Design of a composite multi-aperture array circular directional coupler for high power gyrotron

A composite multi-aperture array circular-rectangular waveguide directional coupler is designed for measuring gyrotron output power and monitoring spectrum with loose-coupling theory. The coupling characteristics of 20-aperture directional coupler with the distribution of Chebychev-equal radius are performed by theoretical analysis and CST microwave studio (CST-MWS) simulations. Results indicate that the composite distribution possesses both the advantages of smaller ripple, higher directivity and wider bandwidth. Simultaneously, we also discover that increasing the thickness and declining the radius of the apertures will be beneficial for weakening the ripple of coupling coefficient. In addition, proper rectangular waveguide size and distance between the last and penultimate aperture of the array can improve the directivity effectively.