Abstract: A circular waveguide TE11-HE11 mode transducing antenna loaded with radial double mediums is reported based on measurements and simulations. The mode transducing antenna includes a novel broadband circular waveguide TE11-HE11 mode converter and a radiator. The circular waveguide TE11-HE11 mode converter is inserted with radial double mediums, and the radiator adopting an open circular waveguide or a small angle conical horn will radiate out HE11 mode. Using the commercial software HFSS to simulate and optimize two kinds of circular waveguide TE11-HE11 mode transducing antennas, the numerical results show that the two antennas have a highly symmetrical radiation pattern and a relatively low sidelobe level, no matter they work at linear polarization or circular polarization. The standing wave ratio (VSWR) and the gain of the two kinds of mode transducing antennas are tested by using the vector network analyzer and the spectrum analyzer. The measured results reveal that the gain of the antenna which uses an open-ended circular waveguide as the radiator is 11.21 dB, the gain of the antenna which uses a small angle conical horn as the radiator is 15.58 dB, and the VSWR is less than 1.05 at the center frequency of 9.4 GHz. The experimental results are in agreement with the simulation results, which validates the feasibility and correctness of the antenna technology of the mode conversion. 