[Milsurplus] Wide FM

[John Vendely]

Wideband FM does provide especially good audio and AM noise rejection 
when above the limiter threshold.  Spectrum occupancy considerations 
were the reasons for the progressively narrower deviation limits imposed 
in the land mobile bands, not because narrower deviation FM was 
inherently better.  The extremely narrow deviation FM currently in vogue 
(modulation index of about 1) sacrifices much of the advantage of FM .  
Present-day military radios use 6 kHz deviation, very wide by current 
standards, for this reason.

Back in the 1970s, 15 kc deviation FM radios were in common use on 10 
meters above 29.0 Mc and 6 meters above 52.5 Mc, the frequency ranges of 
primary interest to military radio enthusiasts.  Note that FCC part 97 
does not set FM frequency deviation limits in these frequency ranges.  
Below 29.0, however, angle modulated signals (i.e., FM and PM) must not 
exceed a modulation index of 1 at the highest modulating frequency.  
This obviously precludes wideband FM, (including 5 kc deviation) but 
this limitation does not apply from 29.0-29.7 Mc, or anywhere above 50.1 
Mc in the 6 meter band, for that matter.

Then, there's the notoriously vague 97.307 (a), which states, "No 
amateur station transmission shall occupy more bandwidth than necessary 
for the information rate and emission type being transmitted, in 
accordance with good amateur practice".  This can be interpreted in 
various ways, but there has never been any action taken against the many 
"enhanced SSB" operators in the lower HF bands who run unnecessarily 
wide bandwidths.

Several years ago, I inquired about our use of 15-20 kc deviation 
military radios on 10 and 6M with ARRL representatives at Hamvention.  
They asked if we were using repeaters, which of course we were not.  
They asked about transmit power levels, and I replied "500 mW to 50W 
max".  The answer was, "Ah, that's no problem, go ahead and operate".

The bottom line is that there is no specific FM deviation limit imposed 
by FCC part 97 above 29.0 Mc.  I think it's reasonable to say that 
careful use of wideband military FM sets of modest power output by 
knowledgeable operators on 29.4 and 51.0 Mc is not a problem.  Exercise 
great care to avoid interference, and be certain the old sets are within 
limits for out-of-band spurious emissions (a lenient 43 dB below average 
power output for transmitters built before April 15, 1977).

73,

John K9WT


On 2/1/2026 8:16 PM, Charlie L. wrote:

> To be honest, I do not know the current rules for FM deviation limits 
> on VHF.  But, I used to run an RCA Fleetfone station on 52.525 with 
> 15KC deviation and talked to my good friend about 50 air miles away 
> all the time, he was using a Motorola station, wideband too, and it 
> was like listening to the FM BC band, the audio was so perfect, and 
> both of us using carbon mics. .  I still have that FleetFone and it 
> still works, but have it throttled back to 5KC deviation, not because 
> of any regulation, but modern rigs looking for 5KC deviation, the 15KC 
> just goes right past them with odd sounding audio on their end.  I 
> retuned the IF and discriminator in order to get enough audio from the 
> 2 NBFM stations I have worked on 52.525 over the past 34 years. Nobody 
> seems to know about 52.525 anymore.  Worked all over the Caribbean, 
> into Alaska on FM  one time, but now the only thing on 6 is FT8. (And 
> me and another buddy on 50.4AM once a week.)  The Fleetphone is 
> normally on almost all the time, but silence is all I get.
>
> Charlie in NC
>
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