In this work, the broadside radiation condition of a class of finite-length bidirectional leaky-wave antennas (Bi-LWAs) is investigated, and resulting that a boundary curve is found in the complex wavenumber plane. By further jointly analyzing the derived conditions and the dispersion curve of a leaky-wave, it is explicitly revealed that the boundary curves hold an opposite shift tendency with the fast wave structure. In such a way, one can easily determine the operating frequency from the derived broadside boundary curve of this fast wave-based bidirectional LWA.