impedance calculation with voltage source

I have been trying to calculate the impedance of a dipole pole antenna that is excited by a voltage source in HFSS v9.2.1. I can use the fields calculator to calculate the current at the center of the voltage source line. I should then be able to get the impedance by dividing the voltage by the current. I have designed the dipole for 130Mhz and made the dimensions of the dipole to match. I should get around 75 ohms at 0 degree phase but instead im getting around 40 ohms at about 30 degrees. I have been using the fields calculator cookbook that Ansoft released. I am not sure if my closed line to calculate current is too big or too small and if I have it placed correctly. I have made all sorts of variations on size and position of the closed line but with no succes. I have noticed that this is also a common issue for many people who want to excite their structure with a voltage source and find impedance. I have attached the program that I am currently working on. I would really appreciate any help with this. Ideas or answers are very welcome. And I think this would be a very useful technique for many people who want to use a voltage source.