A Low-loss Coaxial Comb-line Filter Design and Implementation for Aircraft Autopilot FMCW Radar Altimeter

Pulse or FMCW altimeters are used for height or distance measurement. For both types, a radio frequency signal is sent and after few micro seconds the reflected signal from the ground or sea is received and then by signal analysis the height can be measured. Because of wide frequency bandwidth of mixer in receiver and for better sensitivity, frequency modulation, jamming and noise, a narrowband low insertion loss bandpass filter should be designed. In this paper a new structure stepped impedance resonator (SIR) narrowband bandpass filter is proposed that has special specifications such as %4.5 tunable bandwidth (%3 up to %7), sharp and wide stopband frequency characteristic, less than 0.7 dB insertion loss in passband and more than 80 dB rejection in stopband. With new coaxial resonators the seven pole Chebychev bandpass filter is designed and fabricated with more than %15 frequency tuning capability. Good agreement between measured and simulated results validates this design process.