The method of mode matching (MM) is used to construct ideal waveports in the context of the method of moments (MoM). The waveports can provide not only excitations as sources but also perfectly matched loads for traveling-wave slotted waveguide antennas. Additionally, the application of the higher order basis functions (HOBs) in MoM significantly reduces the number of unknowns compared to the RWG basis functions. Numerical results show that the proposed method is accurate and efficient for traveling-wave slotted waveguide antennas. Finally, a Ka-band waveguide array with 1976 narrow-wall slots is solved using a high-performance cluster.