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Drawing of three sailors working on a radio.

CHAPTER 16
BACKGROUND TO MODERN RADIO
INTRODUCTION

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.

 
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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 that was-

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.

Damped wave produced by an electric spark.
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
 
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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. GENERATOR

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.

Continuous wave produced by a transmitter.
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 GENERATOR.

THE ANTENNA OF A TRANSMITTER REPLACES THE ELECTROMAGNET

Instead of using an electromagnet to produce the magnetic field, the transmitter uses a single wire or ANTENNA,

 
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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 strange.

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 position.

HOW DOES A MESSAGE GET FROM A TRANSMITTER TO YOUR RECEIVER?

All of you are familiar with the movement of waves in water. When a stone is dropped into a pool, the waves

How radio waves pass from the transmitter to receiver.
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

 
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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 message.

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 forces.

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

 
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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.

 
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