[asac][almanews] ALMA Memos 456 and 459 Released

[Stacy Oliver]

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