| - |
Matter, Energy,
and Direct Current |
| - |
Alternating Current and Transformers |
| - |
Circuit Protection, Control, and Measurement |
| - |
Electrical Conductors, Wiring Techniques,
and Schematic Reading |
| - |
Generators and Motors |
| - |
Electronic Emission, Tubes, and Power Supplies |
| - |
Solid-State Devices and Power Supplies |
| - |
Amplifiers |
| - |
Wave-Generation and Wave-Shaping Circuits |
| - |
Wave Propagation, Transmission Lines, and
Antennas |
| - |
Microwave Principles |
| - |
Modulation Principles |
| - |
Introduction to Number Systems and Logic Circuits |
| - |
- Introduction to Microelectronics |
| - |
Principles of Synchros, Servos, and Gyros |
| - |
Introduction to Test Equipment |
| - |
Radio-Frequency Communications Principles |
| - |
Radar Principles |
| - |
The Technician's Handbook, Master Glossary |
| - |
Test Methods and Practices |
| - |
Introduction to Digital Computers |
| - |
Magnetic Recording |
| - |
Introduction to Fiber Optics |
| Note: Navy Electricity and Electronics Training
Series (NEETS) content is U.S. Navy property in the public domain. |
NEETS Module 17 − Radio−Frequency Communications Principles
Pages i,
1−1,
1−11,
2−1,
2−11,
2−21,
2−31,
3−1,
3−11,
3−21,
3−31,
3−41,
4−1− to 4−10,
4−11,
5−1,
5−11, Index
Chapter 5
Introduction to Miscellaneous Communications Systems and Equipment
Learning Objectives
Upon completion of this chapter you will be able to:
1. Describe the basic operation of communications systems that
operate at medium frequencies and below.
2. Describe the basic microwave line-of-sight communications
system.
3. Describe the basic tropospheric scatter communications system.
4. Describe the objective/purpose of the naval tactical data
system (NTDS).
5. Describe the naval tactical data system (NTDS) data transmission
subsystems in terms of links.
6. Explain the various applications of portable communications
equipment.
7. Define the term laser.
8. Describe the basic theory of operation of lasers
9. Describe the possible applications of lasers in communications.
Introduction
In the previous four chapters we've looked at communications equipment and systems
that were used in several frequency ranges. Some have had many applications. In
this chapter you will look at systems used in some portions of the RF spectrum that
have not been covered in detail. We will also discuss the naval tactical data system
(NTDS), which operates in the high-frequency and ultrahigh-frequency regions. Various
portable communications equipment used in the military and an introduction to the
laser and its uses in communications are included. Some of the applications presented
are fairly new to the military community.
Systems
As discussed in chapter 1, the frequency range from elf to SHF is from below
300 hertz up to 30 gigahertz. The first area we will cover is the lower frequency
bands (medium frequency [mf] and below). You will then get a look at the microwave
region and the high-frequency and ultrahigh-frequency range as it pertains to the
naval tactical data system (NTDS).
MEDIUM Frequency and BELow
Most of the receivers and transmitters that you will see used in the mf portions
of the RF spectrum and below are very similar in design. In chapter 1 we discussed
the operational uses of the equipment; now let's look at the equipment itself.
Equipment items covered in this and other chapters are meant to be merely representative
of equipment that may be encountered in naval communications. No attempt will be
made to include all of the possible equipment or equipment configurations.
Transmit Equipment
You should realize the transmitters used in bands of medium frequency and below
are similar to those you studied in chapter 2. In other words, a transmitter used
in one frequency range is basically the same as one used in another range. However,
there are some differences. Two of the differences are component size and the use
of a technique called Doubling Up.
The components used in bands of medium frequency and below are much larger physically
than the ones previously discussed. This is because of the higher operating voltage
and current levels required to produce the very high-powered RF outputs needed for
the uses covered in chapter 1. a given resistor used in an hf application may be
rated at 1/2 watt, whereas the same resistor used in a lower frequency application
would probably be rated in tens or even hundreds of watts.
Figure 5-1 - Doubled-up transmitter block diagram.
A block diagram of a doubled-up transmitter is shown in figure 5-1. Remember,
bands of medium frequencies and below are used almost exclusively for broadcast
and are on the air continuously. Doubling up increases reliability. As you can see,
two transmitters are located in the same equipment cabinet. This allows you to quickly
transfer circuits if one should fail. This dual installation also allows both amplifiers
to be used together to double the output power. When you use this application, you
sacrifice the doubling-up capability of only the power amplifier. All the other
components are still available as backups. Let's go through figure 5-1 and describe
the block functions.
The frequency generator part of the frequency generator and FSK block is an oscillator.
It provides the carrier frequencies for the CW mode. The FSK part is a Frequency
Synthesizer (a frequency source of high accuracy). It makes both the mark and space
frequencies from a very stable clock oscillator. The keying pulses determine which
FSK frequency the keyer chooses to transmit. This signal is then sent to the transmitter
control console where it is distributed to the first RF amplifier. This amplifier
is referred to as the preliminary intermediate-power amplifier (pre-IPA). The pre-IPA uses linear, untuned, push-pull, RF amplifiers to provide amplified RF to drive
other RF amplifiers. The pre-IPA output goes to the intermediate power amplifier
(IPA).
