Hi Basem,

I used this paper to calculate the gain of the hon recently.

"Pyramidal Horn Gain Calculation

with Improved Accuracy",Michael J. Maybell,  and Peter S. Simon, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 41, NO. 7, JULY 1993

The estimate of the gain is 

G0 = 32/pi*A*B/lambda^2

where A and B are aperture dimensions. 

Also have a look at the optimal horn section of http://en.wikipedia.org/wiki/Horn_antenna 

Regards

Dmitry

Thanx for reply Mr.Zelenchuk 

What i need especially for is E-plane horn antenna !!

Do you think it will be same formaula for that

thanx in advance 

The formula is for cosine distribution on an aperture, TE10 exciting horn aperture. I think it should provide good estimate.

Ok. Many thanx for your help

And check out the book"Antenna theory,Analysis and Design" by Constantine A Balanis.Its chapter 13,has all you neee about horn antenna especially the E-plane horn antenna.A fried of mine said that he found a copy of iit online.It is a nice book,i consider as an antenna dictionary.Good luck

Thanx Mr.Lambert

Dear ,

please check these link and you will get whatever is required 

https://smartech.gatech.edu/bitstream/handle/1853/34414/PG_TR_040813_YL.pdf

2. http://www.ijetae.com/files/Volume4Issue5/IJETAE_0514_107.pdf

3. http://www.ijetae.com/files/Volume4Issue5/IJETAE_0514_107.pdf

thanx Mr.Saeed

to design a wave guide or antenna , yes formulas is available ,pls refer the microwave wave theory(rectangular W G and mode analysis ).