NEETS Module 17 — Radio-Frequency Communications Principles
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automation, the NTDS provides commanders with a broad picture of the current tactical situation. It also assists them in directing their operations in time to intercept and destroy all potential enemy threats. The use of digital computers and digital data processing techniques reduces reaction time and increases force effectiveness.
NTDS uses a variety of equipment. This equipment includes transmitters, receivers, cryptographic equipment, high-speed digital computers, magnetic tapes, disks, and a variety of displays. Figure 5-10 shows the NTDS equipment grouping and how it interfaces with the weapons and sensor systems of a ship. Figure 5-11 shows the NTDS system inputs and outputs. As you can see, large amounts and various types of information are provided to or taken from the NTDS. Now that you have seen the types of information associated with the NTDS, let's look at how this information is transmitted and used.
Figure 5-10.—NTDS equipment grouping.
Figure 5-11.—NTDS system inputs/outputs.
NTDS Data Transmission Subsystems
NTDS uses three separate data transmission links to maintain tactical data communications between tactical units. Figure 5-12 illustrates these links. Each link is able to transfer data rapidly to other ships, aircraft, and shore facilities without the delay of human interface (link 14 receive is an exception to this). The data processing subsystem formats the messages for each of the data links. These messages are based on shipboard inputs (from displays, sensors, and other data links).
Figure 5-12.—NTDS communications links.
The automatic data communications links provide the operational commander with a high-speed, accurate mode of tactical communications. Link 11 provides high-speed, computer-to-computer transfer of tactical information, command orders, and unit status to all tactical data systems. View A shows you this configuration. The type of tactical information currently transferred is surface, subsurface, air, and EW track information. Data is provided on friendly, hostile, and unknown identity tracks. This broadcast originates through console button actions by the console operators.
Link 14 provides a means of transmitting track information to those units not capable of participating in the link 11 network. View B shows this network. This is a one-way broadcast of information.
Link 4A permits the computer to take control of the autopilot in an equipped aircraft. Also this link can control a plane under other situations. It may control a flight out to a strike area and return it to base without the need for pilot action. The pilot also has the option of overriding the link. The pilot may use the visual display to aid in understanding the intercept controller, or to totally ignore the link 4A
transmission. View C shows this link used in conjunction with the automatic carrier landing system
Figure 5-13 is a drawing of an intersystem communications employment diagram. It shows the overall possibilities and flexibility of the NTDS. The new terms shown are defined below:
Figure 5-13.—Intersystem communications employment.
· MTDS-Marine tactical data system
· AFDS-Amphibious flagship data system
· ATDS-Airborne tactical data system
· ASWTDS-Antisubmarine warfare tactical data system
Now that we've looked at a complex and stationary system, let's study some fundamental portable equipment.
Q7. What is the primary advantage of NTDS over conventional systems?
Q8. What are the three NTDS data transmission subsystems?
Portable and pack radio sets must be lightweight, compact, and self-contained. Usually, these sets are powered by a battery or a hand generator, have low output power, and are either transceivers or transmitter-receivers. A transceiver consists of a transmitter and a receiver that share common circuits and are housed in the same case or cabinet. A transmitter-receiver is the combination of two separate pieces of equipment that are used together. Navy ships carry a variety of these radio sets for emergency and amphibious communications. The numbers and types of equipment vary according to the individual ship.
One piece of emergency equipment is shown in figure 5-14. It is a rugged emergency transmitter carried aboard ships and aircraft for use in lifeboats and life rafts. The transmitter operates on the international distress frequency (500 kilohertz) and the survival craft communications frequency (8,364 kilohertz).
Figure 5-14.—Typical emergency lifeboat transmitter.
The complete radio transmitter, including the power supply, is contained in an aluminum cabinet that is airtight and waterproof. The cabinet is shaped to fit between the legs of the operator and has a strap for securing it in the operating position.
