NFM on the HF bands?

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Does someone in the community know if a deviation of +/-2.5kHz is legal in the HF bands as an A3 mode or not?  I believe +/-5kHz is legal above 29.0MHz but what about +/-2.5?

Back in the 90's I did some research on NBFM in the FCC rules book and called and discussed it with one of the FCC engineers.  They classified it to be an A3 mode like AM and SSB as long as the total BW occupied was not more than that specified for AM.  If so you could operate NBFM anywhere you could operate AM/SSB.  I don't remember what BW was spec'ed but maybe someone at Flex can answer the question.
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Posted 2 years ago

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Photo of WX7Y


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AM is 6KHz Wide, FM narrow is 5KHz, .
Actually it is F3 not A3 emmissions, so not sure how the FCC Engineer would think Amplitude Modulation is anywhere close to Frequency Modulation emissions.  

I googled this topic and this is the answer I found, 

It is not a restriction on FM per se but a restriction on the allowed bandwidth.  The applicable section is 97.307(f)(1):

"No angle-modulated emission may have a modulation index greater than 1 at the highest modulation frequency."

Since FM (and PM) is angle-modulation and you are restricted to a modulation index no greater than 1, you are effectively limited to using it.  Above 29MHz, 97.305(f)(1) does not apply.  See

here is the google link

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Ken - NM9P

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Back in the 50's and 60's, typical FM on VHF was 15 KHz deviation, and very wide. Then they adopted "Narrow Band" FM, which is the 5 KHz deviation we now call standard. But some HF ops on 160 and 75 meters were experimenting with using "sliver band" FM, which is the 2.5 KHz deviation we now call "Narrow Band" FM and is the new standard for commercial and public service frequencies, and FRS & GMRS. It never caught on, primarily because SSB was the "new thing" and was much more efficient for most HF communications.
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Bert, thank you for the correction, I misquoted the FCC rules book, NBFM is a F3 mode as you state, but must not occupy a BW greater than AM (A3 mode) in the HF phone ham bands, I forgot the F3 part in my original post.

Ken, I have heard the term "Sliver Band" used but never ran across the term when I was doing my research in the 90's.  Yes, SSB killed NBFM dead, being a much better mode in many ways.

Well, I think that answers my question, +/-2.5KHz can be used in the HF phone ham bands.

Regards, Dennis, k0eoo
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Kevin WB4AIO

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Actually, if you use 5 kHz of audio bandwidth as the modulating signal, then 5 kHz deviation becomes legal on HF, since you would be no wider than an A3 station using the same audio spectrum. As far as I know, though, the Flex rigs/software don't allow adjusting the _audio_ bandwidth when in the FM mode, so the point is moot. Also, FM on the static- and QRM-filled HF bands is not optimal anyway, no matter what the bandwidth, because the broadband noise bursts are deafening and maddening on all but the strongest stations.

73, Kevin.