Program and Abstract Book - SRON

19 th International Symposium on Space Terahertz Technology
9-7
Measurement of Emissivity of the ALMA Antenna Panel at 840 GHz
Using NbN-Based Heterodyne SIS Receiver
S. V. Shitov 1,2 , J. Inatani 1 , W.-L. Shan 3 , M. Takeda 4 , A. V. Uvarov 2 and Y. Uzawa 1
1 National Astronomical Observatory of Japan, Mitaka, Japan
2 Institute of Radio-Engineering and Electronics, Moscow, Russia
3 Purple Mountain Observatory, Nanjing, China
4 Kobe Advanced ICT Research Center (KARC), Kobe, Japan
The design of the dish for ALMA telescopes is subject to a number of special
requirements. For example, focusing of the infrared portion of solar radiation must be
restricted via diffused scattering from the surface of the panel, while RF loss, which cause
the in-band noise emission, must be below 1%. These are somewhat contradictory
requirements, since the surface of the antenna must have appropriate roughness (matte
finish) providing the scattering of IR-radiation. The noise emission of a reflective surface is
usually measured by a bolometer-based radiometer, which can provide a very high
sensitivity [1]. Another technique is evaluation of the Q-factor of a resonator formed by the
reflector under test. However, both techniques have their own drawbacks. A sensitive
radiometer suffers usually from the low dynamic range while the resonant method assumes
both precision design and adjustment of a resonator circuit.
We tested ALMA Band-10 heterodyne SIS receiver as an antenna switching
radiometer with 8 GHz instantaneous bandwidth (DSB receiver, IF band 4-8 GHz). The
receiver is mounted within a vacuum 4-K cryostat and contains one of the experimental
waveguide-type SIS mixers. The particular Band-10 SIS mixer employed the resonant type
junction made of epitaxial NbN/AlN/NbN trilayer [2]. The noise temperature of the receiver
referred to the antenna load is about 560 K (DSB) or about 330 K, if corrected for 25-µm
Kapton beamsplitter. The receiver’s antenna switch is balanced to terminate the receiver
input to a common 80-K absorber via two different paths. In this configuration, the
emissivity of the surface is detected via the imbalance of the antenna switch occurring when
the sample-under-test is inserted to produce the extra reflection. Since the large dynamic
range of the receiver, the response can be calibrated with traditional 300-K and 80-K
antenna loads.
The value of (0.25±0.10)% has been measured for the sample of the antenna panel.
To confirm both the measured value and the general feasibility of the method, samples
made of phosphor bronze and stainless steel are measured using the same technique. The
values of (0.30±0.10)% and (1.10±0.10)% are obtained for these samples correspondingly
that is consistent with data obtained with bolometer radiometer [1]. The accuracy of the
described technique will be discussed, that includes the effects of non-thermostatic
environment.
[1] A. E. Lange, S. Hayakawa, T. Matsumoto, H. Matsuo, H. Murakami, P. L. Richards, and
S. Sato, "Rocket-borne submillimeter radiometer," Appl. Opt. 26, p. 401 (1987)
[2] Y. Uzawa, Z. Wang; A. Saito, A. Kawakami, M. Takeda, “Development of a waveguide
NbN-based SIS mixer in the 900-GHz band,” IEEE Transactions on Applied
Superconductivity, Vol. 13, Issue 2, June 2003, pp. 692-695.
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