The Big Hertz
When I was young, there always seemed
to be a radio playing. In those pre-television days it was the mass medium, and my
grandmother in particular was a devoted fan. I was fascinated from the earliest age I can
remember, and I did what I could to figure out how it worked.
In the city I had some vague idea that the wire that was
plugged into the wall brought the sounds, perhaps the way you could yell into a garden
hose and have your voice come out the other end. What mystified me, however, was that she
could listen to the same programs at the summer cottage, which was on an island.
The radio there was operated by massive batteries that sat
in a wooden box under the radio. I entertained the notion briefly that there were little
people in the box, but even as a toddler I knew that was nonsense.
Eventually, an uncle explained that the radio signals
traveled invisibly through the air, and were caught by a long wire antenna strung under
the rafters of the cottage. That explained how it could reach the island, but not much
more.
It was many years before I began to grasp the fundamentals
of radio, and a brief thumbnail survey of friends and relatives suggests that, even though
radio in some form or other affects practically everything we do, not many of us know how
it works. Even non-technical people might understand what's going on under the hood of a
car, in a vague sort or way, or how a refrigerator keeps the milk from spoiling, but radio
remains a black art.
It is a hard concept to grasp. Alternating-current (AC)
electricity has some unusual properties. The voltage fluctuates between a maximum amount
in the positive direction, through zero to a maximum in the negative direction, then back
to the positive maximum, and so on. The magnetic field that surrounds any wire with
current flowing through it fluctuates in step.
At low frequencies the effect of this varying magnetic
field is called inductance, and can drive electric motors, transformers, and the like. At
higher frequencies, if a signal is fed to an appropriate antenna, it radiates outward over
very long distances.
Much of the early work on this phenomenon was done by
Heinrich Hertz, whose name is used as the unit of frequency: the number of complete
positive-negative cycles over a period of time. One hertz (abbreviated Hz) is one cycle
per second, one kilohertz (kHz) a thousand cycles, one megahertz (MHz) a million, one
gigahertz (GHz) a billion, and so on. (Note the quirky use of capital letters: when a unit
is spelled out, it is all small letters; in the abbreviation, Hz always takes a capital H
because it is derived from a name; k for kilo is always small while M and G for mega and
giga are always capitals.)
In general, the process is called electromagnetic
radiation, and the waves created by it travel away from the antenna at the speed of light:
roughly 300,000 kilometers (182,000 miles) per second. A suitable combination of antenna
and tuned electronic circuit can detect these waves at a distance.
In the early days of radio, that's all a receiver could do.
But the ability to sense the alternating presence or absence of a signal was adequate for
the transmission of Morse code, and that sort of communication was adopted, especially at
sea, almost the moment it was invented.
But right from the start a way was sought to transmit more
complex information. The first efforts were to broadcast sound, and Canadian Reginald
Fessenden is considered to have done it first. Instead of switching the radio signal on
and off, an audio signal is used to vary its level, which is a process called modulation.
The receiver first locks onto the basic radio signal -- called the carrier -- and turns
its variations in amplitude into sound. That basic form of radio is called amplitude
modulation, or AM. In reality, any kind of information can be used to modulate a carrier.
What makes radio so useful is that a vast number of
different signals can be transmitted simultaneously from a number of sources. As long as
their carrier frequencies are different, or they are located far enough apart, there will
be minimal interference. The air is teeming with these radio signals, and all of them are
affecting a given antenna at the same time; it's the radio receiver's job to lock onto one
carrier and reject all the others.
To do this it uses a tuned circuit that is sensitive to one
frequency and insensitive to others. An analogy is blowing across the top of a bottle: The
noise your breath makes contains numerous frequencies -- it's essentially a form of white
noise -- but the volume of the bottle makes it resonate at only one frequency, and a sound
wave builds up at that note. If you put a little water in the bottle, changing its volume,
the resonant frequency will change. The ability of a receiver to emulate this is why it is
called a tuner. Usually there's a variable capacitor that allows the operator to change
resonant frequencies to pick up a number of different stations.
There is a wide range of frequencies that can be used for
radio communication; taken together, they're called the radio spectrum, which ranges in
frequency from a few kilohertz to something like 300 gigahertz. Governments and
international agreements closely regulate what services go where. The spectrum is divided
up into "bands," some broadly based on their frequencies, others by their use.
Where possible, regulators like to group stations within a service on a continuous chunk
of the spectrum.
To some extent, a service's position is determined by its
function. At the lower frequencies, for instance, the bandwidth of the material that can
be modulated is decidedly limited, so simpler low-quality signals tend to be there. As
frequency rises, so does information-carrying capacity, and so does directionality.
Another characteristic comes into play here. You can
describe a carrier by its frequency, as we have done so far, or by its wavelength: the
distance from the peak of one wave to the peak of the next as it radiates away from the
antenna at the speed of light. The lowest frequencies in the radio spectrum have
wavelengths measured in dozens of kilometers, while the highest are in millimeters.
Reception efficiency is directly affected by the
relationship between an antenna's length and the wavelength of the signal it's trying to
pick up. Portable devices, therefore, must use high frequencies so they can have small
antennas and still be efficient. The lower frequencies can be reserved for those services
where it's practical to use extra power to overcome the impracticality of making antennas
kilometers long.
