Physics

How the Radio Spectrum Works



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The radio spectrum is an integral part of every life on Earth.  Every time a person makes or receives a telephone call using a cellular device, that person uses a small segment of the radio spectrum.  Using a microwave oven, remotely changing the channel on a television receiver, and listening to music are some of the ways in which people use the radio spectrum to accomplish work.


Radio, a small part of the whole


Radio is a function of the production of magnetic fields by moving electrical currents.  What is called the “radio spectrum” is one tiny part of a much larger electromagnetic spectrum, whose parts cover the gamut from radio waves at the “low” end to gamma rays at the very energetic “high” end. 


Despite the different names they carry, electromagnetic waves all share three characteristics:  frequency, wavelength, and energy.  Frequency is familiar to everyone who listens to commercial or public radio; along with the stations’ call signs, their frequencies are announced every half-hour or so.  This is what you hear:


Broadcast radio


“Thank you for listening today.  This is WCBU-FM, Peoria, Illinois, at 89.9 on your radio dial.”


The “89.9” is a part of the frequency measurement mandated by the Federal Communications Commission for all radio and television broadcasters in the United States.  The full technical specification would be “89.9 Megahertz (MHz),” and this places your car radio in the lower end of the radio spectrum for radio receivers.


If you wish to move even lower, you probably can use that same radio to switch to a local “AM” station.  This means that the car radio’s amplitude modulation receiver will be placed into service, and you will now be treated to talk radio, the farm reports, and local preachers.  For commercial and public radio service, FM (frequency modulation) and AM use assigned frequencies of about 88-108 MHz and 500-1600 kilohertz (kHz), respectively.  In simpler days, nearer the inception of popular use of radios, “one megahertz” was defined as a frequency of one million cycles per second, and “one kilohertz,” one thousand cycles per second.


How big is it?


The radio spectrum, since people use it so frequently, tends to be the part of the electromagnetic spectrum with which they are most familiar.  For comparison of the radio segments with the entirety of the spectrum, imagine the wavelength of your local AM radio stations as a bit longer than a football field.  Cellular telephone signals are much shorter, measuring only a few inches at more than 1,000 MHz.  On that scale, gamma rays, at the extreme upper end of the electromagnetic spectrum, are smaller than the nucleus of an atom.  As a very broad approximation, the lower limits of the wavelengths of the different pieces of the spectrum are:


100 meters for the radio spectrum; 0.1 meters for microwaves; 0.0001 meters for infrared; 0.000001 meters for visible light; 0.00000001 meters for ultraviolet; 0.0000000001 meters for x-rays; and 0.000000000001 meters for gamma rays.


Why use different radio frequencies?


Many people have never given this a thought, despite the fact that frequency choices affect their lives every day. Due to the wavelength principle of radio design, extremely high frequencies allow cellular phones to disappear into tiny pockets, and very low frequencies require huge antenna structures at AM broadcast facilities.  In short, these frequencies are chosen for utility:  How well will this signal cover the area to which the broadcast or phone call is targeted?


An example that is very close to home for many is the Amateur Radio Service (ARS).  Radio enthusiasts who are active members of this service have access to many different frequencies, some for local communications, others for experimentation, and still others for regular conversations with distant radio amateurs.  The properties of radio at different frequencies change markedly.


The lowest amateur radio band includes signals from 1800 to 2000 kHz, and radio waves at these frequencies act very differently in the daytime from their nighttime character.  An operator may use 1900 kHz to call a friend twenty-five miles distant at 10:00 in the morning.  That same call may be made over a distance of 2,000 miles at 10:00 in the evening.  These radio waves will bounce off higher layers of the atmosphere at night, and that advantage is not available in the morning.


Another operator, wishing to maintain close contact with a family member in France, may choose to call using 14.3 MHz (ARS 20-meter band), and he has a better chance at 3:00 in the afternoon than at midnight.  Different frequency bands display characteristics useful for various purposes, depending on time of day and intended target area of the signal.


Radio is your friend


As with any technological advance, radio spectrum use has been driven by what became popular along the way.  People were angry when FM radio first came out; they believed someone was attempting to shove aside their old favorite programs.  In other areas, such as the use of cellular phones, people have demonstrated that they need to mature a bit to determine when such use constitutes a public danger.  Still, the technology is moving forward.  Satellite radio and TV are two of the radio spectrum’s uses that are driving further innovation.  People have not seen the end of what can be done with radio.

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