BACKGROUND TO MODERN RADIO
How many of you remember the first radio receiver
that was in your home town? Probably none, because
that was about the time many of you were born.
In 1920, there were fewer radio receivers in American
homes than ships in the Navy. Yet by 1940, only 20
years later, there were more home receivers in everyday
use than automobiles on the highway.
Radio is such an important part of every American's
daily life that we are inclined to think that it has always
existed in its present form. Actually, the discovery of its
principle dates back to only a few years before 1900.
DISCOVERY OF THE PRINCIPLE OF RADIO
Marconi is usually given credit for the invention of
radio. Actually, he was just the first man to send a
message successfully. The principle of wireless communication had been discovered at an earlier date by another
European scientist, Hertz.
In 1888, Hertz observed that a compass needle when
placed near a magnet would move each time the magnet
was moved. Now that may seem like a simple observation, but he saw something NEW in an OLD principle, and
ENERGY CAN BE TRANSMITTED THROUGH SPACE IN THE
FORM OF A MAGNETIC FIELD.
Further experiments revealed that the range of transmission could be increased by using a.c. with an electromagnet to produce the magnetic field. It was also observed that still greater ranges of transmission were possible when the a.c. used was of HIGH FREQUENCY.
From this point on, the development of radio turned
toward the development of a high frequency a.c. generator. Many devices were tried, but most of them failed.
SPARK GAP TRANSMITTERS
One of the first successful transmitters was an ELECTRIC
SPARK. When an electric spark jumps from one terminal to another, the full discharge does not leap across
the gap and stay there, but it jumps BACK and FORTH
THOUSANDS of times before eventually coming to rest.
Each time the spark completes one round trip between
the gaps, one cycle of a.c. is generated. If the spark
jumps back and forth at a rate of 50,000 times a second,
an a.c. of a 50,000 cycle frequency is generated.
Figure 114.-Damped wave produced by an electric spark.
The oscillations produced by a spark are not uniform.
They start strongly, but soon die out completely as illustrated in figure 114. When the next spark jumps the
gap, they start the process all over again. The a.c.
wave produced by a spark is called a DAMPED wave.
A RADIO TRANSMITTER-ONLY A HIGH FREQUENCY A.C.
Although the electric spark was widely used with early
transmitters, it was not completely satisfactory. An
ideal generator must produce an arc that does not
periodically die out, but rather a CONTINUOUS and uninterrupted chain of vibrations as illustrated in figure 115.
Figure 115.-Continuous wave produced by a transmitter.
The problem of building a useful high frequency a.c.
generator was finally solved by the invention of the
VACUUM TUBE. The vacuum tube, with a few wires, coils,
condensers, resistors, and other little gadgets, provides
the basis for the transmitter.
Always remember that regardless of how many wires,
vacuum tubes, resistors, and other parts a transmitter
may have in its circuit, it is essentially a HIGH FREQUENCY
THE ANTENNA OF A TRANSMITTER REPLACES THE
Instead of using an electromagnet to produce the magnetic field, the transmitter uses a single wire or ANTENNA,
but still the magnetic field is produced in the same way-by a flow of electrons.
WHAT ARE RADIO WAVES?
You are familiar with water waves, sound waves, and
the WAVES you used to pilot around a dance floor-but
what are RADIO WAVES?
Here's an exact definition-radio waves are vibrating
ELECTROMAGNETIC FIELDS IN THE ETHER. You know the
meaning of vibrations and electromagnetic fields-but
what is this thing called ETHER? Seems to be a little
The ETHER is an IMAGINARY substance. It is present
EVERYWHERE, even in a vacuum. Like the wind, no one
has ever seen the ether, or probably ever will.
The ether's reaction to magnetic fields indicates that it
is an ELASTIC substance, capable of being pulled or pushed
out of shape. But when the force used to produce the
distortion is removed, the ether springs back to its normal
HOW DOES A MESSAGE GET FROM A TRANSMITTER TO
All of you are familiar with the movement of waves in
water. When a stone is dropped into a pool, the waves
Figure 116.-How radio waves pass from the transmitter to receiver.
move outward in all directions until they either die out
or reach the edges of the water.
Electrons in moving through a transmitter's antenna
cause a disturbance in the ether just like a pebble being
tossed into a pond of water. As illustrated in figure 116,
the electromagnetic field caused by the moving electrons
expands outward in every direction and eventually will
strike against a receiving antenna and deliver the
Because it is the electromagnetic radio wave that
CARRIES the message to your receiver, radio waves are
also called CARRIER WAVES. Many times they are merely
referred to as the CARRIER.
There are many forces and outside influences that interfere with the perfect transmission of radio waves
from the transmitting antenna to your receiver. Occasionally these outside forces are so strong that the
wave is unable to reach the intended receiver, just as a
strong cross-wind may prevent the ripples from a stone
from reaching the opposite shore. In the last chapter of
this book you will find a discussion of these interfering
HOW FAST DO RADIO WAVES TRAVEL?
Radio waves travel at the speed of light-186,000 land
miles or 164,000 nautical miles per second. That speed
is fast enough to circle the earth at the equator about 7½
times in a single second. It may be useful to you in
making adjustments on certain tactical equipment to know
that radio waves travel at the rate of 382,000,000 yards
in a second, or one mile in six microseconds,
6 / 1,000,000 of a second.
SOMETHING ABOUT FREQUENCIES
Several times you have read in this chapter that the
frequency of the a.c. used with radio transmitters is
high, but no definite values have been given. The actual
frequencies used extend over a wide range, from 30,000
cycles per second at the lower end to greater than
30,000,000,000 cycles a second at the top of the band. At
the present time the upper limit is being raised rapidly so
that within a year the present high may be far below the
top frequencies then being used.
Frequencies greater than 30,000 cycles a second are
called RADIO FREQUENCIES. For the purpose of reference,
the full radio frequency band had been divided into eight '
parts, as listed in the following table.
|BAND ||CYCLES PER SECOND ||KILOCYCLE ||MEGACYCLE
|Very low ||Below 30,000 ||Below 30 ||--
|Low ||Up to 300,000 ||30-300 ||--
|Medium ||Up to 3,000,000 ||300-3,000 ||.3-3
|High ||Up to 300,000,000 ||-- ||3-30
|Very high ||Up to 300,000,000 ||-- ||30-300
|Ultra-high ||Up to 3,000,000,000 ||-- ||300-3,000
|Super-high ||Up to 30,000,000,000 ||-- ||3,000-30,000
|Microwave ||Above 30,000,000,000 ||-- ||Above 30,000
Remember-if you use the expression 30 kilocycles, you
actually mean 30,000 cycles. And if the expression 10
megacycles is used, it means 10,000,000 cycles.
Each of the various frequency bands possesses
characteristics that are of an advantage for certain types of
communication. The bands below 300 kc. are most used
by shore stations for long-range communications.
The frequencies between 300 and 3,000 kc. contains
the commercial broadcast band and some of the long-range,
medium-wave communication frequencies.
Frequencies greater than 300 mc. are used most
frequently with certain types of Navy tactical equipment.
That's enough on frequencies for the present. The last
chapter in this manual contains a complete discussion of
frequencies and radio transmission.