From soliver at nrao.edu Wed May 21 15:29:08 2003 From: soliver at nrao.edu (Stacy Oliver) Date: Wed, 21 May 2003 12:29:08 -0700 Subject: [asac][almanews] ALMA Memos 456 and 459 Released Message-ID: ALMA MEMO #456 Characteristics of a Reflector Antenna : Parameters, graphs and formulae for Cassegrain systems with Mathematica expressions for numerical computation Jaap Baars April 2003 Keywords: Antenna theory, Cassegrain antenna, Mathematica, Radio antenna, Radio telescope In this report I collect data on the characteristics of a reflector antenna in the form of Figures and Formulae. The goal is to make available in one place information which is useful for the analysis of antenna measurements and radio astronomy observations. I discuss the illumination efficiency, axial and lateral defocus, pointing aspects, aperture blocking and random surface errors. The computations and the derived illustrations are made with the aid of Mathematica and all Mathematica expressions are included at the end of the report. This collection of data is an extension of my earlier summary of antenna parameters, issued as and NRAO Electronic Division Internal Report (EDIR), No. 57 of August 1966. Some of the figures of that report are included here. View a pdf version of ALMA Memo #456. http://www.alma.nrao.edu/memos/html-memos/alma456/memo456.pdf _______________________________________________________________________________________________________ ALMA MEMO #459 Investigation of Anomalous Fast Phase Fluctuations in the Site-Test Interferometer Data from Chajnantor Sally Hales, Richard Hills, Yasmin Robson, John Richer (MRAO, Cambridge), Guillermo Delgado, Angel Otarola (Onsala Space Observatory and ESO), Simon Radford (NRAO) 2003-05-14 Keywords: Phase fluctuations, site-test interferometer, ionosphere, scintillation ALMA Memos 332 and 361 reported test comparisons between phase fluctuations predicted by line-of-sight PWV measurements from 183-GHz water-vapour radiometers at Chajnantor and the phase signal measured by the NRAO site-test interferometer observing an 11.2-GHz. geostationary satellite beacon. Though data from the two methods showed good correlation for long periods, an unexpected strong high-frequency fluctuation was found to occur in the interferometer phase, sometimes persisting for several hours. No counterpart for this phenomenon was found in the radiometer data, but a similar effect was seen on the ESO site-test interferometer located on the same baseline but observing a different 11.2-GHz satellite. If the effect proved to be directly proportional to frequency it would lead to complete decorrelation of the astronomical signal at wavelengths of order one millimetre with no prospect of correction using the radiometer measurements. This memo reports investigations of the phenomenon to establish its likely physical origin and hence how it would scale to higher frequencies. We show that the phase variations are accompanied by amplitude variations, indicating some kind of scintillation effect. Moreover the inferred velocity of the underlying disturbances (or waves) is so high as to rule out their being any kind of small-scale tropospheric irregularities not visible to the radiometers; rather, it is consistent with typical propagation speeds of ionospheric disturbances. The occurrence and time of onset in the interferometer data appear to exhibit some agreement with Range-Time-Intensity (RTI) plots from the JULIA back-scatter radar experiments from the Jicamarca Radio Observatory in Peru. We present some statistics showing the incidence of the effect in the interferometer data over a 5-year period, revealing seasonal and diurnal patterns which suggest that the predominant cause is the post-sunset ionospheric disturbances well-known to occur at low magnetic latitudes and collectively termed Equatorial Spread F. View a pdf version of ALMA Memo #459. http://www.alma.nrao.edu/memos/html-memos/alma459/Memo459.pdf _______________________________________________ Almanews mailing list Almanews at listmgr.cv.nrao.edu http://listmgr.cv.nrao.edu/mailman/listinfo/almanews From soliver at nrao.edu Fri May 23 11:37:50 2003 From: soliver at nrao.edu (Stacy Oliver) Date: Fri, 23 May 2003 08:37:50 -0700 Subject: [asac][almanews] ALMA Memo 458 Released Message-ID: ALMA MEMO #458 Preliminary tests of Cartridge-Type Receiver System at Atacama Site M. Sugimoto (The University of Tokyo/NAO-J), Y. Sekimoto (NAO-J), S. Yokogawa (The Graduate University