An alternative configuration for an infinite coaxially end- fed waveguide phased-array antenna is explored. In the proposed con- figuration, the integration of the end-fed launcher and the output waveguide, rather than the waveguide in itself, acts as a basic radiator. This allows the design of more compact antenna elements. In order to analyze and optimize the structure, an accurate full-wave simulation tool based on the multimode admittance matrix representation is developed. Results are validated against state-of-the-art commercial CAD software. Numerical simulations involving singly and doubly tuned con- figurations are presented and discussed to illustrate the potential convenience of the proposed configuration and of the CAD tool developed.