The Planck-LFI flight model composite waveguides

Abstract

The Low Frequency Instrument on board the PLANCK satelliteis designed to give the most accurate map ever of the CMB anisotropyof the whole sky over a broad frequency band spanning 27 to 77 GHz. Itis made of an array of 22 pseudo-correlation radiometers, composed of11 actively cooled (20 K) Front End Modules (FEMs), and 11 Back EndModules (BEMs) at 300K, each FEM and BEM set comprising tworadiometers. The connection between the two parts is made withrectangular Wave Guides (WGs). Considerations of very differentnature (thermal, electromagnetic and mechanical), imposed stringentrequirements on the WGs characteristics and drove their design. Fromthe thermal point of view, the WG should guarantee good insulationbetween the FEM and the BEM sections to avoid overloading thecryocooler. On the other hand it is essential that the signals do notundergo excessive attenuation through the WG. Finally, given thedifferent positions of the FEM modules behind the focal surface andthe mechanical constraints given by the surrounding structures,different mechanical designs were necessary. A composite configurationof Stainless Steel and Copper was selected to satisfy all therequirements described. Given the complex shape and the considerablelength (about 1.5-2 m) of the LFI WGs, manufacturing and testing theWGs was a challenge.This work deals with the development of the LFI WGs, including thechoice of the final configuration and of the fabrication process. Italso describes the testing procedure adopted to fully characterizethese components from the electromagnetic point of view and the spacequalification process they underwent.The Scattering parameters of the WGs were obtained in a one portconfiguration, minimizing instrumental errors. The space qualificationprocess required manufacturing ad-hoc facilities to support the WGsduring vibration tests and to allow electromagnetic tests withoutremoving them.Results obtained during the test campaign are reported and comparedwith the stringent requirements.The performance of the LFI WGs is in line with requirements, and theWGs were successfully space qualified.