Broadband fin-line directional couplers

The design of various circuits of millimeter-wave technology requires the employment of directional couplers, in particular 3dB dirctional couplers. Such directional couplers can be conveniently designed on the basis of fin-line approaches by making use of 'periodic coupling' procedures considered by Marcatili (1969). Another successful method for constructing a directional coupler is based on the utilization of the microstrip-slotline coupler (hybrid coupler) of microwave technology. A suitable modification of a hybrid coupler for an application in fin-line circuits is discussed. Coupler designs obtained with the aid of both methods provide broadband directional couplers with low insertion losses. An analysis of the circuit of a hybrid directional coupler is discussed in detail, and the design of a 3dB directional coupler according to both considered procedures is described.