A Low-Noise Double-Dipole Antenna SIS Mixer at 1 THz S. V. Shitov Institute of Radio Engineering and Electronics (IREE), Russian Academy of Sciences 101999 Moscow, Russia B. D. Jackson, A. M. Baryshev Space Research Organization of the Netherlands (SRON) P.O.Box 800, 9700 AV Groningen, the Netherlands N. N. Iosad, J.-R. Gao and T. M. Klapwijk Department of Applied Physics, Delft University of Technology (DIMES) 2628 CJ, Delft, the Netherlands Abstract A quasi-optical mixer employing a Nb/Al/AlO x /Nb twin-SIS junction with a NbTiN/SiO 2 /Al microstrip coupling circuit is tested at 800-1000 GHz. The mixer design is developed as an option for HIFI frequency bands 3 and 4. The double-dipole antenna is made from NbTiN/Al; a Nb film is used for the back reflector. The mixer design is optimized for the IF band of 4-8 GHz. The receiver noise temperature T RX = 250 K (DSB) is measured at 935 GHz for the bath temperature 2 K at IF=1.5 GHz; T RX remains below 350 K within the frequency range 850-1000 GHz. The double-dipole antenna beam pattern demonstrated good symmetry with sidelobes below –16 dB. Introduction Low-noise THz-band heterodyne receivers are needed to realize the full potential of airborne and space-based telescopes currently being developed for sub-millimeter spectral astronomy, e. g. for HIFI [1]. To consider the possibility of designing an effective SIS mixer at 1 THz, it is worth to have a brief look at the most critical parameters of SIS mixers. The Nb/Al/AlO x /Nb SIS mixers are known as the quantum (photon) noise limited heterodyne down-converters [2], which are tested within frequency range 30-1500 GHz and shown to yield receiver noise temperatures as low as (2-3)*hf/k B below 680 GHz, the gap frequency of Nb [3], [4]. Theoretically, the frequency range of all-Nb SIS mixers as quantum limited detectors can be extended up to twice the gap frequency, i. e. up to about 1300 GHz [5]. However, a single junction SIS mixer can not cover the entire band because of its high specific capacitance, C, yielding the Q-factor of the circuit of order of 10 at 1 THz. The high Q-factor leads to at least two distinctive problems: a relatively narrow instantaneous bandwidth and increased influence of loss. The real part of the high frequency impedance of a superconducting film [6] is responsible for losses, which are growing as the square of the rf current density in the tuning circuit, i. e. proportional to