Chuckey: I agree with FVM, a 50 Ohms source with a 50 Ohms cable, shows a 50 Ohm source impedance to the load. In other words the load doesn't see any difference between direct connection to the generator or connection via the coaxial cable (except for some loss and some time delay). Due to the internal attenuator, RF generators for measurement show impedance close to the specified value (mostly 50 Ohms). In case of Power Amplifiers, the situation can be completely different.
Mateusbatera: To give any useful feedback, you should post the circuit. To be sure, you may verify correct operation of your cables and generator (with the spectrum analyzer).
Your 50 ohms source definitely doesn't model the small electrical antenna. If the input impedance of the amplifier is really far above 50 Ohms, your setup at least gives the voltage gain of the amplifier.
To model the antenna, best is to determine the capacitance (via measurement, empirical formulas or EM simulation) and put this capacitance in series with your amplifier (between the coaxial cable and your circuit not between the generator and the cable).
Re(Zant) for the real whip antenna will probably be far below 50 Ohms (for a very short whip < 0.1 lambda), but as a first start, you can just use the generator with the capacitor.
Next step may be modelling Re(Zant) better as Re(Zant) may affect stability of the circuit.