  In order to better understand the variation mechanism of ozone abundance in
the middle atmosphere, the simultaneous monitoring of ozone and other minor
molecular species, which are related to ozone depletion, is the most
fundamental and critical method. A waveguide-type multiplexer was developed for
the expansion of the observation frequency range of a millimeter-wave
spectroradiometer, for the simultaneous observation of multiple molecular
spectral lines. The proposed multiplexer contains a cascaded four-stage
sideband-separating filter circuit. The waveguide circuit was designed based on
electromagnetic analysis, and the pass frequency bands of Stages 1-4 were
243-251 GHz, 227-235 GHz, 197-205 GHz, and 181-189 GHz. The insertion and
return losses of the multiplexer were measured using vector network analyzers,
each observation band was well-defined, and the bandwidths were appropriately
specified. Moreover, the receiver noise temperature and the image rejection
ratio (IRR) using the superconducting mixer at 4 K were measured. As a result,
the increase in receiver noise due to the multiplexer compared with that of
only the mixer can be attributed to the transmission loss of the waveguide
circuit in the multiplexer. The IRRs were higher than 25 dB at the center of
each observation band. This indicates that a high and stable IRR performance
can be achieved by the waveguide-type multiplexer for the separation of
sideband signals.
