| MadSci Network: Physics |
Greetings, Amy:
The primary waves associated with television are known as
"electromagnetic waves". These waves have many names:
radio waves, short-waves, microwaves, infrared radiation,
visible light, ultraviolet light, X-rays, gamma rays, and
so on. All of them travel at the speed of light, since all
of them are merely "varieties" of ordinary light.
Here are two crude sketches of two waves:
. .
. . , note shorter wavelength
. .
. .
. . , note longer wavelength
. .
Whenever two electromagnetic waves can be called different
from each other, a simple way to recognize that difference
is to note their wavelengths. It is possible for one
wavelength to be many kilometers long! --And it is possible
for another wavelength to be smaller than the diameter of
an atomic nucleus.
A second and more common way to distinguish two different
electromagnetic waves involves measuring them in terms of
time. Recall that all these waves move at the speed of
light: that is about 300,000 kilometers per second. If we
had an electromagnetic wave that was 1000 kilometers long,
and if we measured the time it took to pass by, that would
turn out to be 1/300 of a second! In other words, one
second is enough time for 300 waves, each 1000 kilometers
long, to pass by, one after another. Or we can say that
one 1000-kilometer electromagnetic wave could oscillate 300
times in one second. Normally, each full oscillation of a
wave is called a 'cycle'; we can call a 1000-kilometer wave
a 300-cycle-per-second wave. And of course a different
wavelength would be associated with a different number of
cycles-per-second, or 'frequency'.
Take another look at the two waves sketched above: they are
each only a few centimeters long. BILLIONS of these waves
could pass you by, one after another at the speed of light,
in a single second. Their frequencies can therefore be
expressed in terms of "gigacycles per second".
Electromagnetic waves are generally associated with
electrically charged particles. These particles are found
wherever you find static electricty, for example. If you
scuff your feet on a carpet in a cold dry place, you can
often build up a shocking amount of static-electric charge.
The tiny spark that occurs when you surprise a friend is
harmless, startling, and ALWAYS generates electromagnetic
waves. Some of those waves are the visible light of the
spark; others are radio waves, microwaves, and infrared.
When you turn on an ordinary A.M. radio, the crackling you
hear is caused by electromagnetic waves generated by
lightning bolts -- Nature's giant and far-from-harmless
static-electric sparks.
The ordinary and understandable sounds coming from a radio,
and of course the stuff you encounter on television, get
there via radio waves and microwaves, respectively. Again,
electrically charged particles generate these waves, but
here there is dynamic electricity at work, not static.
Please examine this sketch:
| | magnetic pole
|____N____|
wire is part of
wire ----------------------------- a large
_________ loop (not shown)
| S |
| | magnetic pole
A section of a loop of wire is shown; lets pretend that the
rest of the loop comes out of the screen and connects
behind your back. Also, pretend that you are holding the
portion of wire shown, near yourself. Next, pretend that
you can move the wire, still holding it as shown, all the
way through the gap between the magnetic poles. Due to the
laws of electromagnetism, this will cause some electrons to
start moving (they ACCELERATE for a moment) inside the wire
loop, in a particular direction. If you now imagine
yourself pulling the section of wire back towards yourself,
through the gap, then this would cause electrons to move/
accelerate the other way around the loop. Since electrons
are electrically charged particles, each time you move the
wire through the gap, BECAUSE you are causing electrons to
accelerate, you would be generating some electromagnetic
waves. If you could do this steadily, through-and-back
once per second, then you would naturally be generating
waves at a frequency of one cycle per second. They would
have wavelengths of 300,000 kilometers!
We generally use special electronic circuits to accelerate
electrons back-and-forth, thereby producing electromagnetic
waves. Depending on the circuit, we may generate waves of
a few cycles per second (the Navy does this, because very
low-frequency waves can travel through water to reach
submarines) -- or we may generate waves having frequencies
of many gigacycles per second. Almost every frequency in
that range is used by somebody, somewhere, for some sort of
communications. Even higher frequencies are also in use:
infrared waves have frequencies of hundreds of gigacycles
per second, and are used in lots of remote-control units
(communicating with devices like TVs, VCRs, etcetera).
Visible light waves oscillate many trillions of times per
second, and a major new way to communicate is to send laser
beams through many miles of optical fiber.
It should be obvious that to generate electromagnetic waves
is to do only about one-fourth of the process of using them
to communicate. Some sort of data must be associated with
the waves; then they must be detected and, finally, the
data must be extracted.
The simplest kind of electromagnetic communication is to
merely generate and then not-generate waves. Long and
short bursts of waves were used for years to represent
information in the classic form of Morse Code. Gathering
the information happens automatically, as the waves are
detected....
