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(Modulation Index = Deviation divided by Modulation Frequency.) That’s one of the reasons that ham radio still uses it. This high modulation index improves the Signal-to-Noise (SNR) for voice signals. This type of normal FM works the best because the 5 KHz deviation has a modulation index of 1.7. 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. Today we just call it normal FM, plain old FM, or regular FM. Channel frequencies could be more closely spaced together. But, this required new radios. 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). In 1962, the problem was (and still is) that there was only so much radio spectrum of two-way radio VHF-UHF bands available. So, the number of users of two-way radio systems increased. The demand for more radios required more channels to handle it. Normal FM in Two-Way RadioĪs technology moved forward with transistors, electronics became less expensive. Wideband Narrowband FM Radio Communications. 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. We still see some of the older military surplus radios in use with WFM, such as the PRC-77. But, the plain old WFM mode worked fine with unstable radios when the frequency drifted around. Frequency stability was also fairly sloppy in the old radio sets. Back in those days, it was expensive to manufacture a radio with FM deviation of +/- 5 kHz (plus and minus five kilohertz).
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The old wide FM equipment (mostly vacuum tube gear) was somewhat less refined compared to today’s high technology communication systems. But, it was just called plain old “FM” before 1962. 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). Prior to the 1962, two-way radio systems used Frequency Modulation mainly on VHF and some UHF channels. 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. FM works by deviating that frequency when you talk into the microphone.
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The display frequency that shows on your radio is really just the center frequency or average frequency of the FM frequency channel. 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. 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. The original source of this article is RadioMaster Reports. In an emergency, you may need to communicate with others by radio… In this RadioMaster Reports article, we unravel the secrets of the various FM modes and explain how… FM versus NFM for Best Radio Communications? 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.