Halligan's Hallicrafters International

    Halligan's Hallicrafters International
maintained by Scott Neader, KA9FOX

From: WHITEHEADG@aol.com
Date: Sun, 9 Dec 2001 21:11:06 EST
Subject: Re: [AMRadio] Narrow Band FM
Message-ID: <149.6078bf5.294573ba@aol.com>

http://www.amfone.net - Home of the AM Radio List.

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It's refreshing to see some really technically sound tutorial info on the net 
regarding a topic as important to our hobby as modulation theory. 
Congratulations and thanks for taking the time to create this.

One issue that I think bears emphasis is the relative spectral bandwidth 
required for the various modulation modes. I suspect that many believe, 
especially those who operate primarily in the VHF domain, that narrowband FM 
takes less bandwidth that AM. Correct me if I'm wrong, but I recall that NBFM 
requires about 16khz of spectrum to get enough sideband energy to deliver 
reasonable audio power, where AM requires at least 6 to 8 kHz, and sideband 
about half of that (Let's encourage more use of traditional AM in the VHF 
bands. There's a reason the commercial air traffic control system still uses 
it!). Angle modulation (FM or PM), in fact, always generates an infinite 
number of sidebands, although, depending on modulation index, most are highly 
attenuated and insignificant for small mod indices. As you point out the 
Bessel function determines the relative amplitudes of carrier and the 
multiple sidebands as a function of mod index. I think there is a widespread 
misconception that peak deviation equals bandwidth which is not the case. The 
bandwidth of the significant sidebands of an angle modulated signal is 
determined both by the rate of frequency deviation, i.e., the modulating 
frequency, and by the peak deviation, the latter being determined by the 
amplitude of the modulating signal in a given phase or frequency deviation 
circuit. Commercial FM broadcast allows a much higher modulating frequency 
and much higher modulation index which results in a much wider transmission 
bandwidth (200 kHz as I recall) in order to achieve higher received audio 
frequency fidelity with a very good signal-to-noise ratio.

It's curious to me that the single most important parameter determining 
effectiveness of communication whether eyeball-to-eyeball or via radio or 
light, i.e., Signal-to-Noise Ratio, is hardly ever mentioned in our hobby. 
The nonlinear relationship of receiver output S/N Vs received signal S/N for 
angle modulation, which gives us "capture effect," compared to a linear 
relationship for the same in AM modes would be a topic of interest for future 

Thanks again and please feel free to comment, or correct.

George, W1BOF


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This page last updated 18 Jul 2001.