antenna correction factor

Hi to all.
I need your help.
It is well know that a generic antenna can be characterized in term by Antenna Correction Factor (ACF) or by Gain (G).
ACF is a receiving parameter.
G is a transmitting parameter.
By reciprocity, theory tell us that the two parameters are related each to other. In linear term and we have:

1/G=(R/ξ)*λ2/(4*pi)*(ACF)2

Where:
ξ is the impedance of the medium where the propagation is.
R is the system impedance (usually 50 Ohm)
λ is the wave length

I'd like to verify the relation between ACF and G with FEKO.
I select a dipole antenna for testing. I want to divide the test in two part:
1) Receiving mode:
I Excite the dipole with a plane wave of electrin field E and i measure the voltage (Vr) across a load of 50 Ohm placed on the gap of the dipole. The ratio E/Vr is the ACF.
2) Transitting mode:
FEKO has the PW card that permits to excite the dipole with a real generator. I try to excite the gap with this option. After, always with FEKO, i plot the Gain of the dipole antenna (the same of the receiving mode). But.....the result is different: the Gain obtained in trasmitting mode and the ACF obteined in receiving mode don't match the equation in the top of the paper.

I'm sure that the simulation of the receiving mode is correct (other verifies tell me this,). I think the problem is the transmitting mode. In particular i think the problem is the feeding simulation of the dipole.

Can you help me?
Has someone use the PW card with FEKO to simulate a real generator that feeds an antenna?
Has someone try to simulate the Gain of a generic antenna?

Thanks a lot.
Marco