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A low-noise wide bandwidth detector and a spectrometer are key instruments for millimeter wave radio telescopes to unravel the redshift distribution and physical nature of submillimeter galaxies (SMGs). With the purpose of searching highly redshifted spectral lines from galaxies of unknown redshift, a new receiver system (called z-machine) has been developed for the Nobeyama Radio Observatory (NRO) 45-m radio telescope in 100 GHz band (Nakajima et al. submitted to PASJ). The atmospheric contribution to the system noise temperature at NRO is more than 100 -150 K around 100 GHz. Under this circumstance, the sensitivity cannot be improved by making lower-noise receivers in the case of it is already limited by atmospheric noise. To improve the sensitivity, we need to build a dualpolarized receiver system. When both polarizations are received simultaneously, there is a √2 improvement in the signal to noise (S/N), or a reduction factor of two in observing time on a telescope. Therefore, a wide-band dual-polarization operation is required for the NRO z-machine receiver system. One of the major components for a waveguide based dual-polarization receiver is an orthomode transducer (OMT). An OMT is a passive microwave device which separates two orthogonal, linearly polarized signals within the same frequency band. It is worth mentioning there has been substantial progress in the field of applications for millimeter and sub-millimeter wavelengths radio telescopes in the past year.

Double ridge Bøifot-type junction waveguide OMTs have been developed for ALMA Band 4 (125-163 GHz), and Band 8 (385-500 GHz) in National Astronomical Observatory of Japan. A stepped double-ridge waveguide transition has been adopted because it is particularly suitable for a split-block wave guide construction on a CNC (Computer Numerical Control) milling machine using end-mills. With three sections stepped double-ridge waveguide transitions, those OMTs exhibited more than 26 % of bandwidth to the center frequency. Further wide-band operations can easily be achieved with increasing number of steps. As a result of an infinite number of steps, smooth and continuous ridge transitions are desirable wherever feasible. Recently, quadrupleridge waveguide OMTs with smooth taper transitions have been developed and demonstrated in low frequencies. Thanks to CNC Machining advances in recent years, smooth taper ridge transitions became possible for split-block construction in millimeter-wave range.