calculating the focal point of parabolic reflector antenna

hi
can any body help me to calculate focal point of a parabolic reflector antenna?i dont have f/d ratio ! i need a parabolic reflector at 5.6GHz with the gain of about 27dB

http://www.antenna-theory.com/antenn...tors/dish3.php

Here's a link re geometry of a parabola:

http://www.tpub.com/math2/13.htm

The key understanding to very high gain on parabolic reflectors is the sensitivity to error in curvature and focal point error of the the sensor element effectively limit that maximum efficiency of the antenna. You ought to do a sensitivity analysis for surface roughness and shape error with the sum of all points in error. You need extreme control of the surface shape to achieve what Andrew gets in a larger antenna with 33.4 dBi gain... > in order to get 27dB gain.

Consider a goal of trying to achieve 33dBi gain and hope you get control of process variances to achieve 27dB guaranteed in a smaller dish. It is not easy to get <0.005% focal error over the entire surface to the sensor element. The same precision is used in quad helix antenna to achieve high gain.

Just ask ANDREW how they get to be #1 in microwave antenna field for performance.

I would expect to pay S1500 for a 30dB antenna. and S4K for a 40dB antenna for qty 1. If you can make it cheaper and guarantee results. kudos to you.

you know i want to design and simulate(in feko) a parabolic reflector and it's feeder as my end of study project. i want to use pyramidal horn and i could find it's dimensions for 12dB gain and diameter of the reflector about 52cm for 27dB gain. problem occur when i put the reflector in front of the feeder.i dont know exactly where should i put the reflector.when i see the VSWR graph it has more than one minimum and out of frequency range of 5.4G-5.8Ghz
thanks for your help

u can find some formula and technique at antenna handbook by lee volume 2 chapter 15
reflector antenna by yahya rahmat samii