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Drawing of sailor adjusting a transmitter.

CHAPTER 17
INTRODUCTION TO TRANSMITTERS
MANY TYPES-BUT STILL SIMILAR

The Navy uses many different types of transmitters. Some are small enough to be carried away in your sea bag; others are large enough to bunk in. In spite of the variance in physical size, most transmitters are basically alike. To get off to a good start on transmitters, you should first see what the differences and similarities are among various types.

First, why the differences? Two things are largely responsible for determining the size of a transmitter-the FREQUENCY at which it transmits, and the AMOUNT OF POWER it delivers.

HIGH FREQUENCY transmitters are smaller, because the parts used-coils, vacuum tubes, etc.,-are small.

HIGH POWER transmitters have to be made large to prevent their destruction. Low POWER transmitters may be very small. You can see this difference by comparing the big TBK, 500 watts, with the portable TCS, 50 watts, and the SCR-536 Handy-Talkie of only .027 watts.

 
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Second, what are the basic similarities? The general plan of all transmitters can be seen in figure 117, which shows the stages of a typical transmitter.

Every transmitter has an OSCILLATOR. It may be one of the SELF-EXCITED jobs already studied, or it may use a QUARTZ CRYSTAL as a source of high frequency a.c. At best, the a.c. generated by any transmitter is feeble, and it must be strengthened (amplified) before it has sufficient power to carry a message any great distance.

Hence, AMPLIFIERS come next in a transmitter. There are two types of amplifiers-VOLTAGE AMPLIFIER and POWER AMPLIFIER. As you can see in figure 117, the voltage amplifier is the stage immediately following the oscillator. The last or OUTPUT stage is the POWER amplifier.

Stages of a typical transmitter.
Figure 117.-Stages of a typical transmitter.
The stage next to the oscillator is called the BUFFER-so named because this stage ISOLATES the oscillator from the POWER AMPLIFIER. Without the buffer, changes in the power amplifier due to keying or roll of your ship would be REFLECTED back to the oscillator and cause it to change frequency. And if the oscillator changed frequency, the person copying your messages will lose contact momentarily and thus receive an incomplete message.

The POWER AMPLIFIER lives up to its name. It increases the flow of current in the antenna so that a strong electromagnetic carrier wave will be produced.

 
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CONTINUOUS WAVE TRANSMISSIONS

The electromagnetic wave of a transmitter is like the sound wave produced by a steamship's whistle. As long as the valve is held open and the steam pressure holds steady, the whistle will send out sound vibrations at one strength and one frequency. A CONTINUOUS WAVE (C.W.) TRANSMITTER operates in a similar manner. It sends out a continuous stream of vibrations at radio frequency.

Output wave of a c.w. transmitter.
Figure 118 -Output wave of a c.w. transmitter.
The wave in figure 118 was produced by a c.w. transmitter. The oscillations are the same distance apart and have a CONSTANT AMPLITUDE.
How Morse code message is produced.
Figure 119.-How Morse code message is produced.
 
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HOW TO SEND A MESSAGE WITH A CONTINUOUS WAVE TRANSMITTER

By blowing a series of long and short blasts, you can use your whistle to communicate with another ship. You can do the same thing with a c.w. transmitter. By inserting a switch, or KEY, in the transmitter circuit, you can send out long and short blasts of r.f. energy.

The output from a KEYED c.w. transmitter is given in figure 119. When the key is held down for a short interval, a DIT will be formed. If the key is held down for a longer period of time, a DAH will be produced. By making a combination of dits and dahs with a key, you can produce a Morse code signal. The (dit, dit, dah, dit) in figure 119 is the signal for the letter F.

HOW SOUND WAVES ARE TRANSMITTED

The device used to transmit sound waves is a combination c.w. transmitter and audio amplifier.

Block diagram of a modulated transmitter.
Figure 120.-Block diagram of a modulated transmitter.
 
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In figure 120, the OUTPUT of the SPEECH AMPLIFIER is fed into a MODULATOR STAGE. In this stage, the RADIO FREQUENCY and AUDIO FREQUENCY VOLTAGES are COMBINED to form a CARRIER WAVE that contains characteristics of both. Notice in figure 121 that the CARRIER WAVE varies
Block diagram of a low-level modulated transmitter.
Figure 121.-Block diagram of a low-level modulated transmitter.
in amplitude in direct proportion to the SIZE and FREQUENCY of the AUDIO component.

The process of COMBINING the audio and radio frequency waves is MODULATION. The part of the modulated carrier wave that comes from the transmitter section is the R.F. COMPONENT, and the portion from the speech amplifier is the A.F. COMPONENT.

If the modulation voltage is sent into the POWER AMPLIFIER stage, such a transmitter is said to be using HIGH-LEVEL MODULATION.

When the modulation voltage is sent into the BUFFER stage, as in figure 121, the transmitter is said to be using LOW-LEVEL MODULATION.

 
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MODULATED C. W. CODE TRANSMISSION

Some code transmitters combine the characteristics of the C.W. and MODULATED C.W. transmission.

Modulated C.W. code transmitter.
Figure 122.-Modulated C.W. code transmitter.
Figure 122 is a block diagram of a modulated c.w. code transmitter. An AUDIO FREQUENCY OSCILLATOR, generating a constant frequency note, is inserted in place of a speech amplifier.

The r.f. section of the transmitter produces a CONTINUOUS CARRIER WAVE. When the key in the audio oscillator is pressed, the audio frequency signal is sent into the power amplifier, modulating the carrier wave.

The sound produced at the receiving end of a modulated c.w. signal is at the frequency of the audio oscillator. If the oscillator is generating a frequency of 400 cycles, the frequency of the dots and dashes that you receive will also be 400 cycles.

TUNING THE TRANSMITTER

About the only contact you will have with transmitter; will be to tune them up when you change from one

 
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frequency to another. If you are on a large ship where the transmitter room is considerable distance from the radio room, you may not get the opportunity to work with the transmitter very often. The ETMs will have that job.

In spite of the fact that you may not have the chance to tune them very often, you should know the correct tuning procedure. Then you can do the job yourself in an emergency.

Tuning a transmitter is largely a matter of following a routine of turning dials, and closing and opening switches. Unlike receiver operation, the operation of transmitter controls MUST be in the PROPER SEQUENCE, otherwise you will damage the transmitter.

In the chapter on Navy transmitters you will find brief instructions explaining the proper method for tuning some of the more frequently used transmitters. WHENEVER you desire more information about these or other transmitters, study the INSTRUCTION BOOK that comes with each set.

699198°-4,12

 
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