My wording ("above the noise floor") was not well chosen. What I mean is simply this: Your receiver will have a certain noise floor which you will see/receive

My wording ("above the noise floor") was not well chosen. What I mean is simply this: Your receiver will have a certain noise floor which you will see/receive even without an antenna connected to it (internally generated by imperfect components). If you connect an antenna, this antenna has a certain "antenna factor" which describes how much of the field strength of a signal "in the air" will be converted an electrical signal at your antenna terminal. For a very short antenna, this factor is small, for a "good" antenna this factor increases. Assuming both your "bad" and "good" antennas are still good enough to pick up background noise and desired signals above your receiver noise floor (which is usually the case, even if you put your finger into the antenna terminal!), all you get is a difference in absolute signal levels while the SNR (ratio between the desired signal and the noise) will not change. So this principle is universal, independently of the modulation scheme.