You absolutely need an RF generator, because trying with the transmitter you will have so many leakings that the measurement will be not true.
I remember one time a guy called me from a different country and told me that the receiver I designed had a - 140 dBm sensitivity.
Knowing the project I can expect about 1 - 1,5 db Spread in the characteristics ( with a normal sensitivity of - 108 dBm), but not - 140 dBm.
He swear that the measurement was done properly, but after some explanations he told me that he was without a RF generator and he used the transmitter ( a 2 W transmitter ) as generator, with some attenuators.
But the trouble was that the transmitter and the receiver were about 20 centimeters one to the other.
Explained him the mistake he found ( lean ) an Rf generator and he found the sensitivity be - 70 dBm due to overload ( and failure ) on the first preamplifier after the preselector (someone connected directly the transmitter to the receiver !!!!!!!!!!!!!!!! ).
Mandi