GB> 75th Anniversary of FM Radio Special Event

Glen Zook gzook at YAHOO.COM
Wed Dec 3 01:11:03 EST 2008

It is certainly possible to get a modulation index of less than 1 and it will definitely sound fine.  For a maximum transmit audio frequency of 3000 Hz you can run up to +/- 3 kHz deviation.  In Europe virtually all 2 meter FM now is using a deviation of +/- 2.5 kHz which produces a modulation index of 0.83333.  Also, a number of "modern" transceivers have a "narrow" FM mode for HF operation that produces a deviation of +/- 2.5 kHz deviation.  Remember that most "modern" equipment limit the maximum modulation frequency to 3000 Hz or less.

It is possible to have deviations considerably less than +/- 2.5 kHz.  However, you also have to reduce the receiver bandwidth accordingly and increase the audio gain to get a "loud enough" received signal.  There are people who claim that it is not possible to reduce the deviation substantially and get communications quality reception.  Now it can depend on the type of detector as to how narrow a signal may be useful.  But, a Foster-Sealy type of discriminator can detect pretty low deviation IF the bandwidth of the receiver is reduced accordingly.

Remember that commercial two-way FM is being reduced for +/- 5 kHz deviation to +/- 2.5 kHz deviation within the next few years.  That will reduce the modulation index from 1.66667 to 0.83333.  Now on the 896 MHz commercial two-way FM band the deviation has been restricted to under +/- 2.5 kHz for well over a decade.

When the commercial two-way FM bands had the deviation reduced from +/- 15 kHz to +/- 5 kHz (modulation index of 5 down to 1.6667 where 3000 Hz is the maximum modulating frequency) various individuals predicted that the audio quality, etc., would suffer greatly.  But, with proper redesign of the audio circuits in both the transmitter and receiver and with the reduction in receiver bandwidth the reduction in deviation was not a problem.

I keep wanting to try some very narrowband FM (i.e. less than +/- 0.5 kHz deviation) on HF just to see how well it works.  I have the Collins NBFM adapter in my 75A-3 and by using the crystal phasing the bandwidth of the receiver can be reduced in proportion to the transmitted bandwidth.  Now I am not certain as to if there is enough audio gain in the receiver, at least enough gain to produce a comfortable audio level in a speaker.  As for headphone operation I am pretty certain that there will definitely be enough audio gain.

It wouldn't be that much trouble to start with an old tube type lowband FM transmitter using the crystal oscillator and modulator and then use something like a Heath DX-20, DX-35, DX-40, or DX-60 to provide the Class "C" amplification.  Depending on the actual lowband frequency many of those transmitters used crystals within the 75 meter phone band.

Frankly, I just have to find the time to experiment.

Remember, NBFM, which used deviation of +/- 3 kHz was being "pushed" by the ARRL and others in the late 1940s and well into the 1950s.

Glen, K9STH


--- On Tue, 12/2/08, kd4e <doc at> wrote:

From: kd4e <doc at>

I recall discussing this back when I was contemplating the restoration of a Sonar HF exciter and it was determined that the legal restriction made it functionally impractical to run FM below 10M, despite the technicality that it was theoretically possible - not sure if it was that the modulation would be too pinched or if it was that FM was insufficiently efficient to make sense there.  Memories fails other than that I was persuaded that it would be a Quioxtic undertaking.


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