GB> 75th Anniversary of FM Radio Special Event

Glen Zook gzook at YAHOO.COM
Wed Dec 3 10:20:58 EST 2008

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" equipment.

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


--- On Wed, 12/3/08, Perry Ballinger <w8au at SSSNET.COM> wrote:

> From: Perry Ballinger <w8au at SSSNET.COM>

FM today would not make it on our "amplitude" HF bands where SSB is predominant.  The "capture effect" would favor the SSB signals.
However, FM can be used in an amplitude fashion if one wants to talk to AMers.  NBFM modulation of "first order" sidebands must be kept to the same bandwidth as a communications grade AM signal, or +-3 kHz to be legal. You have to park your carrier about 2.7 kHz off the AM frequency so that one of your FM sidebands rides up and down the AMer's receiver selectivity slope, thereby converting it to AM.
I did this a number of years ago on 75 meters (with a Hallicrafters HT-19) to see if anyone would notice any difference.  Most did not, but a few thought my "AM" sounded a little different. (depending on the characteristics of their specific receiver passband).  When I told them what I was doing, they were amused by it.
The old name for this technique was "slope detection."


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