The measurements and calculations (coupled mode equations) on the conversion of the transverse magnetic TMo1 vircator (virtual cathode oscillator) mode at 4 GHz to the linearly polarized TE11 mode are presented by means of mode converter systems using a circular waveguide with curvature. The mode converter is composed of the 38.78°bend with 39.07cm curvature and of the 50.78° inverse bend with 25.24cm curvature in the circular waveguide of 9cm inteal diameter. The efficiency of conversion from TMo1 to TE11 at 4 GHz exceeds 90%, and the overall etfficiency from TM01 to TE11 exceeds 90% in the calculated range from 3.72 to 4.8 GHz.