Survivalist radio communications: FM or NFM? Which FM mode is best for emergency use? What is NFM? What is NB? What is narrowband FM or wideband FM? FM goes further than NFM, but why? The answer to these questions are important for the comms prepper when programming a VHF/UHF walkie talkie for emergency communications. In this RadioMaster Reports article, we unravel the secrets of the various FM modes and explain how…
In an emergency, you may need to communicate with others by radio…
The original source of this article is RadioMaster Reports.
In this RadioMaster Reports article, we unravel the secrets of FM modes and explain how to know when to use NFM and FM. In an emergency, you may need to communicate with others who are using various types of radios, on many different channels. Are they using FRS, GMRS, HAM, MARINE, MURS, Business Band, PMR446, UHF CB, FreeNet, or KDR 444? Not familiar with all these acronyms? RadioMaster Reports explains what they are, who uses them, and what type of FM to use on each one. TL;DR: FM good. NFM not so good.
Background of Early WFM in Two-Way Radio
The display frequency that shows on your radio is really just the center frequency or average frequency of the FM frequency channel. FM works by deviating that frequency when you talk into the microphone. Basically the width of the spectrum or bandwidth is what the transmit signal needs to modulate (wiggle or deviate) above and below the display frequency when you talk. Prior to the 1962, two-way radio systems used Frequency Modulation mainly on VHF and some UHF channels. We now call the older type of transmission mode WFM or Wideband FM, and it was a frequency deviation of +/- 15 kHz (plus and minus fifteen kilohertz, with a bandwidth of about 36 kHz at channel spacing 50 KHz). But, it was just called plain old “FM” before 1962. The old wide FM equipment (mostly vacuum tube gear) was somewhat less refined compared to today’s high technology communication systems. Back in those days, it was expensive to manufacture a radio with FM deviation of +/- 5 kHz (plus and minus five kilohertz). Frequency stability was also fairly sloppy in the old radio sets. But, the plain old WFM mode worked fine with unstable radios when the frequency drifted around. We still see some of the older military surplus radios in use with WFM, such as the PRC-77. The old wide bandwidth WFM required the channels to be separated by at least 50 kHz, because the transmitters on those channels would interfere with each other and slop over to the adjacent channels if the channel frequencies were spaced any closer together.
Normal FM in Two-Way Radio
As technology moved forward with transistors, electronics became less expensive. So, the number of users of two-way radio systems increased. The demand for more radios required more channels to handle it. In 1962, the problem was (and still is) that there was only so much radio spectrum of two-way radio VHF-UHF bands available. It is a limited resource, there are only so many megahertz to go around! To solve this problem, brilliant engineers developed economical equipment that could work with a narrower FM deviation, +/- 5 kHz, and this could double or triple the capacity of the same finite megahertz of radio spectrum (bandwidth approximately 16 kHz with channel spacing 25 kHz). Channel frequencies could be more closely spaced together. But, this required new radios. In the 1970s, this was called narrow FM. Today we just call it normal FM, plain old FM, or regular FM. This mode of FM required better radios, and the quality of those radios also had the added advantage of better signals and going further than the older WFM equipment. This type of normal FM works the best because the 5 KHz deviation has a modulation index of 1.7. This high modulation index improves the Signal-to-Noise (SNR) for voice signals. (Modulation Index = Deviation divided by Modulation Frequency.) That’s one of the reasons that ham radio still uses it. Ham radio operators still use normal FM mainly because they were not required by government regulators to change to narrowband, and regular FM continues to work well for them. MURS radios can still use regular FM only on MURS channel 4 and MURS channel 5. GMRS can use normal FM, but most of the so-called GMRS bubble pack radios are now using NFM.
But, don’t let your eyes glaze over yet, because the real point of this article is coming up.
FM Bandwidth Channel Spacing Deviation Table
||Years of Use
||Channel Spacing kHz
Note: the above table applies mainly to public safety radio and business radio. Ham radio continues to use regular FM +/-5 kHz deviation.
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.
The original source of this article is RadioMaster Reports.
Disclaimer: Content provided in RadioMaster Reports is included for the sole purpose of educational information on a passive basis. This information may be useful to the public in the event of emergencies or disaster recovery, especially when normal techniques are not an available option. Users of this educational information are solely responsible for their actions.
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