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Used as a precision dual balanced mixer with circular waveguide input, the trimode turnstile waveguide junction is the key to a simple microwave polarimeter technique which permits instantaneous viewing of input polarization. Through linear mixing, the relative phase and amplitude of orthogonal components of an arbitrarily polarized input signal are preserved in the IF outputs of the two mixers. After amplification, these two IF signals are applied to orthogonal deflection planes of a cathode-ray tube to obtain an accurate, instantaneous "picture" of input polarization. Circular polarization generates a circle; elliptical polarization, an ellipse; and linear polarization, a line which indicates the plane of polarization. In certain applications, a variation of the method permits direct IF recording of the polarization information. Instrument errors and ways to minimize these are discussed. The polarimeter technique has application in microwave communication, radar, countermeasures, radio astronomy, antenna studies, and in laboratory measurements of polarization.