mode-transducing antenna are in good agreement with the simulational results. The mode of the radiated microwave is TE11 mode, and the direction of the radiated microwave overlaps with the axis of the MILO device. The - antenna gain is about 17.4 dBi at 1.76 GHz. The 3-dB beam widths are 21.2deg in E-plane and 26.3deg in H-plane, respectively. No obvious breakdown appeared in the region of the mode-transducing antenna and the region of the interface of the vacuum-air in the experiments. The experimental results confirm the ones predicted by simulation.