FM on hf

Nigel Holmes Holmes.Nigel at ABC.NET.AU
Thu Dec 4 00:05:48 EST 2008

 A good discriminator detector preceded by some hard limiting i.f.
amplifiers will ignore am components such as ssb, random noise etc.

Both wide and narrow FM have been used in hf comms. 

The Larkspur series of front line comms equipment used by British
Commonwealth countries through mid-'50's to '70's was very efficient.
The C42 & C45 transceivers used approx 100 kHz deviation in the range
25-60 MHz with 0.5-15 watts. Other transceivers (C13?) used narrower
(3-10 kHz) deviation in the 2-20 MHz range. The communication efficiency
often bettered ssb. 

Slope detection is the easy way for an AM receiver to listen to nbfm,
but loses the considerable advantage of rejecting amplitude

73 Nigel VK3DZ

-----Original Message-----
From: Boat Anchor Owners and Collectors List
[mailto:BOATANCHORS at LISTS.TEMPE.GOV] On Behalf Of Glen Zook
Sent: Thursday, 4 December 2008 2:21 AM
Subject: Re: [BOATANCHORS-TEMPE] GB> 75th Anniversary of FM Radio
Special Event

If the receiving bandpass filter is good enough (sharp skirts, etc.)
then the capture effect would not be a problem.  Remember that the
amplitude of the SSB signal is varying all over the place whereas FM is
a constant carrier.  Slope detection has been around since FM has been
recognized but, of course, is no where near as good as a Foster-Sealy
discriminator.  Also, a product detector can be used for FM reception.  

In my opinion a Foster-Sealy is considerably better than most, if not
all, of the "modern" techniques used for FM detection (i.e. ratio
detector).  However, they definitely are considerably more expensive to
manufacture these days and therefore are not generally used in "modern"

One of the benefits of an SSB signal is the reduced bandwidth and by
"pulling in the skirts" by considerably reducing the deviation of the FM
signal and using sharper filters in the receiver this effective increase
in effectiveness should benefit the super narrow FM signal.  Now
experimenting with super narrow band FM has not been a priority because
of the widespread use of SSB.  But, I believe that such experimenting
would provide some useful data.  My primary background has been in the
FM realm (I owned the Motorola reconditioned equipment center for the
south-central U.S. from 1970 until Motorola went out of that end of the
business in 1979 as well as having been the first FM Editor of CQ
Magazine starting in January 1971 and ending in September 1973) and I
have always been interested in super narrow band FM.

Again, I know that there are a number of persons who believe that
reducing the modulation index beyond a certain point would not work.
However, I think that experimenting with reducing the deviation of the
FM (or PM) signal might prove them wrong.  In any case, it should be an
interesting experiment.

Remember back in the days of modulated oscillator transmitters the
question of are you running AM or FM was best answered with yes!

Glen, K9STH
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