What does a parasitic layer do?

Hi all,

I am using a colleague's model of a bowtie antenna with a parasitic layer on top. Can anyone tell me what the purpose of this layer may be and how to recognize if I should try using one or not? thanks in advance.

The parasitic layer may act as a director (as with a yagi) to radiate most of the energy in its direction. It also might function to improve the return loss.