Early FM Mobile Radio Tests
Here is some of the developmental history from 1941 of how FM as we
know it today got started and tested by GE, with FCC Engineers in tow. FM
was only invented 1935 by Major (name, not rank) Edward Armstrong. Of note
is that these FM mobile radio tests started at +15 kHz FM deviation and on
40 kHz channel spacing! That means it was only re-farmed once in the mid
1960's to what was called "Narrowband" of +5 kHz FM deviation and to 20
kHz channel spacing on VHF Low-Band (30-50 MHz), which was called "UHF"
back then. On VHF High Band (138*-150-174 MHz) it became 15 kHz channel
spacing as the filters became better by the time 150 MHz became available
(after WW II). I had always thought there were two re-farms to get down to
+ 5 kHz deviation, starting at + 30 kHz.
Any way, here is the article that describes the history of the Tests
by GE that set FM as better than AM. Enjoy!
see magazine page 5, pdf page 7. If it only goes to the Home page, select February 1942.
In 1995, the FCC announced the "Land Mobile" communications bands
(Police, Fire, Business, TRC, etc.) radios would be re-farmed again to +
2.5 kHz FM deviation = 100% modulation on 7.5 kHz channel spacing in 2000.
Well, it took until 2013 to effect all the changes necessary! And the plan
to re-farm again in short order to + 1.25 kHz FM Deviation on 3.0 kHz
channels was in that same plan for the future (2020??).
Where AM has natural electronic limits to 100% modulation (where the
waveform "crosses 0" on the negative-going waveforms, FM does not.
FM "100% modulation" is a man-made construct. For
different services, (Land Mobile, FM broadcast radio, TV audio) can and did
have different "100% modulation" limits to the deviation amounts.
* The Federal Government equivalent of the FCC is the NTIA (National
Telecommunications and Information Agency for the FBI, Military, Secret
Service, Postal Inspectors, etc.) They have their own bands around the FCC
bands for the same purposes (Land Mobile two-way radios). The NTIA and the
FCC often move together on such issues as radio specs so manufacturers can
make radios to common and equivalent specs.