The IPA receives the pre-IPA output, amplifies the signal, and drives other selected
power amplifiers. The IPA is a single-stage, untuned, linear, push-pull, RF circuit
that uses water and forced-air cooled tubes.
Signals are then sent through the amplifier control, where they are used for
signal monitoring purposes before being applied to the final RF amplifier (pa).
The pa amplifies the signal to the final desired power level. The pa also contains
variometers (variable inductors) for coupling. This coupled output is fed to the
RF tuning unit.
The RF tuning unit consists of variable oil-filled capacitors and a fixed inductor
for frequency tuning. The signal is then sent to a knife switch. This switch simply
routes the signal to the Dummy Load or the antenna by way of the Helix House. (A
dummy load is a nonradiating device the absorbs the rf
and has the impedance characteristics of the antenna.) The dummy load is impedance
matched to the pa. It allows testing of the pa without putting a signal on the air.
When the equipment is in an operating mode, the dummy load is not used. The helix
house is a small building physically separated from the transmitter location. It
contains antenna loading, coupling, and tuning circuits. The main components consist
of a Helix (large coil) and variable inductors. The signal is fed from the helix
directly to the antenna. Sometimes two antennas are used.
Figure 5-2 - Simplified VLF transmitting antenna.
Figure 5-3 - Cutler, Maine antenna installation.
Figure 5-4A - Triatic type antenna.
Figure 5-4B - Triatic type antenna.
Figure 5-5. - Typical VLF to mf receiver.
Figure 5-6 - Receiver block diagram.
Antenna designs vary with the amount and type of land available, desired signal
coverage, and bandwidth requirements. Figure 5-2 shows a simplified transmit antenna.
The Navy uses TOP-HAT (flat- top) capacitive loading with one or more radiating
elements. Typical top hat antennas consist of two or more lengths of wire parallel
to each other and to the ground, each fed at or near its mid point. The lengths
of wire are usually supported by vertical towers. These antennas may take many shapes.
The matching network shown is in the helix house. Figure 5-3 shows the installation
at the naval communications unit in Cutler, Maine. The Navy has several of these
types of installations. They are used primarily for fleet broadcasts and have power
outputs in the .25- to 2-megahertz range. You should notice the transmitter, the
location of the helix houses, and the dual antennas. You should also notice the
transmission line tunnel. It is underground and over a half-mile long. Figure 5-4,
view (A) and view (B), shows another antenna configuration. This array of monopoles
(quarter-wave, vertically polarized stubs) is referred to as a TRIATIC antenna.
a triatic antenna is a special form of a rhombic-arranged monopole array. This type
of array is designed to transmit from a particular location. Triatics are all basically
the same but have some design differences at each site. The physical differences
compensate for differences in terrain. Now that we have looked at the transmit side,
let's look at the receive side.
Receive Equipment
The receiver you will study here is fundamentally the same as those we covered
in chapter 2. a receiver used in this frequency range is about the same electrically
as one used in any other range. Figure 5-5 shows the receiver we will discuss. It
is a highly sensitive, special purpose receiver because it is capable of splitting-out
multiplex signals for detection and reproduction. This receiver covers the frequency
range of 3 kilohertz to 810 kilohertz in five bands. It will receive most types
of signals, including AM, CW, SSB, FM, and FSK. All operator controls are on the
front panel, and a speaker and headset jack permit monitoring.
Our receiver has five basic stages excluding the power supply. With the exception
of a video amplifier in place of an RF amplifier, the circuits perform the functions
normally associated with a typical receiver. Figure 5-6 is a block diagram showing
the signal paths of the receiver. The input stage consists of a low-pass filter,
an attenuator, a calibration oscillator, and a video amplifier. The low-pass filter
passes input frequencies below 900 kilohertz. These frequencies are passed to the
attenuator, which sets the signal to the proper level to drive the mixer. This minimizes
noise and distortion. The calibration oscillator produces a 250-kilohertz output.
It is used to calibrate the receiver level and to check for tuning dial accuracy.
The input signal is direct-coupled from the attenuator to the video amplifier. This
amplifier is a broadband, constant-impedance driver for the mixer. The oscillator-mixer
stage consists of a mixer, phase splitter, local oscillator, and frequency control
circuits.
A Hartley configuration is used for the local oscillator. The oscillator output
is equal to the tuned frequency plus 2.215 megahertz. Two voltage-variable capacitors
are used in the local oscillator to stabilize small frequency variations. a phase
splitter is used to drive the mixer diodes into conduction during half of the local
oscillator cycle.
The mixer circuit uses the diodes to heterodyne the input signal with the local
oscillator signal from the phase splitter. The diodes short the signal to ground
during half the local oscillator cycle.
The IF amplifier stages consist of the mixer amplifier, four selectable bandwidth
filters, three IF amplifiers, and an IF buffer amplifier.
The output of the mixer is directly coupled to the mixer amplifier. The IF signal
is then directed through one of four bandwidth filters to the first IF amplifier.
