I want to add a capacitor in yagi antenna in CST so that I can control its complex impedance. i have achieved 36-3j but I want it to be 36+24j. In order to change just complex part, I want to add a capacitor.
I find it strange that you need to make your antenna inductive, but if you do, then you need to use a series inductor not a capacitor. If the impedance is 36-3j and you want it to be 36+24j then you need a 27j inductor in series (with the centre conductor), or if it is a balanced feed, 13.5j inductors in series on both sides of the feed loop. You might be able to achieve the same thing by changing the perimeter of the feed loop if you have one - are you using a folded dipole feed for the Yagi?
nH times GHz cancels powers of ten, so for a 27j inductor you need inductance(nH) =27/(2 pi f(GHz)).
If you are feeding this antenna with a 36 ohm transmission line it would be better to leave the impedance as 36-3j, which is a good match, and use a series inductor at the other end of the feed line to reduce the reactance of the generator, which I presume is 36-24j. Then there will not be a standing wave on the feed line bouncing off the not very good match at both ends. Matching as you describe is really only the answer if the receiver or transmitter is connected directly to the antenna port.
Have you tried Smith chart for impedance matching . Long ago I used paper , now I hope there are software for that . We know that for every lamda/4, C and L interchange .
Beta= 2*pi /lamda (is the starting point)
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