Design technique for multiport combline couplers with single port excited

The design and construction of a five-line ten-port codirectional coupler, with both the amplitude and phase characteristics constrained to taking a chosen from over the 2¿¿4 GHz octave, is discussed. The equations defining an n-line system of coupled transmission lines are reduced to a form where the mode propagation constants may be handled independently of the other parameters, removing the need for an n ¿¿ n diagonalisation. Error integrals, representing the deviation of the response from an arbitrarily specified required response for both phase and amplitude over any chosen bandwidth, are defined. A numerical method for the simultaneous reduction of these errors is outlined. A five-line equal-power-splitting design is given as an example. A physical coupler having an operational bandwidth of 2¿¿4 GHz is constructed and its characteristics measured using a time-domain automatic network analyser (TDANA) [1]. Theoretical and practical results are found to be in good agreement.