Development of Forward-wave Directional Couplers Loaded by Periodic Shunt Shorted Stubs

In this paper a new procedure for designing forward-wave directional couplers using periodic shunt short circuited stubs is proposed. A new type of cell using these stubs, which enlarge the phase difference between even- and odd modes of a uniform microstrip coupled line is introduced. Using the equivalent circuit model for even- and odd-modes of the proposed cell, the elements of the  ABCD transmission matrix of the cell are derived. The design procedure of the Forward-Wave Directional Couplers (FWDC) is disccused using the equivalent circuit model. To verify the accuracy of the proposed method, three FWDCs with different levels of coupling are designed and studied at the central frequency of 1.5 GHz. The couplers are numerically simulated using High Frequency Structure Simulator (HFSS) and the results including S-paramerters, coupling level and the overall bandwidth are compared with those obtained by equivalent circuit model. Moreover, a prototype of the proposed 0 dB coupler is made using a single layer of substrate and it is succesfully tested. Measured results show that 0.62 dB level of coupling is obatined with 1 dB flatness from 1.45 GHz to 1.63 GHz, which corresponds to 11.7% fractional bandwidth and the length of the fabricated coupler is 0.26λg.