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Your RF Cafe
Airplanes and Rockets:
FUNDAMENTAL SYSTEMS EQUIPMENT
Upon completion of this chapter you will be able to:
1. State the function of a radio communications handset, a radio set control, and a transfer switchboard.
2. Describe the functions and interrelationships of a radio transmitter.
3. Describe the functions of receive and transmit multicouplers.
4. Describe the differences between the codes used for manual telegraphy and teletypewriter transmissions.
5. Describe the two basic modes of teletypewriter operation.
6. Describe the two types of teletypewriter dc circuits.
7. State the two types of radio teletypewriter shift systems and describe their basic differences.
8. Describe the functions and interrelationships of radio-frequency-carrier shift send and receive systems.
9. Describe the signal flow in an audio-frequency-tone shift system.
10. State the function of the tone terminal set in an audio-frequency-tone shift system.
11. Describe the basic multiplexing process.
12. Describe the three operations performed by a facsimile system.
13. Describe the functions and interrelationships of facsimile equipment.
14. Describe the countermeasures that can be used to eliminate compromising emanations.
A communications system is a collection of equipment used together to do a specific job. You may see this
equipment used to send or receive voice communications, or both, or to send, receive, or send and receive
Figure 3-1 is a basic block diagram of a voice system. You can see how this equipment is interconnected to form a basic communications system. We are going to look at several of the equipment blocks in detail.
Figure 3-1.—Voice system.
The handset converts acoustical energy (your voice) to electrical energy for use in modulating a radio transmitter. It also converts electrical energy to acoustical energy for reproduction of a received signal. When the push-to-talk button is depressed on the handset, the dc keying circuit to the transmitter is closed, placing the transmitter on the air.
Handsets are normally connected to a radio set control unit.
RADIO SET CONTROL UNIT
The radio set control unit shown in figure 3-2 provides a capability to remotely control some radiophone transmitter functions and the receiver output. Some of the controls are used for turning the transmitter on and off. Others are used for voice modulating the transmission (or keying when CW operation is desired). You can even control the audio output level of the receiver and silence the receiver when transmitting.
Figure 3-2.—Radio set control unit.
Under standard operating conditions up to four of these units can be used in parallel with a single transmitter
and receiver group to provide additional operating positions. This setup is often found aboard ship where a
transmitter and/or receiver is controlled and operated from several locations such as the bridge or the combat
Figure 3-3.—Transmitter transfer switchboard.
Each switch operating knob corresponds to a remote control station. Each switch position (1 through 6) corresponds to a transmitter. One switch position, X, provides for transfer of all circuits to additional
transmitter transfer switchboards when more than six transmitters are installed in the system. When the rotary switch is placed in the OFF position the remote control station is removed from the system.
Let's look at an example of one transfer switchboard application. When remote control station number two is to have control of transmitter number three, the switch knob designated number two is rotated until its pointer indicates position three on its dial plate.
The receiver transfer switchboard permits the operator to transfer the audio output from a receiver to a remote control station audio circuit. A representative receiver transfer switchboard is shown in figure 3- 4. This switchboard contains ten seven-position switches. Each switch is connected to a remote control station, and each switch position (one through five) is connected to a receiver.
Figure 3-4.—Receiver transfer switchboard.
The X position on each switch allows transfer of circuits to additional switchboards just like with the
transmitter transfer switchboard.
Q1. What are the basic functions of a handset?
Q2. What capability does a transmitter transfer switchboard provide?
Q3. What function does a receiver transfer switchboard perform?
You learned earlier that transmitters may be simple with low power (milliwatts) capabilities. These may be used to send voice messages a short distance. You may also use highly sophisticated units that use thousands (even millions) of watts of power to send many channels of data (for example voice, teletypewriter, television, telemetry) simultaneously over long distances. Let's look at a complete transmitter set.
