Antenna for RFID system at 865 MHz.

Hi Folks,

I want to do power budget analysis for RFID system operating at 865MHz.

I want to use dipole antennas at the reader and Tag end.
If 4W EIRP at reader antenna is transmitted, how much power is received by the Tag antenna, considering the path loss? After that, I want to check the power consumption of each block like rectifier, Voltage multipliers etc used in Tag IC.

In a vague manner, I know that HFSS is used for antenna structure simulation. ADS is also used for simulation purpose.
How do I check the power recieved by the Tag antenna from 4W EIRP transmitted?
How should I consider and analyze the pathloss?

Considering my requirement, what is the best way to proceed?
Please help me at the earliest.

Thanks