The only operating controls are a three-position selector switch and a push-button telegraph key. A hand crank screws into a socket in the top of the cabinet. The generator, automatic keying, and automatic frequency changing are all operated by turning the hand crank. While the hand crank is being turned, the set automatically transmits the distress signal SOS in Morse code. The code consists of six groups of SOS followed by a 20-second dash. It is transmitted alternately on 500 kilohertz and 8,364 kilohertz. The frequency automatically changes every 50 seconds. These signals are intended for reception by two groups of stations, each having distinct rescue functions. Direction-finding stations cooperating in long- range rescue operations normally use 8,364 kilohertz, whereas aircraft or ships locally engaged in search and rescue missions use 500 kilohertz.
Besides the automatic feature, you can key the transmitter manually on 500 kilohertz only. This is done by means of a push-button telegraph key.
Additional items (not shown) packaged with the transmitter include the antenna, a box kite, and balloons for supporting the antenna. Hydrogen-generating chemicals for inflating the balloon and a signal lamp that can be powered by the hand-crank generator are also included.
The equipment floats and is painted brilliant orange-yellow to provide good visibility against dark backgrounds.
A transceiver is shown in figure 5-15. It is portable, battery powered, and has two channels. It provides homing information and two-way voice communications between life rafts and searching ships and aircraft. This transceiver is a microminiature, solid-state, hand-held radio that operates on the 121.5-megahertz and the 243-megahertz guard channels. The transceiver has four operating controls. These are the volume (VOL) control, the two-position FREQUENCY SELECTOR, the PUSH-TO- TALK/ CODE button, and the three-position MODE switch.
Figure 5-15.—Emergency portable transceiver.
The antenna is a rubber covered, omnidirectional, flexible whip antenna that is 7.74 inches long. The batteries supplied with the radio set are lithium D cells. Each cell is fused to protect against damage from external short circuits. Two cells are installed in the transceiver and four are packaged as spares.
An operational transceiver is shown in figure 5-16. It is watertight, lightweight, portable, and operates in the vhf and uhf range. You can use any of 1,750 channels, spaced 200 kilohertz apart, in the 225-400 megahertz range. Its mode of operation is AM voice and it supplies an average output power of 3 watts. It was designed mainly for manpack (backpack) use, but it may also be used at a fixed station or in vehicles when certain accessories are added. When not in use, the equipment is disassembled and stowed in a special aluminum case similar to an ordinary suitcase.
Figure 5-16.—Typical vhf/uhf backpack transceiver.
Figure 5-17 shows a typical vhf miniaturized manpack radio set. View A shows the pack frame, the handset, and the canvas accessory pouch. The pouch contains two antennas (one flexible steel band-type whip and one collapsible rigid whip). The handset fits in the pouch when not in use. View B shows the transmitter-receiver.
Figure 5-17.—Vhf receiver-transmitter.
Now that you have learned about portable equipment, let's look at one of the newest areas of communications. You are going to learn the fundamentals of how a laser works and how it may be used in the field of communications.
Q9. What are the three main design considerations of portable equipment?
The word LASER is an acronym for light amplification by stimulated emission of radiation. The laser is widely used in industry, and experimental work is being done with it in communications. You will find a laser is similar to uhf and microwave power sources and could replace either of them in point-to-
THEORY OF OPERATION
Lasers take energy at (or near) the visible light spectrum and convert it to a very narrow and intense beam in the same region. A close relative of the laser is the light emitting diode (LED). The LED takes dc or low frequency ac power and converts the energy into visible light.
The principle of the laser is somewhat similar to that of a very high-Q cavity resonator. Chapter 1 of NEETS, Module 11, Microwave Principles, explains cavity resonators. The laser is shock-excited by a spark transmitter. This transmitter is called a spark transmitter because it uses the discharge of a capacitor through an inductor and a spark gap as a source of rf. While the input energy of the laser covers a wide band of frequencies, the output is on one frequency. Energy outputs of the laser are either INCOHERENT or COHERENT. For example, if you turn on a transmitter with no modulation, you will get coherent radiation. When you connect a noise source to an antenna, the result is incoherent radiation.
