A compact waveguide isolator was developed, providing full reverse-power protection for TWTAs with 100W RF CW output, to be qualified for use in the next generation of Q-band multi-beam satellite missions. Using a classical quarter-wave coupled resonator Y-junction circulator design with a mixed metal housing and high conductivity material bonding allowed the full 100W of RF power to be handled in full reflection condition without overheating the junction ferrite discs.
A dual-channel waveguide load was designed to dissipate the 100W RF power in ceramic absorber elements over a large area. Thermal analysis was employed to optimise the mechanical element design of the RF-characterised load material to average out the heat distribution over the elements whilst maintaining RF performance characteristics.
Although RF finite element analysis was used in the design of the circulator, manufacturing tolerances were critical in realising the s-parameter performance over the desired bandwidth. The isolator design was targeted to achieve a 5GHz bandwidth centred on 40GHz with 22dB return loss and 23dB isolation over the -10 to +85°C temperature range, however the manufactured isolator was centred 2.5GHz lower in frequency. It was tested beyond the rated maximum power in TVAC at Honeywell’s Aylesbury facility and confirmed low loss, high isolation performance even under full reflection conditions.
The key challenges of the project were deciding on a design to manage the thermal energy in the isolator while maintaining good s-parameter performance whilst realising something that was manufacturable.
The developed part operates at a higher power level than previous designs, allowing the junction isolator to work where previously only a far larger differential phase-shift isolator could work.
The developed isolator features a quarter-wave coupled ferrite 3-port junction, developed to optimise the heat flow out from the ferrite resonators into the surrounding metal body. A two channel load was designed to maximise the area for the heat to be dissipated during fault condition to minimise the temperature rise of the isolator.
The architecture of the system is a 3-port ferrite junction circulator plus a dual-channel load – combined to form a junction isolator.
The project was carried out in two defined phases; in Phase 1 a detailed review of existing high power isolator technologies and breadboard development was undertaken, in Phase 2 the design, manufacture and test of a 100 W Q Band isolator was carried out.
High Power Q Band isolator has been designed and successfully tested in TVAC at 100W power with full reflection.