30 dB microstrip coupler with high directivity

Due to inhomogeneous structure of a microstrip directional couplers, i.e. partly dielectric substrate, partly air, they mostly present property of poor directivity and low coupling level. High directivity is achieved by a capacitive compensation by gap coupling of open stub formed in sub-coupled line. Nevertheless, these couplers have advantage of easy fabrication, lightweight and incorporation with other microwave devices and are validated via design using Sonnet software. The main goal was to obtain coupling around -30 dB, meaning that almost all power is passed to the output, with a wide band; from around 3.5GHz to nearly 9GHz. Desired values have been obtained, including isolation and input match reaching -70 dB.