The signal proceeds to the second and third IF amplifiers for amplification before
demodulation. An IF buffer amplifier is used to pass the IF to the IF OUT jack and
to isolate this jack from the rest of the circuitry.
Three demodulators are used in this receiver. They are the AM detector, product
detector, and FM detector. The AM detector is used to demodulate AM signals. The
product detector demodulates SSB, CW, and FSK signals, and the FM detector demodulates
FM signals only. An output from the FM detector is provided to the FM OUT jack.
This FM output may be used for recording or detailed analysis.
The output from the selected demodulator is amplified by the audio amplifier
and presented simultaneously to the Headset jack, Audio OUT terminals, and the speaker.
You should note that this receiver, as with most others, requires no other special
equipment. It uses a standard DF loop or a whip antenna. If it is installed in a
submarine, a trailed, (towed) long-wire antenna may be used.
Microwave
Communications systems in the 1 gigahertz to 10 gigahertz portion of the radio
frequency spectrum use line-of-sight propagation. Propagation takes place in the
lower atmosphere (troposphere). It is affected by factors such as barometric pressure,
temperature, water vapor, turbulence, and stratification (forming of atmospheric
layers).
A typical microwave transmitter includes an exciter group, a modulator group,
a power amplifier, and power supplies. The transmitter usually has a power output
of about 1 watt. When a higher output is required (about 5 watts), a traveling-wave
tube (TWT) is used as the amplifier. (A TWT is a high-gain, low- noise, wide-bandwidth
microwave amplifier. It is capable of gains of 40 decibels or more, with bandwidths
of over an octave. The TWT was discussed in chapter 2 of NEETS, Module 11, Microwave
Principles.) a typical microwave receiver contains an RF-IF group, local oscillator,
demodulator, and amplifier. Both transmitters and receivers contain special circuits
because of the high operating frequencies and critical frequency stability requirements.
Line-of-Sight System
Figure 5-7 - Typical hop-link and section allocation.
A line-of-sight (los) microwave system consists of one or more point-to-point
hops as shown in figure 5-7. Each hop is designed so that it can be integrated into
a worldwide communications network. Los systems have many characteristics. In these
systems, propagation is only affected by changes in the troposphere. The distance
between microwave system hop points ranges from 50 to 150 kilometers (31 to 95 statute
miles). These systems are capable of handling up to 600 4-kilohertz voice channels
and can also transmit television. These signals can usually be transmitted with
less than 10 watts of power. Both the transmit and receive antennas are horn-driven
paraboloids that provide high gain and narrow beam widths. In some applications,
as shown in figure 5-8, plane reflectors are used with the paraboloids. These systems
are very reliable. They are designed to operate over 99 percent of the time. These
systems are well adapted to multichannel communications and closed circuit television.
Figure 5-8 - Parabolic antenna and passive reflector combination.
Now let us take a look at another system. It is called the tropospheric-scatter
microwave system. But first, you may want to review tropospheric propagation in
NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas.
Tropospheric Scatter System
A tropospheric-scatter (tropo-scatter) microwave system gets results similar
to those of the line-of- sight system. It does it in a different way. The los system uses towers to relay information.
Figure 5-9 - Mobile 30-foot tropospheric-scanner antenna.
The tropo system uses the turbulence in the layer between the troposphere and
the stratosphere to bounce signals back to earth. This method provides several hops
and communications beyond los. The propagation reliability and communications capability
is the same. The transmission range is up to 800 kilometers (500 statute miles).
Transmitter output power may be up to 75 kilowatts depending on the operational
requirements. The antennas are horn-driven paraboloids and may be as large as 50
to 60 feet in diameter. Figure 5-9 shows a typical tropospheric-scanner antenna.
Remember that hf has a hop distance (skywave) of about 1,400 miles; the distance
of one hop for a line-of-sight system is between 31 and 95 miles. The tropospheric-scatter
system conveniently fills the gap between these distances.
Both of these systems are used ashore. You're now going to get a look at a shipboard
data information exchange system.
Q1. What is a dummy load?
Q2. What is the function of a product detector?
Q3. What is the frequency range of the mf band?
Q4. Microwave systems use what portion of the atmosphere?
Q5. What is the voice channel capacity of an los communications system?
Q6. What is the one-hop transmission range of a tropospheric-scatter
system?
NAVAL TACTICAL DATA System
In recent years, the Navy has introduced several new highly technical and effective
combat weapons systems. However, these weapons systems did not solve the basic combat
command problems that confront our Navy. In combat, a fleet continues to be involved
in close-range offense and defense. During close-range combat, the shipboard combat
information center (CIC) is involved in complex tactical situations. These situations
require intelligent and highly important decisions. Each decision has to be made
in a short period of time. You will find the speed at which these combat situations
must be solved is inconceivable to someone thinking in terms of typical CIC operations
of the recent past. Therefore, the NTDS was developed by the U.S. Navy as a command
tool for commanders in tactical combat situations.
Objectives
The naval tactical data system (NTDS) is based on the interaction of humans and
machines. The NTDS helps coordinate fleet air defense, antisubmarine warfare, and
surface defense operations. Through
Posted November 5, 2021
|
