I don't think you can make a flat statement either way with the information
supplied. The only thing that's for sure is NBFM has a constant power
envelope whereas AM has a highly variable envelope. And why do you say AM
has excessive bandwidth?. NBFM only can only approaches the bandwidth
efficiency of AM when the FM modulation index gets very small.
When FM gets narrow enough (expressed by modulation index, not deviation),
it's interference rejection is no better than AM.

Joe
W3JDR

...
Hi, guys,

ISTR.......
(through the mists of time) that the excessive bandwidth of an AM
transmission signal can interfere with the reception of a NBFM signal,
though the converse is not true. Can anyone confirm this (or else
flatly deny it/affirm it)?

Thanks,

p.
--

The BBC: Licensed at public expense to spread lies.