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In this study, we designed and experimentally evaluated a series-connected array of superconductor-insulator-superconductor (SIS) junctions in the 100-GHz band mixer for the multi-beam receiver FOREST on the Nobeyama 45-m millimeter-wave telescope. The construction of the junction chip comprised a waveguide probe antenna, impedance matching circuit, SIS array junction, and choke filter, which were made from a superconducting niobium planar circuit on a quartz substrate. The multi-stage impedance matching circuit between the feed point and the SIS junction was designed as a capacitively loaded transmission line, and it comprised two sections with high (~90 Ohm) and low (~10 Ohm) characteristic impedance transmission lines. The structure of this tuning line was simple and easy to fabricate, and the feed impedance matched with the SIS junction in a wide frequency range. The signal coupling efficiency was more than 92% and the expected receiver noise temperature was approximately two times the quantum limit for 75-125 GHz based on quantum theory. The array junction devices with 3-6 connected junctions were fabricated and we measured their performance in terms of the receiver noise temperature and gain compression in the laboratory. We successfully developed an array junction device with a receiver noise temperature of ~15-30 K and confirmed that the improvement in the saturation power corresponded to the number of junctions. The newly developed array junction mixer was installed in the FOREST receiver and it successfully detected the 12CO (J = 1-0) molecular line toward IRC+10216 with the Nobeyama 45-m telescope., Comment: 10 pasges, 8 figures, Accepted for publication in PASJ

We have developed a two-beam waveguide-type dual-polarization sideband-separating SIS receiver system in the 100-GHz band for {\it z}-machine on the 45-m radio telescope at the Nobeyama Radio Observatory. The receiver is intended for astronomical use in searching for highly redshifted spectral lines from galaxies of unknown redshift. This receiver has two beams, which have 45$^{\prime\prime}$ of beam separation and allow for observation with the switch in the on-on position. The receiver of each beam is composed of an ortho-mode transducer and two sideband-separating SIS mixers, which are both based on a waveguide technique, and the receiver has four intermediate frequency bands of 4.0--8.0 GHz. Over the radio frequency range of 80--116 GHz, the single-sideband receiver noise temperature is lower than about 50 K, and the image rejection ratios are greater than 10 dB in most of the same frequency range. The new receiver system has been installed in the telescope, and we successfully observed a $^{12}$CO ({\it J}=3--2) emission line toward a cloverleaf quasar at {\it z} = 2.56, which validates the performance of the receiver system. The SSB noise temperature of the system, including the atmosphere, is typically 150--300 K at a radio frequency of 97 GHz. We have begun blind search of high-{\it J} CO toward high-{\it z} submillimeter galaxies., Comment: 15 pages, 14 figures, accepted for publication in PASP