Directivity coupler with a very low coupling coefficient

The paper describes a double - dielectric layer S band high directivity coupler with a very low coupling (-40 dB and -60 dB) designed on microstrip transmission line technology. The four coupler ports are used as: one input, two outputs and one isolated port terminated by 50 Ω SMD resistor. The directivity coupler consists of two substrates with a common ground plane and with microstrips situated on the opposite sides of the individual substrates. The coupling is provided by two circular slots in the common ground plane. The couplers were designed and simulated in the CST Microwave Studio. Finally experimental samples were fabricated and measured. In the paper the simulation and measured results of the couplers are presented and discussed.