preventing an LNA from oscillations

Here is the situation....when i designed an LNA operating at 11.5 GHz, i found it unstable at 2 GHz, so when i tried to put a parrallel 50 ohm resistor at the gate terminal with an RFC, i expected the noise figure to degrade so much, but surprisingly, it was the same, all the difference is that the LNA is stable at all frequencies.
SO, my question is, how the noise figure didn't degrade ?!

The reason is because your resistor probably has some parasitic inductance that will not change the Gamma optimum (Γopt) of your LNA, at the frequency where are you measuring the noise.