How to determine the Aperture radius of Reflector Antennas?

Hi all,
I intend to design a feed horn - Reflector assembly as a research topic my masters program . I am currently working on HFSS , Grasp and have just started with FEKO too . I am facing a difficulty in determining the Aperture radius in the reflector , as to how do we determine the aperture radius given the focal length and diameter of reflector and the frequency of operation .I am unable to find any such formulas which will help in determining Aperture radius. Please do guide me in this regard .

Thanks and regards,
Mukund

Hello there:

I think you should read a good antenna textbook before trying to design antennas. An "aperture radius" apparently is one-half of a round aperture diameter. Aperture antennas can be horns or optical systems like a prime-focus parabolic dish, Cassegrain or Gregory.
All aperture antennas are usable over a wide bandwidth and their dimensions related to a signal wavelength determine their radiation pattern and directivity.