GB> 75th Anniversary of FM Radio Special Event

Perry Ballinger w8au at SSSNET.COM
Wed Dec 3 01:12:47 EST 2008

At 11:53 PM 12/2/2008, kd4e wrote:
>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. I was persuaded that it would be a 
>Quioxtic undertaking.

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."

Perry  w8au   

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