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Design, Development and Fabrication of Post-coupled Bandpass Waveguide Filter @ 11.2 GHz for a Radiometer
Microwave methods, both passive and active, play an important role amongst modern diagnostics of fusion plasma research. Microwave Reflectometer is used to measure plasma density profile and its fluctuations. Electron Cyclotron Emission (ECE) waves which has a frequency range in the microwave as well as millimeter range is used to measure plasma temperature and its fluctuation. ECE frequency depends on toroidal magnetic field used for plasma confinement. Toroidal magnetic field has radial profile. This radial profile gives good radial correlation between cyclotron frequency and spatial location of ECE.
The power level in cyclotron radiation is very low (nW - &umW). A heterodyne radiometer can be used to measure such a low power level. In the Radiometer IF section, the combination of power divider and filter bank is used to resolve the spatial location of the cyclotron radiation. The existing E-band radiometer has 1-14GHz IF frequency. For this frequency range, only 1-10 GHz filter bank is available. Next filter frequency required is 11.2GHz.
With the above requirements, the authors designed, developed and fabricated a post-coupled Band pass waveguide filter @ 11.2GHz. The dimensions of the filter were decided using certain calculation and criteria. After carrying out various simulations using ANSOFT-HFSS, the filter dimensions were optimized for required electrical parameters. Although the attenuation is higher in the post-coupled design in comparison to an iris-coupled construction, the new design outperforms the latter with regards to ease of fabrication and thus time and cost constraints