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FM on HF?

posted Aug 31, 2013, 11:34 AM by Charles Boling
A question was raised this morning after the 75m MNW net regarding the legality of the use of FM below the 10m band.

FCC part 97 provides the answer.  There is no prohibition of FM, but sections 305 & 307 give clues as to why it's not popular.
97.305(c) contains a nice "easy to read" table (about as easy as you're going to get) showing the restrictions for each frequency segment of the amateur bands.
Below it, 97.307(f)(1) says "No angle-modulated emission may have a modulation index greater than 1 at the highest modulation frequency."
http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=336ab7469b61ecbfa15086dbf1bf2c59&rgn=div5&view=text&node=47:5.0.1.1.6&idno=47#47:5.0.1.1.6.4.157.4
and that restriction (1) appears in the aforementioned table for all frequencies below 28.3 MHz.
With a max audio frequency of 3kHz, a now-Amateur-standard 5kHz deviation would be illegal, but narrow-band 2.5kHz signal would be acceptable.  Basically, you shouldn't have a signal wider than a classic AM transmission -- it's just not nice on a crowded band.

Narrowband FM is a fairly recent addition to most amateur radios, and like most things recent, unless it offers a very compelling improvement over what's already in use (as PSK31 did, for instance), it's not going to take the amateur world by storm.   Narrowband FM has less noise immunity than its wider siblings, and while great for close-in ground-wave communication, its advantage quickly disappears as signals weaken, or start changing in creative ways after bouncing off the ionosphere.  But don't let that stop you from playing with it!  Try it, learn from it, and if it offers an advantage in a particular situation, use it!

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