Arcetri Astrophysical Observatory

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logosmallarcetri Arcetri Technical Reports 5/2001


An X/K band Dichroic System for Radioastronomical Applications

A. Monorchio(2), F. Palagi(1), D. Panella(3), P. Leto(4), C. Nocita(4) and N. Fognani(1)

1) CAISMI - CNR
(2) Department of Information Engineering, University of Pisa
(3) Arcetri Astrophyysical Observatory
(4) IRA - CNR, Noto VLBI Station

Abstract

Within the framework of the RADSPEC project funded by the Italian National Research Council, a dichroic screen for radioastronomical applications has been designed and realized.

Its design, although based on the thin type approach, shows the same loss characteristics of those based on the thick type approach, but is much more robust against variations of the radiation incidence angle.

The dichroic screen is realized with two thin parallel Frequency Selective Surfaces (FSS). Its transmission band goes from 21 GHz to 26 GHz and the reflection low-frequency cut-off is at 10 GHz. The main advantage of the dual FSS configurations is a wider transmission bandpass as compared to the transmission band of the single FSS.

Laboratory tests show a very good agreement with the numerical simulations, confirming the reliability of the design methods and programs.

Measurements have been done on each single FSS first and then on the two coupled FSS's set at different distances. The best compromise between the wider transmission bandwidth and the lower average attenuation in the band is obtained with d = 4.3mm.

An astronomical test has been performed at 22.235 GHz with the Noto VLBI Radiotelescope, measuring the antenna temperature of the radio source Taurus A, with and without the dichroic screen. At the same time the system temperature increase due to the dichroic screen was measured.

While the atmospheric noise fluctuations prevent a direct measurement of the dichroic screen attenuation, it can be obtained from the system temperature increase due to the screen. The derived value confirms the design and laboratory measurements which are below the design specification of 0.1 dB as a maximum value.

Postscript version of this report.