In the middle 1800's a physicist named James Clerk Maxwell
discovered the basic mathematical equations describing
electricity, magnetism, and electromagnetic waves. For one
thing, he computed that all these waves move at the speed
of light -- and therefore it seemed reasonable that light
itself was an electromagnetic wave. But he had no way to
prove this. He wasn't even sure how to go about proving
that any sort of electromagnetic waves actually existed.
Later that century, another physicist named Heinrich Hertz
figured out a way. He set up a coil of wire with a small
gap in it, and forced electricity to spark across the gap.
Nearby he placed another coil, with another gap in it.
When a strong spark jumped the gap in the first coil, Hertz
expected that invisible electromagnetic waves would cross
the distance to the second coil, and some of them would be
absorbed. The absorption process is simply backwards to
the generation process: If causing electrons to accelerate
produces electromagnetic waves, then when electrons absorb
electromagnetic waves, they must become accelerated! And
since accelerated elctrons are MOVING electrons, they must
constitute a flowing electric current! Hertz expected to
detect this current by watching it jump the gap in the
second coil. It worked! Today we honor this physicist by
having formally named "one cycle per second" "one Hertz".
It is certainly easier to say "five hundred megahertz" than
"five hundred megacycles per second", and considering how
valuable we find the consequences of his initial detection
of electromagnetic waves, the honor has been well earned.
An antenna is a device that is specially designed to either
emit or absorb electromagnetic waves efficiently. The more
efficiently we can produce or detect these waves, the less
power we need, to either send a signal from the trasmitter,
or to extract useful information at the receiver. The
coils that Hertz used were rather inefficient. In theory,
the most efficient antenna is a simple straight wire that
is exactly as long as the wavelength of the electromagnetic
wave that you want to deal with. This would obviously be
impractical for transmitting/receiving 100-kilometer waves!
Also, we often want a single antenna (especially for a
receiver unit) to handle a whole range of wavelengths as
efficiently as possible. So even today, after more than a
century of advances, an improved antenna design still makes
its debut every so often. (The latest improved antenna
designs use special geometric shapes known as fractals.)
Any primary electromagnetic wave that carries information
is known as a "carrier wave". A great many carrier waves
of different frequencies are being generated by radio and
TV stations; electronic circuits known as "filters" allow a
receiver of electromagnetic waves to select the particular
frequency that the user wants. If you tune your A.M radio
to 800 kilohertz, then you are setting it to filter out all
except a carrier wave of that frequency. "A.M." stands for
"Amplitude Modulation", and this describes the way data is
added to the carrier wave. Amplitude modulation is really
just a more refined version of turning the carrier wave on
and off, as described earlier for Morse Code. Because the
receiver has an easier time detecting the carrier wave if
that wave is transmitted continually, the information is
transmitted in the form of a sequence of higher-power and
lower-power waves:
. .
. . , note lower amplitude (power level)
. .
note same wavelengths (carrier wave)
. .
. .
. . , note higher amplitude (power level)
. .
. .
The rate at which the power level of the carrier wave
changes is very rapid, thousands of times per second. The
overall shape of each wave in the carrier wave is very
smoothly wavelike, but the wave preceding or following it
may have rather greater or lesser amplitude. Each change
in the power of the carrier corresponds to a small piece of
the overall message being transmitted. The receiver just
stays synchronized with the carrier, and uses variations
in the detected power level to control a speaker.
When you switch your radio to receive F.M., or "Frequency
Modulation" signals, you might notice that the markings on
the tuner are numbers of Megahertz. Suppose a particular
radio station transmits its carrier wave at the frequency
of 88.80 Megahertz. Frequency Modulation means that this
carrier wave frequency is deliberatly modified. Sometimes
the radio station may transmit waves of 88.81 Megahertz,
and sometimes it may transmit waves of 88.79 Megahertz. Of
course, a whole range of frequencies surrounding the main
carrier of 88.80 Megahertz would be used. An F.M. radio is
able to constantly convert the changes in the freqency of
the carrier wave into appropriate sound waves. The major
reason that we widely use F.M. technology is that lightning
doesn't interfere very much. While lightning generates
electromagnetic waves all across the spectrum, those waves
are not immediately followed by more waves of slightly
different frequencies. And since F.M. radios are designed
to pay attention to changes in the frequency, the emissions
of lightning are mostly ignored. (Well, ok, there are
multiple-flash lightning strikes, and a small amount of
static does get through -- but it is nowhere near as
bothersome as what an A.M. radio picks up.)
One of the physical phenomena of electromagnetic waves is
that they can don't have to be absorbed; they can bounce.
