Design and Development of Multimode Tracking Coupler for Satellite Communication

Abstract

Spacecraft platform stability error in roll and pitch axes is of the order of (0:150) causes edge of coverage gain degradation to the extent of 3-4dB due to narrow beam. In order to reduce gain degradation problem, RF tracking system is required with tracking antennas and tracking feed. A multimode tracking feed system to track on board geostationary satel- lites is proposed which has components such as tracking coupler, Ortho Mode Transducer (OMT) and Horn etc. The tracking coupler is the device which is essentially required to ensure tracking using monopulse technique. The tracking system is based on amplitude and phase characteristics of the higher mode TE21 only, generated on the aperture of the antenna when the boresight axis is shifted from the satellite direction. Multimode techniques may provide full monopulse capability from a single aperture compared to the other techniques such as four or twelve horn feed system. In the tracking coupler design, various design options are available with us such as TE21 or TM01 mode coupler. But we proposed TE21 tracking coupler which provides both the polarization linear and circular. The task of TE21 tracking coupler is to extract the tracking mode (TE21) from dominant mode (TE11) in circular waveguide to provide full autotrack capability by using different coupling distributions. Thus, design of multimode multihole tracking coupler at 29:5 GHz according to specifications and compact tracking coupler at 29:5 GHz is proposed with variation of different parameters. Also, we can successfully reduce length of the coupler up to the half without compromise tracking accuracy of multimode tracking feed system. Therefore, multimode multihole tracking coupler provides same performance as literature and compact tracking coupler provides poor coupling but not affect our tracking performance of multimode tracking feed. Therefore, our approach provides same mode rejection as multimode multihole tracking coupler (i.e. 50 dB for TE11 mode) with acceptable coupling and good tracking accuracy also with reduce length up to the half of the multimode multihole tracking coupler design.