link budget calculation

hello guys,
why is aperture efficiency of antenna is taken allways a value between .5 and .7 for link budget calculation.?how we can get the aperture efficiency of our antenna.?pls help me out ....its urgent
deepthy

Hi. Aperture efficiency is a parameter for analysing antenna arrays. In simple terms, lets take an array of dipoles in which the aperture efficiency is a measure of how close the dipoles are placed in the array with respect to the total area. Smaller the distance between dipoles, greater will be the aperture efficiency.