Design of Waveguide Cavity Filters for Ku-band Satellite Applications

Abstract

In this master's thesis several waveguide cavity filters are designed and simulated and three of them are constructed, two third order and one fifth order filter. One third order filter is fabricated in brass, and the two other filters are fabricated in aluminium. Each order filter is simulated using brass, aluminium and gold as surface material. This is done to investigate and compare the different insertion loss due to materials with different conduction. The insertion and reflection loss of the physical filters is measured on an automatic network analyzer. The measured results are compared to the simulated results where applicable. It is found that the advantage of using gold plating is small. Simulations indicate a difference in insertion loss of only 0.006 dB compared to an aluminium filter. The difference in insertion loss between brass and aluminium is simulated to 0.2 dB. The measured insertion loss for all filters is higher than simulated results. The brass filter is seriously flawed, and can not be compared to simulated results. The pass-band insertion loss of the two aluminium filters is found to be 0.4 dB higher than simulated results. This discrepancy between measured and simulated results is large enough that the physical filters do not comply with the design specification, while the simulated filters do. The smallest pass-band insertion loss measured is 0.433 dB for the third order filter, and 0.814 dB for the fifth order filter. The highest allowed pass-band insertion loss in the filter specification is 0.25 dB over a bandwidth of 200 MHz. The reason for the difference between simulations and measurements is thought to be both due to small machining faults and to a large extent due to a suboptimal placement of a joint between the two parts comprising the filter.