#### The Transition between Radiative and Reactive Region for Leaky Waves in Planar Waveguiding Structures

The transition from a radiative to a reactive regime is studied for leaky modes supported by planar waveguiding structures such as Fabry-Perot cavity leaky-wave antennas (FPC-LWAs). These are typically studied above the \emph{leaky cutoff}, i.e., the frequency for which the leaky phase constant $\beta$ equals the leaky attenuation constant $\alpha$. However, an FPC-LWA can efficiently radiate even below the leaky cutoff, the condition for minimizing the beamwdith at broadside occuring for $\beta<\alpha$. Nevertheless, well below the \emph{leaky cutoff}, reactive phenomena may dominate radiative phenomena. These aspects are investigated here, providing a criterion to determine whether the relevant leaky waves are responsible for radiation rather than reactive phenomena. It is seen that the minimum-beamwidth condition occurs in a region where reactive effects prevail over radiative effects. It is concluded that, although directivity is maximized, the gain may be adversely affected by impedance matching issues. These results are useful for LWA design.