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A low coupling impedance beam position monitor based on microwave spectroscopy is proposed. A coaxial cavity is coupled to a circular beam pipe through four small longitudinal slots. Displaced beams excite TE cavity modes that can be detected by two antennas. Applying the modified Bethe's theory, the TE111 mode power has been derived as a function of the beam displacement. An aluminium prototype has been constructed with the support of numerical (HFSS) simulations. Bench measurements using a coaxial wire excitation show good agreement with both theoretical and numerical expectations.