Check Antenna Theory: Analysis and Design book Ch 14, you will see the realtion between the voltage and current then you can see what is the simulated impedance you got and in which side you need to move your feeding to find the 50ohm position.

*From Modern Antenna Design by Thomas A Milligan

As the query is 'to find the feed point of a probe fed microstrip antenna ', we should restrict our analysis on probe excitation only. I have applied this procedure (as mentioned below) to find the feed position for optimal coupling. This technique is quite general and is normally used to find single feed position to excite 3 or mode modes simultaneously.

Read Microstrip Antenna Design Handbook_Ramesh Garg_2001',open Chap 4, sec. 4.2.2 (Cavity Model Analysis), pp. 257, Evaluation of Zin has been explained in detail in a lucid way. After the write code (c, c++, MATLAB or fortran any language) to find Zin for fixed probe position and Say, it is fun1. Then write fun2, where vary probe position and for each probe position, call fun1 to find the Zin, just extract the real (Zin) and store this data. After that, you will get Real(Zin) Vs probe position data. PLot is and locate the probe position for which real(Zin) is equal to 50-60ohm. That the optimal probe position.

Hello sir thanks for your reply but sir I want to know feed point calculation using HFSS.if u know please share with us

You can run parametric study, obtain the real(Zin) vs probe position plot. Then, repeat that step as mentioned above.

Hi Shally,

Thanks Nitin Kumar Sir for sharing the snapshot of the book that you referred. Special thanks to Sudipta Maity Sir for always helping the beginners. You always show the right direction and explain the problem in a very lucid way.

As far as the problem with coaxial probe in HFSS is concerned, I have solved this kind of problem in the following manner.

1. First of all, make the coaxial cable in HFSS. A step-by-step procedure is given at https://www.youtube.com/watch?v=w9SA9LjRuPA

2. While designing this, instead of entering the coordinates directly, use some variable (say $x for x-axis coordinate and $y for y-axis coordinate). All the cylinders that are used while designing the coaxial cable (e.g. proble, outer conductor, inner-conductor etc.) will have the same coordinates. (You can see and edit the value of the variable at 'Project' ---> 'Project Variables'. The unit of $x and $y variables is 'mm').

3. Now place the coaxial cable with your antenna (or design your antenna by first designing the coaxial cable as discussed in the paper).

4. Now apply the 'Optimetrics' on these variables (both $x and $y).

*****Steps for applying 'Optimetrics' in HFSS' are given in the file attached.*****

Thanks Munish Kumar for appreciating my reply. I will try to help in future also.

Thanks sudipta sir and munish sir.you both are very helping.

Regards

Dear Sudipta Maity ,

By using the referred formula for position determination, how can I identify the position when the results give values like 0.0294 - 0.0292i or 0.0286 - 0.0286i?

Best Regards.

Computing input impedance (Zin using Coaxial probe): I have solve this problem analytically (plz consult: Microstrip Antenna Design Handbook_Ramesh Garg_2001) and we got excellent agreement between theory and experiment. The formula may not be available explicitly for different MA (circular, rectangular, triangles, elliptical, etc.), but we can easily compute it by extending the theory.

If required, you can consult my paper on 30-60-90 triangular MA, where I have shown, we can excite 5 broadside radiating modes using single coaxial probe fed.