Voice Communication in Business Volume 2
Essays on telecommunications, 1981-2002

I prepared this, along with a diagram, as a sidebar for Selling off the Spectrum. As it appeared in Business Communications Review for September, 1991, the diagram looked a lot better than my sketch.

Survey Of The Spectrum
(1991)

Just what is the radio spectrum that the lawyers want to auction off? The whole thing is spread out before us in the attached diagram, although no effort has been made to show all the different radio services which are presently in use. To clarify the diagram, some words of explanation may be helpful.

Electromagnetic signals are usually identified by their frequency, measured in cycles per second, or Hertz, abbreviated Hz. One cycle starts with a value of zero, increases to a maximum, falls back to zero, goes to a negative maximum and returns again to zero, in the form of a (hopefully) familiar sine wave. Prefixes are used with Hz to simplify the description of higher frequencies:

There is another way to identify an electromagnetic signal: by its wavelength. A wavelength is the distance a radio signal can go at the speed of light (which is a form of radio signal) during the time required for one cycle. The speed of light is usually taken as 300 million meters per second. To find the wavelength (in meters), we simply divide speed of light by frequency. For example, the middle of the AM broadcast band is 1 MHz. If we divide 300 million meters per second by 1 million cycles per second, we come out with 300 meters (per cycle). That is, the wavelength, the distance the signal can go in the period of one cycle, is 300 meters or about 1000 feet. AM broadcast antennas are usually one quarter of a wave length (for reasons which need not concern us here). Thus the radio towers just outside of town, about 250 feet high, are almost certainly for a local AM radio station.

As an interesting corollary, if we have wavelength in meters, we can also divide it into the speed of light to get frequency: the wavelength of an FM signal is about 3 meters, so 300 million meters per second divided by 3 meters per cycle comes out 100 Megacycles per second, or MHz. And yes, the FM band goes from 88 to 108 Mhz. Obviously, the speed of light is equal to the frequency in cycles per second multiplied by the wavelength, meters traveled per cycle. As frequency increases, wavelength decreases so that the two multiplied together remain constant at 300 million meters per second. Neat, huh?

In our diagram, Figure 1, I have used m for meter, so that it won't be confused with M for Mega. A kilometer is km, while a centimeter is cm and a millimeter is mm. To deal with the shorter wavelengths we need as we approach light frequencies, the term micron is used. A million microns equal one meter, or, to put it another way, one thousand microns equal one mm. Having used both m and M already, I have just called a micron a micron on the diagram. For even shorter wavelengths, the Angstrom Unit is used. One micron is 10,000 Angstrom Units. The proper abbreviation is A with a small o on the point. My computer won't do that, so I just used AU.

One more point. To get the whole spectrum on one page, I have used what is called a logarithmic scale. Instead of going 1, 2, 3, 4, 5, as in a linear scale, it goes 1, 10, 100, 1000, and so on. That is, each division, or decade, is ten times as big as the one below it. There are ten times as many frequencies between 100 Hz and 1000 Hz as there are between 10 and 100 Hz. Log scales are particularly useful for handling information that deals with ratios. For instance, a 10% increase is always the same size on a logarithmic scale, while on a linear scale it gets bigger and bigger. But log scales have another interesting advantage: the distance half way between 1 and 10 turns out to be the square root of 10 (the geometric mean) or about 3.18. Thus for all practical purposes, the middle of each decade, bounded by 1 and 10, is 3. This turns out to be very handy when we want to show wavelength and frequency on the same scale: we can alternate decade boundaries for wave-length and frequency, each one splitting a decade of the other.

Now we are in a position to run once over the spectrum lightly. It looks like we have a lot of frequencies available, and we do. The only trouble is that we don't know how to use the higher ones, and most of the ones we do know how to use are already occupied. In addition, more and more uses for radio are being found all the time. To add to the fun, new services tend to take up more bandwidth than old ones; an FM channel takes about 20 times the bandwidth of an AM channel, and TV takes 30 times the bandwidth of FM. Although the same channels can be reused in different parts of the country, care must be taken in assigning frequency, power, and often the height of the antenna to prevent two stations on the same frequency from interfering. Services like CB and amateur radio depend on good manners and the ability to change frequencies within a band to effect sharing, while taxi wars have erupted when competing companies have had to take turns using the same dispatch channels.

At the bottom of the column in Figure 1, starting at 10 Hz, we find the audio band. Humans can hear frequencies from about 20 Hz to 16 KHz, but we shouldn't confuse the audio signals we hear with electromagnetic signals until after Mr. Bell's microphone has converted acoustic energy into electricity. If we wanted to find the wavelength of sound waves in air, however, we could easily do so by dividing the velocity of sound (about 1100 feet or 335 meters per second) by frequency.

