What is meant by cavity backed antennas,what is the need for it .

i am designing a cavity backed bowtie antenna.what is the need for cavity in antenna designing.if i am using a cavity box ,should i have to use a groundplane .how to calculate the dimensions of the cavity box.what material should be assigned to the cavity box.

A cavity is just a tuned circuit. Its main advantage is that you you a very wide choice of frequency responses and impedances to play with. It should be made from copper or some other low resistance material, otherwise its Q goes down and the losses increase.
Frank