1,683 research outputs found
Pattern measurements of a low-sidelobe horn antenna
The techniques and results of power pattern measurements of a corrugated horn antenna designed for low sidelobes are reported. The power pattern was measured to levels 90 dB below the main beam maximum in both the E- and H-planes. The measured patterns were found to be in good agreement with predictions from existing theory for the performance of corrugated scalar feeds
Beyond the Small-Angle Approximation For MBR Anisotropy from Seeds
In this paper we give a general expression for the energy shift of massless
particles travelling through the gravitational field of an arbitrary matter
distribution as calculated in the weak field limit in an asymptotically flat
space-time. It is {\it not} assumed that matter is non-relativistic. We
demonstrate the surprising result that if the matter is illuminated by a
uniform brightness background that the brightness pattern observed at a given
point in space-time (modulo a term dependent on the oberver's velocity) depends
only on the matter distribution on the observer's past light-cone. These
results apply directly to the cosmological MBR anisotropy pattern generated in
the immediate vicinity of of an object like a cosmic string or global texture.
We apply these results to cosmic strings, finding a correction to previously
published results for in the small-angle approximation. We also derive the
full-sky anisotropy pattern of a collapsing texture knot.Comment: 23 pages, FERMILAB-Pub-94/047-
Spillover and diffraction sidelobe contamination in a double-shielded experiment for mapping Galactic synchrotron emission
We have analyzed observations from a radioastronomical experiment to survey
the sky at decimetric wavelengths along with feed pattern measurements in order
to account for the level of ground contamination entering the sidelobes. A
major asset of the experiment is the use of a wire mesh fence around the
rim-halo shielded antenna with the purpose of levelling out and reducing this
source of stray radiation for zenith-centered 1-rpm circular scans. We
investigate the shielding performance of the experiment by means of a geometric
diffraction model in order to predict the level of the spillover and
diffraction sidelobes in the direction of the ground. Using 408 MHz and 1465
MHz feed measurements, the model shows how a weakly-diffracting and unshielded
antenna configuration becomes strongly-diffracting and double-shielded as
far-field diffraction effects give way to near-field ones. Due to the
asymmetric response of the feeds, the orientation of their radiation fields
with respect to the secondary must be known a priori before comparing model
predictions with observational data. By adjusting the attenuation coefficient
of the wire mesh the model is able to reproduce the amount of differential
ground pick-up observed during test measurements at 1465 MHz.Comment: 14 pages, 17 eps + 1 gif figures and 4 Tables. Accepted for
publication in A&AS. Fig.7 available at full resolution from
http://www.das.inpe.br/~tello/publications.ht
Primordial Gravity Waves and Weak Lensing
Inflation produces a primordial spectrum of gravity waves in addition to the
density perturbations which seed structure formation. We compute the signature
of these gravity waves in the large scale shear field. In particular, the shear
can be divided into a gradient mode (G or E) and a curl mode (C or B). The
former is produced by both density perturbations and gravity waves, while the
latter is produced only by gravity waves, so the observations of a non-zero
curl mode could be seen as evidence for inflation. We find that the expected
signal from inflation is small, peaking on the largest scales at
at and falling rapidly there after. Even for
an all-sky deep survey, this signal would be below noise at all multipoles.
Part of the reason for the smallness of the signal is a cancellation on large
scales of the standard line-of-sight effect and the effect of ``metric shear.''Comment: 4 pages, 1 figur
2-Point Correlations in the COBE DMR 4-Year Anisotropy Maps
The 2-point temperature correlation function is evaluated from the 4-year
COBE DMR microwave anisotropy maps. We examine the 2-point function, which is
the Legendre transform of the angular power spectrum, and show that the data
are statistically consistent from channel to channel and frequency to
frequency. The most likely quadrupole normalization is computed for a
scale-invariant power-law spectrum of CMB anisotropy, using a variety of data
combinations. For a given data set, the normalization inferred from the 2-point
data is consistent with that inferred by other methods. The smallest and
largest normalization deduced from any data combination are 16.4 and 19.6 uK
respectively, with a value ~18 uK generally preferred.Comment: Sumbitted to ApJ Letter
Power Spectrum of Primordial Inhomogeneity Determined from the 4-Year COBE DMR Sky Maps
Fourier analysis and power spectrum estimation of the cosmic microwave
background anisotropy on an incompletely sampled sky developed by Gorski (1994)
has been applied to the high-latitude portion of the 4-year COBE DMR 31.5, 53
and 90 GHz sky maps. Likelihood analysis using newly constructed Galaxy cuts
(extended beyond |b| = 20deg to excise the known foreground emission) and
simultaneously correcting for the faint high latitude galactic foreground
emission is conducted on the DMR sky maps pixelized in both ecliptic and
galactic coordinates. The Bayesian power spectrum estimation from the
foreground corrected 4-year COBE DMR data renders n ~ 1.2 +/- 0.3, and
Q_{rms-PS} ~ 15.3^{+3.7}_{-2.8} microK (projections of the two-parameter
likelihood). These results are consistent with the Harrison-Zel'dovich n=1
model of amplitude Q_{rms-PS} ~ 18 microK detected with significance exceeding
14sigma (dQ/Q < 0.07). (A small power spectrum amplitude drop below the
published 2-year results is predominantly due to the application of the new,
extended Galaxy cuts.)Comment: 9 pages of text in LaTeX, 1 postscript Table, 4 postscript figures (2
color plates), submitted to The Astrophysical Journal (Letters
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