It’s not immediately clear that Shannon’s theorem is a good point of comparison here, since it’s only recently that coding schemes have really approached the Shannon limits, and FM and AM do not use these.
Even if one does assume a Shannon-perfect coding scheme, as the noise ratio gets greater the benefits of spreading a signal across a higher bandwidth fades. Furthermore, most coding schemes hit their maximum inefficiency as the signal to noise ratio decreases and messages start to be too garbled to be well decoded.
I’d additionally note that folks get near the Shannon noise limit _through_ ‘magic noise rejection’ (aka turbo and ldpc codes). It’s therefore not obvious that FM isn’t gaining clarity due to a noise rejection mechanic. The ‘capture effect’ is well described as an interference reducing mechanism.
Empirically, radio manufacturers who do produce sophisticated long range radio usually advertise a longer range when spreading available power across a narrower rather than wider bandwidth.
Even if one does assume a Shannon-perfect coding scheme, as the noise ratio gets greater the benefits of spreading a signal across a higher bandwidth fades. Furthermore, most coding schemes hit their maximum inefficiency as the signal to noise ratio decreases and messages start to be too garbled to be well decoded.
I’d additionally note that folks get near the Shannon noise limit _through_ ‘magic noise rejection’ (aka turbo and ldpc codes). It’s therefore not obvious that FM isn’t gaining clarity due to a noise rejection mechanic. The ‘capture effect’ is well described as an interference reducing mechanism.
Empirically, radio manufacturers who do produce sophisticated long range radio usually advertise a longer range when spreading available power across a narrower rather than wider bandwidth.