Reinhard Keller rkeller@mpifr-bonn.mpg.de : Max-Planck-Institut f. Radioastronomie, Bonn GERMANY.
Chairman of the RadioNet Networking Activity N4: European Radio Astronomy Engineering Forum.
RADIONET Receivers Workshop on May 30, 2005
New Trends on Receivers Development"
"A new kind of filtering waveguide frontend for cryogenic cooled receivers at low frequencies."
Dr. Reinhard Keller; MPI für Radioastronomie, Martin Dohlus; DESY; Dr. Wolfgang Hauth; Microwave CAD and Consulting
. Abstract. Waveguide components are key elements for radio astronomy receivers. Building the connection between the feed horn and low noise amplifiers they are crucial for the performance of the whole system. At frequencies below some 5 GHz the feed horn normally is at room temperature while the low noise amplifiers are cryogenically cooled. Therefore the waveguide frontend has to provide thermal isolation from 300 K to 15 K and the signal has to penetrate the wall of a vacuum dewar needed to provide high vacuum for low temperature convection. To meet these requirements a vacuum window is needed with high mechanical stability, lowest leakeage rate and ultra low insertion loss. As the diameter of circular waveguides at low frequencies gets big, plastic foils are strongly stressed and tend to break. Therefore we choose a ceramic window to meet the mechanical and electrical requirements mentioned above. These windows are frequently used in high energy applications i.e. particle accelerators like DESY. They have shown high reliability and excellent performance over many years of operations. Although radio astronomy is not a high power application the boundary conditions are very similar. Unfortunately the high dielectric constant of about 10 requires matching networks on both sides of the window to make the ceramic disc invisible for the signal. To match the window to the connecting circular waveguides we choose a filter structure with half wavelength resonators and circular irises. The mechanical outline of the ceramic disc is designed to form a resonator itself and the couplings to the neighbour resonators are adjusted by varying the resonators cross section. This was done by mode matching techniques which are very well suited for uniform cross section problems. In this paper we give a short outline of the theory and the design procedure. Simulation results are compared with measured results.