There exist two regions in open guiding wave structures

bound region and radiation region

and the radiation region is divided into antenna region and reactive region.The transition from the bound region to the radiation region has been extensively studied

however

the definition of the transition for the antenna region and the reactive region is much more complicated due to simultaneous existence of the leaky effect and cutoff effect in the whole radiation region.How to give a reasonable definition for these two regions thus becomes an important problem in investigation on leaky wave phenomenon.We extend the traditional definition of the cutoff region by using the concept of complex effective dielectric constant

and propose a simple but efficient method to separate the two regions.Furthermore

the attenuation caused by the leaky and cutoff effect is determined in the cutoff region and the deduced expression reveals the phenomenon of large attenuation constant but with low radiation efficiency.