Some things they bounce off easier than others; for light,
when we have an especially good light-bouncer, we call it
a "mirror", and the bouncing is called "reflectance". In
general, the ability of anything to act as a reflector
depends on how many unpaired electrically charged particles
are concentrated in a given amount of surface area, and it
depends on what frequency of electromagnetic wave happens
to encounter that surface. The higher the frequency, the
greater the concentration of electric charges are needed to
make a good mirror. Or simply compare the wavelength to
the size of the average gap between electric charges: the
smaller the wave, the smaller a gap is needed to keep the
wave from getting through.
At the top of the Earth's atmosphere is a layer of charged
particles collectively known as "the ionosphere". This is
a very low-density environment, a near-vacuum, and while
the ions move about a lot, there is considerable amount of
space between them, on the average. Nevertheless, they are
close enough together to reflect A.M. radio waves quite
well. Once upon a time I used to think that a neat way to
detect alien civilizations would be to listen for the
electromagnetic waves produced by alternating-current power
-- that's billions of watts and thousands of miles of
powerlines acting as antenna -- but then I realized that
the ionospheres of those alien planets would be reflecting
those waves, and keeping them from getting to us. Probably
those planetary ionospheres are doing all electric-powered
civilizations a favor, preventing vast energy losses from
power grids to outer space....
The ionosphere is not a good enough mirror to reflect the
radio frequencies used in F.M. transmissions. To receive
an F.M. radio signal, you must be in "line of sight" with
the transmitting antenna. (To receive an A.M. radio signal
you can be over the horizon from the antenna, because the
ionosphere reflects the signal from the antenna to you.)
And even if you can't actually see the antenna without a
telescope (or even with a telescope, through smoke or fog
or brick buildings), you can receive an F.M. signal because
most ordinary objects do not reflect those radio waves very
well. Steel does, so a building with a lot of steel in it
can seriously interfere with radio reception.
If you recall what you saw, the last time you looked at a
microwave oven, you might remember that the door to the
oven was glass, with a piece of metal full of small holes.
The microwaves in the oven are too big to get through those
holes, so for them the metal is a great mirror. Meanwhile,
ordinary light consists of quite-small electromagnetic
waves that pass through the holes easily, letting you see
what is happening to the food cooking in the oven.
And if you recall seeing any windows with a special heat-
reflective coating, you might have noticed that the coating
reflects some light, but a reasonable amount gets through.
In such a coating (known as "partially silvered") the
charged particles are close enough together to reflect the
frequencies of electromagnetic waves which are infrared,
but are still far enough apart to let most light through.
As you might expect, a fully silvered coating works as a
full mirror for ordinary light (but it would have to be
even-more-fully-silvered to reflect ultraviolet light...).
The next relevant topic concerns the amount of data that
can be loaded onto a carrier wave. There is a limit, after
all, and this limit exists because any data represents some
kind of change in the nature of the carrier wave. The more
data you try to load onto the wave, the less the carrier
wave resembles its unloaded state. That can make it rather
difficult to detect the carrier wave! Fortunately, the
amount of data that can be effectively carried is directly
related to the frequency of the carrier wave: The higher
the frequency of the carrier, the more data it can carry.
In general, there is a 10-to-1 relationship between the
frequency of the carrier wave, and the maximum rate at
which it can carry data. That is, if a carrier wave has a
frequency of 100 Megahertz, then the most it can carry is
10 million bits per second of data. Any more than that,
and the carrier wave is distorted into unrecognizability.
In our world of backwards-compatible, many-decades-old
radio technology, we usually arrange a 20-to-1 or more
relationship between the carrier frequency and the rate of
data transmission. You can note that while our ears can
detect a maximum frequency of about 20 Kilohertz, the
LOWEST frequency on the A.M. radio band is 530 Kilohertz --
a 26.5-to-1 ratio. A.M. radio frequencies are quite well
able to carry a good-quality audio signal (that is, if
lightning didn't interfere with it so much).
For F.M. radio, the intent is to broadcast in stereo, thus
requiring enough carrier-wave capacity (or "bandwidth") to
handle two complete audio signals. When you note that the
lowest frequency on the F.M. radio band is 87.5 Megahertz,
you can see that there is plenty!
And now...just take a quick look at your television screen,
and imagine how much data per second that represents!
Television waves are Frequency Modulated. Even more than
for F.M. radio, you must be in the line of sight of a TV
transmitter to receive a good signal. The frequencies of
TV-channel carrier waves tend to range from roughly 200
Megahertz to more than one Gigahertz.
The digital revolution of recent years will be utterly
changing television technology. All old TVs will need to
be either replaced or upgraded. The process is starting
slowly, but will gain speed as more and more of the newest
units are sold. Digital technology lets us easily transmit
six times the data, in the same bandwidth, as the older
technology. Furthermore, the data being transmitted will
actually be compressed data. Uncompressed, the results on
your TV screen will be as clear as looking through a simple
window, letting you see the world as never before.
Try the links in the MadSci Library for more information on Physics.