NBFM probably hasn't been used on ham bands in 30 years though the
whisperers on 2m "wide" fm are that narrow. Truly NARROW band FM has the
same band width as AM, about 6 KHz, no second order modulation products.
But copied exalted carrier on an SSB receiver it has a great sound. In
the mid 60's around Dallas, Texas, several of us used that mode on the
bottom of 2m. It sounded super, better than SSB, in my 75S3B. I planned
to, but never did, build a NBFM receive adapter. It can be slope
detected in an AM receiver (not with crystal or mechanical filters) but
that's not the best way to do it.
Mathematically, this truly NARROW fm has the same spectrum as AM, just
the carrier is 90° different in phase. There would be few on 40 with AM
receivers that would understand that blank carrier, though those using
SSB receivers would copy just fine. And the neighbor's TV wouldn't
notice any modulation.
73, Jerry, K0CQ
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