Lasers can be either CW or pulsed. Actually, lasers are little different from conventional oscillators. However, the way lasers convert energy from one form to another is quite different. In conventional oscillators, dc power from the collector is converted to RF energy. The frequency is for the most part independent of the molecular or atomic structure of the generator. This is not true for the laser. Laser conversion takes place directly within the molecular structure of a crystal or gas. The external circuits have little effect on actual output frequency. The fact that the light from an LED is always the same color results from similar conditions. In a laser, incoherent light excites the electrons in the atoms to higher energy levels than they normally would have. The new energy states are unstable and the electrons drop down to lower energy levels. Energy is then released in the form of light.
Figure 5-18 shows the ends of the crystal or glass tube laser with light waves reflecting back and forth between two mirrored surfaces. One mirror is only partially reflective, and light energy is transmitted through it to form the light beam. You will find that power sources for lasers include flash tubes or, in the case of diode-type lasers, dc power supplies.
Figure 5-18.—Simple laser.
High-energy lasers have very small power losses. As a result, a laser with a 2- or 3-inch initial beam can be used to communicate directly with a distant planet. However, from a communications standpoint, they do have some drawbacks. While laser energy is capable of being formed into a very narrow beam, it is still subject to the same difficulties as any other form of light transmission. Fog and other adverse weather conditions can absorb the light. Small atmospheric temperature variations can cause deflection and scattering. An optical waveguide was designed to help overcome this problem. It consists of a thin dielectric fiber clothed by another dielectric coating several wavelengths thick. It has been successfully used to move the beam over considerable distances and around bends. The use of optical waveguides is known as FIBER OPTICS. A number of fibers can be paralleled to reduce the attenuation through the waveguide. This technique has resulted in an inexpensive telephone system with a bandwidth greater than that of previous methods. Researchers are attempting to develop a laser that will operate in the blue-green portion of the visible spectrum. Water offers little attenuation to the blue-green band of frequencies. Because of this, blue-green communications lasers could possibly penetrate the ocean to great depths.
This could give us a very secure undersea communications link.
Q10. Lasers operate in what portion of the frequency spectrum?
Q11. What are the two types of lasers?
Q12. What are the effects of adverse weather on the laser beam?
Now that you have completed this chapter, a review of what you have learned is in order. The following summary will refresh your memory of new terms.
DOUBLING UP is a type of two-equipment installation where one unit can be substituted for another in the event of failure.
FREQUENCY SYNTHESIZER is a frequency source of high accuracy.
DUMMY LOAD is a nonradiating device that absorbs the RF and has the impedance characteristics of the antenna.
HELIX HOUSE is a building at a transmitter site that contains antenna loading, coupling, and tuning circuits.
A HELIX is a large coil of wire. It acts as a coil and is used with variable inductors for impedance matching of high-power transmitters.
TOP-HAT antennas are center-fed and capacitively loaded.
TRIATIC is a special type of monopole antenna array.
LASER is an acronym for light amplification by stimulated emission of radiation.
COHERENT refers to radiation on one frequency or nearly so.
INCOHERENT refers to radiation on a broad band of frequencies.
FIBER OPTICS are conductors or optical waveguides that readily pass light.
MTDS is an abbreviation for the marine tactical data system.
AFDS is an abbreviation for the amphibious flagship data system.
ATDS is an abbreviation for the airborne tactical data system.
ASWTDS is an abbreviation for the antisubmarine warfare tactical data system.
ANSWERS TO QUESTIONS Q1. THROUGH Q12.
A1. An impedance-matched device capable of absorbing all of a transmitters power.
A2. It demodulates SSB, CW, and FSK signals.
A3. 300 kilohertz to 3 megahertz.
A5. Up to 600 4-kilohertz channels.
A6. Up to 800 kilometers (500 statute miles).
A8. Links 4A, 11, and 14.
A9. They must be lightweight, compact, and self-contained.
A10. At or near visible light.
A11. CW or pulsed.
A12. It absorbs it.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics