Sunday, July 16, 2017

Narrow Band FM in Two-Way Radio

Good examples of Narrow NFM are FRS radios, PMR radios, or a modern business-band UHF radios. Narrowband NFM only has a modulation index of .8 which is about half the modulation index of normal FM. You can fit twice as many channels per megahertz (approximate bandwidth 11 kHz with channel spacing 12.5 kHz).

  • NFM may have better spectrum efficiency than FM, but NFM just doesn’t go as far.
The 6 decibels reduction in NFM deviation and modulation index is sadly a 6 dB decrease in signal-to-noise ratio (SNR), and this unfortunately means that NFM causes a loss of about 30% of your distance coverage versus regular FM. Of course Narrowband FM works fine when you are nearby, when you have strong signals (full quieting), but when you get in the fringe zone at far distances, Narrowband NFM goes to static quite rapidly.
  • You would need to double or triple your NFM transmitter Watts of power to equal a regular FM transmitter.
  • That means that those who are interested in emergency, disaster, or SHTF simplex radio communications should stick with regular FM instead of NFM whenever possible. Switching from NFM to FM will boost your range by 30%.

The Future of FM

This recent change to NFM will probably be the last of the FM series. In the future, modulation will be all digital. With digital, the spacing between channels can be even more narrow, and the distance performance can be improved.
Proof positive that installing a digital capable vhf/uhf repeater such as Yaesu's Fusion repeater is an act of a true visionary!

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