calculating field strength
in a general case you should know if yhe target is placed in far field, near field or in the "very" near field (induction rather radiated).
For the target in near or far field illuminated by the main lobe (boresight direction) the gain of the transmit antenna is NOT important.
The 1st approssimation is to ommit the sida lobes, so consider that all the power is radiated in the main lobe.
then consider the target received power density (D)(W/m^2), and then by applyin the Ohm law calculate the field streght E=(D*377)^.5, then convert in dBuV.
But pay attention: math. formulas are not exaustive!
The radiated power call for a "direction" of the radiation. Field strenght is an "on air" quantity regarless the direction, in fact the E measurement instruments are shperic!
an accreditate link for E field calculation is the following:
**broken link removed**