We note that the A on which orchestras tune up is at 440 Hz, and high C which sopranos strain to hit is very close to the standard audio test frequency used in telecommunications. The wavelength of electrical signals in the audio range on telephone lines comes as something of a shock: from 1000 to 100 Km, or 620 to 62 miles.

Between the top of the hi-fi range to the bottom of the AM band, radio signals are usually called very low frequency (VLF) and LF. This is the region where Marconi and other pioneers developed radio telegraphy, sometimes using steam driven generators similar to those used for electrical power. With their very long wave-lengths, these frequencies follow the contours of the earth, and even penetrate below the surface of the ocean, allowing them to reach submerged submarines. VLF and LF radio is extensively used for ship-to-shore and other long distance radio telegraph transmissions.

The AM broadcast band, from .535 to 1.65 MHz, is what most people mean when they say radio. In use since 1920, it is still going strong. Commercial AM radio stations are licensed for 250 watts to 50,000 watts output. In this band, the radio wave still follows the surface of the earth, and coverage during the day is related pretty much to power; high power goes farther than low power. At night, however, the Heaviside layers, layers of ions high in the atmosphere, shift to allow even low powered AM signals to bounce off them and return to earth at great distances.

The "short-wave" band, up to about 10 MHz, has just the right wavelengths (from 150 meters to 10 meters) to take full advantage of reflections off the Heaviside layers. As a result, much international voice communications take place here. It was initially believed that frequencies above the broadcast band were not suited for commercial use. Radio amateurs (hams) were permitted to use these ghetto frequencies, and they did much of the early exploring and development in the region "200 meters and down." With the coming of the second world war and the desperate need for people with knowledge of radio, the military turned in vain to the colleges. Electrical engineering had been considered primarily preparation for work in the electric power industry, with radio and electronics only taught in a few of the biggest schools, and then as graduate subjects. As a result, the military welcomed hams, taking advantage of their almost unique skills. After the war, engineering schools reluctantly allowed students to follow an "electronics" option, and within a few years, "power majors" became an endangered specie.

Above short wave, we find the VHF and UHF bands, used primarily for FM and TV broadcasting. Between the VHF and UHF TV bands, the region from 225 to 400 MHz has been reserved for government use. Apparently these frequencies (about 200 MHz in all) are the ones that NTIA and the FCC want to transfer to civilian use and auction off. One is reminded of Teapot Dome, a geological formation believed to hold oil, and reserved for the use of the Navy. During the Harding administration, it was auctioned off somewhat less publicly and caused a bit of a stir.

For frequencies above 30 MHz, radio waves go pretty much in straight lines, and punch through the Heaviside layers rather than bounce off them. This led to the belief that such frequencies would only be useful for short distances, because people over the horizon would be in the shadow of the earth. As a practical matter, an antenna located on a tall building or, better yet, a high mountain, finds its horizon considerably farther away than if it were located at sea level. As a result, most FM and TV stations today have a greater coverage area than most AM stations, unless you count the freak distances covered with bounces off the Heaviside layers.

For a time, 890 MHz was considered pretty much the top of the usable spectrum, but this time the radar and microwave people did the exploring, and the FCC held its "Above 890" hearings to try to figure how these high frequencies could best be used. They had already managed to get satellites and terrestrial microwave on some of the same frequencies in the 10 GHz range, and other problems could easily be predicted. Today, it seems likely that there will be far more people to exploit these frequencies than there are frequencies, even though there is more bandwidth between 890 MHz and the Ku Band than there is from 890 down to AM broadcasting.

One is pushing the state of the radio art in the millimeter wave region. There are two or three decades here that will take years to explore, and even longer to develop technology to use effectively. When we go a little higher, we come to infra-red, or heat, followed by the visible spectrum and then ultra-violet. The visible spectrum is less than an octave (highest frequency is less than twice the lowest), but this single octave gives us the whole revolution in fiber optics.

Note that the visible spectrum goes from red to blue, where blue, at a higher frequency, has more energy (is "hotter"). People normally think of blue as cold and red as hot, which leads to confusion from time to time. Above ultra-violet, we come to X-rays and gamma rays. Although these high energy waves have their uses, they also have obvious limitations, particularly if we are concerned with communication in its various forms.

This tour has ignored many if not most of the users of spectrum: hams, radio astronomy, navigation, etc. But at least it provides a road map so that any service of interest, given its frequency or wavelength, can be located. For further details, the reader is referred to "Reference Data for Radio Engineers" (the ITT handbook), published by Howard A. Sams & Co., and various books and magazines dealing with radio and TV engineering. 

[ Top ] [ Next ] [ Table of Contents ]

Copyright 2006 Lee Goeller. All Rights Reserved.