The spectrum is divided into eight frequency ranges: VLF
(very low frequency), LF (low frequency), MF (medium frequency), HF (high frequency), VHF
(very high frequency), UHF (ultra high frequency), SHF (super high frequency), and EFH
(extremely high frequency).
The bottom two, VLF and LF, are mostly used for navigation,
although there is some long-wave broadcasting in Europe. About half of the MF range is
taken up by the familiar AM radio band, which stretches from about 0.5MHz to 1.6MHz. Above
that is the marine band, which extends to the lower part of the HF range.
Above the marine band is shortwave radio, which is used for
international broadcasting. Amateur radio is present there (and elsewhere in the spectrum
as well). Up to this point, sound broadcasting is amplitude modulation, although there's
lots of other material there, such as radio teletype, various forms of Morse code, and
single sideband code and voice. Above this frequency range, audio -- analog audio anyway
-- tends to be frequency modulated.
The lower part of the VHF band is used for various sorts of
point-to-point communication. Then comes the lower part of the television VHF band,
channels 2 to 6 (in North America), followed by the FM radio band (88 to 108MHz) and the
air-control band. Another two-way communications band follows, and then television
channels 7 to 13.
The UHF band contains television channels 14 to 69,
followed by cellular phones and various 900MHz consumer devices such as cordless phones
and garage-door openers. The upper part of the band is used for 2.4GHz consumer-electronic
devices, global-positioning systems, digital radio (except in the U.S.), remote television
relays, and so forth.
The SHF range is used in part for terrestrial microwave
links, which ferry telephone and other data from point to point on the ground with
enormous capacity. Three different sections are used for satellite communications: C-band
(those older, large dishes) uses 6/4GHz (the first number refers to the frequency at which
signals are sent to the satellite, and the second the downward frequency. They're
different so the satellite's transmitter won't interfere with its receiver); Ku band
(pizza-sized digital dishes) is at 14/11GHz, and the developing Ka band is at 30/20GHz.
The uppermost range in the radio spectrum, EHF, is used for
radar and for radio astronomy, both of which require the extreme directivity of such
frequencies.
The portion of the electromagnetic spectrum that we use --
the radio spectrum -- while extensive and crowded is only a relatively small part of the
total. The top frequency is about 300 billion hertz, but moving up the spectrum to about
10 trillion hertz, we hit infrared light, and then visible light itself at 1 quadrillion
hertz -- those are really radios in your eyeballs -- then ultraviolet. Above that are
x-rays, and then at the very top are gamma rays.
The whole spectrum is immense. The Encyclopedia
Britannica's description of it says "Going from the [frequency] values of radio
waves to those of visible light is like comparing the thickness of this page with the
distance of the Earth from the Sun, which represents an increase by a factor of a million
billion. Similarly, going from the values of visible light to the very much larger ones of
gamma rays represents another increase in frequency by a factor of a million
billion."
My grandmother, listening to her battery radio at the
cottage, had little idea she was participating in such a big deal.
Glossary
Radio, and electromagnetic radiation in general, shares a
vocabulary with other fields. This brief summary points up the specific uses in this
context.
Band: A group of adjacent frequencies within the
electromagnetic spectrum usually devoted to a single purpose, such as FM radio or cellular
phones.
Bandwidth: The maximum amount of information,
expressed in Hz, that a carrier of a certain frequency can accommodate. A broadcast analog
television channel, for instance, has a bandwidth of 6MHz.
Carrier: A high-frequency radio wave that radiates
outward from a broadcast antenna. Information is modulated on it.
Directivity: The tendency of radio waves to become
more directional as frequency rises. Lower frequencies, which are more or less
omnidirectional, are used for commercial radio, as they reach numerous receivers. Higher
frequencies are useful for point-to-point communication, where the signal can be aimed at
a single receiver, or a small group.
Electromagnetic radiation: The ability of
high-frequency signals to radiate outward from a suitable antenna, to be detected at
considerable distances. Behaves like light, which is one form of it.
Electromagnetic spectrum: The complete range of
frequencies over which the electromagnetic radiation phenomenon occurs. At the low end,
frequencies overlap the audio spectrum (which is not electromagnetic), reaching up to
gamma rays at a frequency of about 10 quadrillion quadrillion hertz.
Frequency: The number of complete cycles of a radio
(or other) wave in a given period of time. Unit is hertz (Hz), which represents one cycle
per second.
Modulation: The use of one signal, such as audio, to
control a radio carrier. A receiver locks onto the carrier and uses the variations in it
to recreate the original signal.
Propagation: Generally, the movement of waves
through a medium. Usually includes information as to strength and radiation pattern.
Radio: The lower end of the electromagnetic
spectrum, which is used for human communication. Ranges from low-frequency marine systems
to radio telescopes; a frequency ratio of about 100 million to 1.
Wavelength: The distance an electromagnetic wave
travels in completing one cycle, radiating outward at the speed of light. A 300kHz signal,
for example, has a wavelength of approximately 1 kilometer.
...Ian G. Masters
ian@mastersonaudio.com
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