The AM Forum

I'm curious if FM is allowed on all bands or just specific like 2m and so forth.  I could not easily tell from band plan at arrl site.

73
NEO

Narrowband FM is allowed in the HF phone bands as long as it is no wider than an AM signal. There's also the FM portion of 10 meters which allows for a wider FM signal than what is allowed below 10 meters, the bandwidth/deviation allowed there is the same as what is allowed on VHF, I think.

NBFM generated digitally is the same width on all bands I believe. In the old analog world it is all dependent upon the frequency it was generated at and the multiplication factor and deviation had to be reduced as multiplication increased.

Ive been on 75 a few times with AMers with the TS-940 on NBFM for G&G and nobody commented and I didnt say anything; also kept the deviation low.

Carl

Hey Bob! Was wondering what happened to you. Long time no hear.

"Narrowband FM is allowed in the HF phone bands as long as it is no wider than an AM signal"

How wide can hifi AM go???

Why not read Part 97?

97.308 Notation (1)
(1) No angle-modulated emission may have a modulation index greater than 1 at the highest modulation frequency.

Keep it handy: PART 97 (http://www.arrl.org/files/file/Regulatory/Part%2097%20-%2004-28-2011.pdf)

And if I remember it right, a modulation index of 1 was the equivalent of the bandwidth of an AM signal modulated to 100%.

I assume that we're talking analog voice communications here.  The BW occupied by a conventional analog NBFM  signal is 2(m+d), where m = the highest modulating frequency and d = maximum deviation.  Your "typical" commercial NBFM voice signal used at VHF Low Band and above with 300-3000 Hz audio and +/- 5 KHz maximum deviation occupies 16 KHz.  Try that below 10 Meters and see what happens.

An DSB AM signal with communications audio (300-3000 Hz) occupies 6 KHz as shown by the same formula (d=0).  How you can do FM voice within the confines of an AM signal BW is beyond me, unless you significantly reduce the modulating frequency and the deviation below practical limits.  Or unless you define AM on the ham bands as having at least 8 KHz modulating frequency so that you can sound a bit more like your neighborhood AM broadcast station.

Going by the section on frequency and phase modulation in my 1953 handbook, that's exactly what they did. Under 10 meters, audio and deviation was limited to no more than 3 khz. BUt they're saying that by cutting the audio off at 3 khz, the maximum bandwidth should be no wider than 6 khz.

I'm not sure if we're saying the same thing or not.  If you limit audio and deviation combined to 3 KHz, then yes, your maximum BW will be 6 KHz.  However, if you let your audio go up to 3000 Hz and you want to keep within 6 KHz BW, then there's no room left to deviate; therefore you can't run a 3 KHz audio modulated signal on FM and stay within 6 KHz BW.

I'm just repeating what it says in the ARRL handbook I have, which also explains how that modulation index of 1 is found. Carrier deviation/Modulating frequency=Modulation index, so 3 khz deviation divided by 3 khz audio is a modulation index of 1.

And that requirement is removed in the 29.0-29.7 segment of 10 meters, as well as on all higher frequency bands.

Thanks for info.