Quality factor of helical coaxial cavity resonators with modulated radius

This paper presents an analytical method for the estimation of the unloaded quality factor of helical coaxial cavity resonators in the case where the radius of the helix is modulated along the direction of its axis. The method exploits closed form expressions available for the electric and magnetic fields in the sheath helix. Appreciating that the ratio of stored versus dissipated power in each helix turn is independent of the field distribution within the resonator, the aforementioned closed form expressions readily enable the estimation of the Q-factor for arbitrary modulation of the helix radius. In earlier works, the reduction of the helical core towards the open end of the resonator has been proposed as means to increase the power handling capability. The work presented in this paper is a critical step towards optimizing the trade-off between Q-factor and power handling performance in helical coaxial cavity resonator filters.