PURPOSE: A present invention relates to a linear/circular polarization converter and particularly to a linear/circular polarization converter to realize linear polarization-circular polarization conversion by using an elliptical waveguide having an elliptical cross-section consisting of long axis and short axis and selecting the long axis size, short axis size and length in the tube axis direction of the elliptical waveguide. CONSTITUTION: A linear/circular polarizing converter of the present invention uses an elliptical waveguide(1) having elliptical cross-section which receives supply of a linearly polarized signal at one end of elliptical waveguide and outputs a circularly polarized signal from the other end of elliptical waveguide(1) When the application frequency wavelength of polarized signal supplied to the elliptical waveguide is lambda 0, length of elliptical waveguide is L, cutoff wavelength of polarizing plane in the long axis direction of polarized signal is lambda CH, cutoff wavelength of the polarizing plane in the short axis direction of polarized signal is lambda CV, phase of the basic mode polarization signal in the long axis direction for the polarized signal is &thgr;H, phase of basic mode polarization in the short axis direction is &thgr;V and phase difference of these phases &thgr;H and &thgr;V is δ&thgr;, &thgr;H is expressed using lambda 0, lambda CH and L, while &thgr;V is expressed using lambda 0, lambda CV and L, and long axis size a, short axis size b and length L in the tube axis direction are selected to meet δ&thgr; = 90 degree.
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