Performance of shaped beam quadrifilar antennas on the METOP spacecraft

The design and performance of two low gain shaped beam quadrifilar helix antennas on a large complex spacecraft are presented. Scattering from the strongly illuminated satellite degrades the antenna performance by distorting the copolar pattern and increasing the crosspolarisation. This paper describes the results of numerical simulations of the installed antenna radiation patterns using the MoM and GTD techniques in the VHF and UHF bands, respectively, In this frequency range the size of the structure makes it difficult to model the antenna performance using either method, and the results must be interpreted carefully. The accuracy of the predictions has been established by comparisons with measurements using a 1/8 scale mock-up of the quadrifilars and spacecraft.