Radio Transmitting Set
The applications, configurations, and components you will become familiar with here are typical of most general purpose transmitter systems used in the Navy. A specific transmitter is used only for ease of illustration and example.
We will be discussing a 1,000 watt, single-sideband radio transmitting set that is available to the Navy in any one of four setups. The normal configuration has a transmitter capable of voice, continuous wave, and radio teletypewriter transmissions in the 2- to 30-megahertz frequency range. Exact spacing and number of channels available within the frequency spectrum, modes of operation, and frequency range depend on the model of equipment and how it is configured for use. Stack or rack mounting is used in a ship or shore permanent installation with accessory equipment (for example an RF amplifier, coupler control unit, or power supply) to form a complete communications system. One of three different three- phase primary power sources can be used (depending on whether the transmitter is land, air, or shore based) to provide operating power to the set. Combinations available are 115 volts, 400 hertz; or 208/440 volts, 60 hertz.
Figure 3-5 shows the major units of this set. They are the radio transmitter, the radio frequency amplifier, the power supply, and the electrical equipment shock mount base. An antenna coupler group (consisting of a coupler and coupler control unit) is normally used to match the impedance of the system to a 50-ohm transmission line. If you want to operate with any 50-ohm antenna system, terminating connections are available.
Figure 3-5.—Radio transmitting set.
The transmitter unit provides an upper sideband (usb), lower sideband (lsb), independent sideband (isb), CW, FSK, or compatible AM signal. The output of the transmitter has enough power to drive the radio frequency amplifier.
Depending on the model, the transmitter tunes across the frequency range in 100- or 500-hertz increments. Digital circuitry is used to accomplish this process. Transmitter outputs are also applied to the RF amplifier to automatically tune it to the correct frequency. We will go through a detailed breakdown of the transmitter unit later in this chapter.
RADIO FREQUENCY AMPLIFIER.—The RF amplifier unit is a two-stage linear power amplifier that produces an output of 1,000 watts with a nominal input of 100 milliwatts. Nineteen frequency bands are used to cover the operating frequency range. The operating band is automatically selected by digital coding generated by the transmitter. The code controls two motor-driven band switch assemblies. Automatic control circuits protect the unit against overload and compensate for variations in system gain, mode of operation, and loading.
All low voltages required for operation (except two of the relay control voltages) are internally produced. The high voltages required in the amplifier stages are produced by the associated power supply (when using 60 hertz primary power) or the optional internally mounted power supply (when using 400 hertz primary power).
Let's take a look at figure 3-6 to see all the operating controls and indicators located on the front panel. Some controls are used only for initial setup and are protected by a hinged access cover. All connections are made at the rear of the case. The amplifiers and the associated interstage broadband transformer assemblies are cooled by forced ventilation. Cooling air is drawn through a filter on the front panel and exhausted through a port on the rear of the case. You should always take particular care to clean or replace any filter in electronic equipment as a regular part of your preventive maintenance program.
Figure 3-6.—Rf amplifier unit.
POWER SUPPLIES.—One power supply produces operating voltages for the amplifier when operating
from a 60-hertz power source. All components of the power supply, except the power transformers, are mounted on a
chassis and panel assembly that is hinge-mounted to a metal case. The power transformers are constructed as part
of the case and there are no operating controls.
The other power supply produces operating voltages for the RF amplifier when a 400-hertz, three- phase, 115-volt primary power source is used.
ANTENNA COUPLER GROUP.—The antenna coupler group is an automatic antenna tuning system. However, the equipment design includes provisions for manual or semiautomatic tuning. This makes the system adaptable for use with other radio transmitters. The manual tuning capability is useful when a failure occurs in the automatic tuning circuitry. Tuning can also be accomplished without the use of RF power (SILENT TUNING). This method is useful in installations where radio silence must be maintained except for brief transmission periods.
