Exactly what John has said above, this page has a lot of information on antenna parameters.

http://www.antenna-theory.com/antennas/aperture/horn.php

I followed it for my designs.

Thanks John and Muhammad. Appreciate your answers!

Good morning Saurav,

I believe that John and Muhammad have offered the most direct answer to your question.

You might also consider the information within the attachment.

Good morning Kenneth,

Thanks for the attachment.

Yeah they have offered the most direct answer. However, in the equation of magnitude of E-field, I cannot see the gain of the horn antenna. I wonder how this equation will relate to different antenna gain.

Saurav

Hello Saurav,

On slide 18 of the presentation, there is a formula for the gain of a pyramidal horn. There is also an approximation for the gain:

G = eff*4*Pi*Aph/lambda^(2). The efficiency (eff) of a horn type antenna is estimated to be between 0.75 and 0.9. Aph is the physical area of the aperture. This is a general equation for aperture type antennas.

An excellent source for antenna design is Stutzman and Thiele, "Antenna Theory and Design".

Kind regards,

Ken

Hello Saurav,

search google.com

Thanks Puran for showing the way :P