L-band output network

In the INMARSAT-3 system, the L-band output network (ONET) interfaces between the SSPAs and the antenna feeds. It provides filtering of the output signal and uses Butler matrix hybrid networks for routing power to one, or all, of the 22 antenna feeds, depending on the relative phasing of the input signals, as set by the preceding beam former. The authors describe the design and performance of the re-entrant cavity bandpass filter, its associated coaxial low pass filter, and the stripline 4×4 matrices. Performance figures are presented for transmission characteristics and phase tracking. Additional to the twenty two identical ONET filters, there is a requirement for a single output filter in the navigation channel. Having higher frequency, lower bandwidth and sharper cut-off than the ONET filter, this was realised in evanescent mode construction. Design and performance details are given