An intuitive and effective analytical method is presented in this letter to investigate the bandwidth properties of the finite-length bidirectional EH1-mode microstrip leaky-wave antenna (LWA) for broadside radiation. First, a broadside radiation boundary curve is theoretically established in the complex kz = β - jα plane through the usage of the array factor. Afterwards, it is distinctively found that as the operating frequency is gradually enlarged, the EH1-mode wavenumber trajectory and the extracted boundary curve show opposite shifting tendency in the complex wavenumber plane, thus generating an intersection point, namely, boundary frequency, which is actually the upper limited frequency for broadside radiation. To verify the presented analytical method, a finite-length bidirectional EH1-mode LWA is practically analyzed and implemented. The theoretical and measured results both demonstrate that this LWA is capable of generating the desired narrow beam at broadside in a wide band up to 5.5 GHz, evidently validated the theoretically-predicted ones.