The radio-frequency spectrum is part of the larger electromagnetic spectrum. They both represent a range or a span of radiated energies. Many things cause radiations: an x-ray machine, a microwave oven, a light bulb, a transmitting radio. The energies radio transmitters produce start above the audible frequency band, things we can hear, and end just below the visible light spectrum, things we can see. In between these two points lies the radio-frequency spectrum.
We measure radio emissions by cycles per second or hertz. Imagine a rock thrown into a pond. Waves radiate from the point of impact. Similarly, audible waves radiate from us as we speak, as air is compressed or displaced by sound. Human speech lies principally between 100 and 250 hertz. Our hearing range extends to roughly 20,000 hertz. Musically speaking, middle “C” on the piano is at 261 hertz.
Radio transmissions are high above all of this, in the hundreds of thousands and millions of cycles per second. Some examples are an A.M. radio station signal at 560,000 hertz, an F.M. station broadcasting at 90,300,000 hertz, and a cellular telephone transmitting at 890,450,000 hertz. In actual practice, we shorten these long numbers by using (and then abbreviating) the words kilohertz and megahertz. A kilohertz is a thousand cycles per second, a megahertz a million. The three figures noted would thus be 560 KHz, 90.3 MHz, and 890.450 MHz respectively. These numbers are known as frequencies. And nearly every frequency in the radio spectrum is assigned a use.
In the United States the Federal Communications Commission assigns and administers non-Federal radio spectrum. Cellular radio, satellite, amateur radio, land mobile (two-way business radio), and broadcasting, are some of the scores of services used by tens of millions of people to do business and to communicate. FCC radio-frequency allocations also license spectrum for emergency services. With users increasing but radio spectrum remaining fixed, there is a tremendous need to use bandwidth efficiently. More later . . .
