  Over the past decade, the issue of gain degradation at broadside in periodic
leaky-wave antennas (P-LWAs) has been resolved, using a circuit modeling
approach, by introducing proper asymmetry in the unit cell of the antenna
structure. This paper provides a more fundamental and insightful perspective of
the problem by showing, using a simple coupled-mode analysis, that the optimal
level of structural asymmetry corresponds to an exceptional point of the
coupling parameter between the two eigenmodes of the P-LWA. This contribution
represents a key step towards the development of a full electromagnetic
resolution of the broadside issue.