The antenna coupler matches the impedance of a 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohm transmission line at any frequency in the 2- to 30-megahertz range. Control signals from the associated antenna coupler control unit automatically tune the matching network in less than five seconds. During manual and silent operation, tuning is accomplished by the operator with the controls mounted on the antenna coupler control unit. A low power (not to exceed 250 watts) CW signal is required for tuning. Once tuned, the coupler is capable of handling 1,000 watts peak envelope power (pep).
The coupler is enclosed in an aluminum, airtight, pressurized case. Six mounting feet enable the unit to be attached to the mast of a ship at the base of a whip antenna. The coupler is pressurized with dry nitrogen to aid internal heat transfer and to prevent corona and arcing. All components of the coupler are secured to a chassis that is mounted to the case so that an air duct exists between the chassis plate and the
case. An internal fan circulates the nitrogen over and through the heat-producing elements and then through the air duct. While passing through the air duct, the nitrogen loses its heat to the bottom of the case. This heat is then transferred by convection through fins on the bottom of the case and by conduction through the mounting feet.
Figure 3-7 shows the antenna coupler control unit. This unit provides the power and control signals required to tune the coupler. Control signals are either automatically produced by the coupler control when a tune cycle is initiated or manually produced with the front panel controls.
Figure 3-7.—Antenna coupler control unit.
All dc operating voltages are produced from a 115-volt, 48- to 63- or 350- to 450-hertz, single-phase primary
power source. Meter and protection circuits are used to give you complete control of the coupler from the remotely
positioned coupler control unit.
Q4. If the RF amplifier discussed has an 80 milliwatt input, what would be the maximum output?
Q5. What are the tuning modes for the coupler group discussed?
Q6. What is the purpose of an antenna coupler?
Q7. Why is the coupler pressurized with nitrogen?
RADIO TRANSMITTER.—Figure 3-8 shows the front panel of the radio transmitter unit. The radio transmitter accepts audio or coded intelligence and uses it to modulate one of 280,000 possible operating radio frequencies in the 2.0- to 29.999-megahertz frequency range. Tuning is accomplished digitally by means of five control knobs and a switch located on the front panel. The transmitter has a normal RF output level of at least 100 milliwatts and is designed to be used with an associated RF power amplifier.
Figure 3-8.—Radio transmitter unit.
When the AM and SSB transmit modes of operation are used, the output from a handset is applied to the transmitter. The voice signals are amplified and used to modulate a 500-kilohertz local carrier that produces a 500-kilohertz IF. The resulting double sideband signal is filtered in the AM mode, amplified, and converted by a triple-conversion process to the desired RF operating frequency. The RF signal is amplified to a nominal 100 milliwatt level. In CW operation, the 500-kilohertz local carrier is inserted directly into the IF amplifiers. The signal is further processed in the same manner as the voice signals in the AM or SSB modes of operation. In FSK operation, the loop current is converted to audio frequencies representing marks and spaces. These audio signals are applied to the audio circuits of the transmitter. Thereafter, these signals are processed in the same manner as the voice signals in AM or SSB modes of operation. A typical radio transmitting set block diagram is shown in figure 3-9.
Figure 3-9.—Typical radio transmitting set block diagram.
The receiver we will discuss is a triple-conversion superheterodyne, tunable from 2 to 30 megahertz. Triple conversion uses three IF frequencies to give better adjacent-channel selectivity and greater image- frequency suppression. Figure 3-10 shows the front panel of this receiver where tuning is done digitally by five controls and a switch. A display window directly above each control provides a digital readout of the frequency setting. The displayed frequency can be changed in 1-kilohertz increments. The front panel switch allows the operating frequency to be changed in 100- or 500-hertz increments depending on the model. This will provide you with 280,000 discrete frequencies locked to a very accurate frequency standard. You can continuously tune each 1,000-hertz increment by selecting the VERNIER position of the hertz switch. When using the vernier, the full accuracy of the frequency standard is sacrificed. The receiver demodulates and provides audio outputs for the LSB, USB, ISB, AM, CW, and FSK types of received signals.