for Advanced Studies/NAO-J), T. Okuda (The University of Tokyo/NAO-J), K. Tatematsu (NAO-J), K. Kohno (The University of Tokyo), H. Ogawa (Osaka Prefecture University), K. Kimura (Osaka Prefecture University), K. Suzuki (Instrument Development Center of School of Science, Nagoya University), T. Minamidani (Nagoya University) 2003-05-14 Keywords: Cryostat, Cartridge, Cryocooler, ASTE We have developed a cartridge-type receiver system composed of three cartridge-type receivers and a cryostat, which is designed to test on the Atacama Submillimeter Telescope Experiment (ASTE). It was preliminary evaluated at Pampa la Bola (alt. 4800 m) in the northern Chile since November 2002. The cryostat, which can house 3 cartridge-type receivers, has been developed with following technologies; a central pipe and bellows structure to reduce mechanical vibration; simple and efficient thermal links for plug-in cartridges; 3-stage Gifford McMahon cryocooler and an outdoor compressor. Engineering models of band 3 (100 GHz), band 8 (500 GHz), and band 10 (800 GHz) cartridge-type receivers were independently developed with cartridge-test cryostats. They were integrated into the cryostat at NAOJ, then the system was shipped to the site. We confirmed that the system including three receivers operates as designed and the concept of cartridge-type receiver system is very promising for the ALMA. View a pdf version of ALMA Memo #458. http://www.alma.nrao.edu/memos/html-memos/alma458/memo458.pdf Download a zipped postscript version of ALMA Memo #458. http://www.alma.nrao.edu/memos/html-memos/alma458/memo458.zip _______________________________________________ Almanews mailing list Almanews at listmgr.cv.nrao.edu http://listmgr.cv.nrao.edu/mailman/listinfo/almanews From soliver at nrao.edu Fri May 23 14:51:29 2003 From: soliver at nrao.edu (Stacy Oliver) Date: Fri, 23 May 2003 11:51:29 -0700 Subject: [asac][almanews] ALMA Memo 466 Released Message-ID: ALMA MEMO #466 Gain Stability: Requirements and Design Considerations Larry D'Addario 2003-05-22 A theoretical framework for discussing the effects of gain stability in radiometers is developed, clarifying the relationship between frequency domain and time domain measurements. The available literature on experimental results is reviewed with respect to the intrinsic gain fluctuations of HFET amplifiers and the temperature sensitivity of various other components used in radio telescope receivers. The results are applied to ALMA, where it is found that the sensitivity of broad-band total power radiometry is likely to be limited by gain fluctuations rather than thermal noise unless the present state-of-the-art is improved. This is true for both 10 Hz beam switching (via subreflector nutation) and rapid on-the-fly scanning. However, it is also found that at frequencies above 410 GHz fluctuations in atmospheric opacity due to turbulence will be worse than both thermal noise and receiver gain fluctuations a large fraction of the time; further study of the site testing data will allow this to be better quantified. View a pdf version of ALMA Memo #466. http://www.alma.nrao.edu/memos/html-memos/alma466/memo466.pdf _______________________________________________ Almanews mailing list Almanews at listmgr.cv.nrao.edu http://listmgr.cv.nrao.edu/mailman/listinfo/almanews From soliver at nrao.edu Fri May 30 16:51:25 2003 From: soliver at nrao.edu (Stacy Oliver) Date: Fri, 30 May 2003 13:51:25 -0700 Subject: [asac][almanews] ALMA Memo 4608 Released Message-ID: ALMA MEMO #460 MEASUREMENT OF GAIN COMPRESSION IN SIS MIXER RECEIVERS A. R. Kerr, J. Effland, S.-K. Pan, G. Lauria, A. W. Lichtenberger and R. Groves 2003-05-15 Keywords: Superconductor-Insulator-Superconductor mixers, saturation, gain compression, dynamic range When an SIS mixer is partially saturated by broadband noise, it continues to exhibit a linear response to a small CW test signal, with the small-signal gain depending on the level of the saturating noise. This allows the CW test signal to be used as an indicator of the receiver gain in the presence of high-level noise. If not taken into account, gain compression can be a significant factor limiting the accuracy of high precision radio astronomy instruments. View a pdf version of ALMA Memo #460. http://www.alma.nrao.edu/memos/html-memos/alma460/memo460.pdf _______________________________________________ Almanews mailing list Almanews at listmgr.cv.nrao.edu http://listmgr.cv.nrao.edu/mailman/listinfo/almanews