impedance matching of a coaxial cable and a microstrip patch

You have to move the location of the coaxial input along the patch center line to match to 50 ohms for a standard half wave (in the dielectric) patch antenna.

If the patch is thin, the coax. needs to be closer to the center of the patch.

The thicker patch requires the coax. to be nearer the edge.

Start half way between the center and the edge, then move the coax. and see if it gets better, adjust accordingly.

The coax. has to come from under the ground plane of course.

kchiggins

I am sorry my question is how to fabricate the coaxial cable to feed a microstrip anteena using the HFFS program , any one can help plz

fabricate means to build something, HFSS means computer analysis. If you are asking how to input this into HFSS, that's;

Center conductor to top patch, outer conductor to patch ground plane. Make sure the length of the input cable is 1/4 wavelength or longer or HFSS gives you erroneous S11. Add a radiation boundary keeping it outside the antenna by ? half wavelength.

HFSS is a complicated beast, lots of errant outputs.
The HFSS latest revisions Rev. 9.5 & 10 added so little to rev 8.5, a friend of mine sold all his stock in Ansoft instantly. Sounds like IE3D and CST Microwave Studio will catch up and pass Ansoft in the coming years. It's still a good program, but it seems the management thinks so much of it that not improving it is adequate.

kch