NEETS Module 17 - Radio-Frequency Communications Principles
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2-21 to 2-30,
2-31 to 2-37,
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manner that allows RF current to flow through the area of contact. Resistance of the skin to the current flow at the areas of contact causes heat. The effect of the heat on a person at the point of contact ranges from noticeable warmth to a painful burn.
The most useful and widespread technique in the reduction of RF burn hazards is the proper bonding and grounding of all metallic objects in the RF radiation field.
In some cases, the RF burn hazard can be eliminated only through the use of restrictive operating procedures. These procedures govern the simultaneous use of transmitting and cargo equipment. Techniques such as operation of transmitters at reduced power and the prohibition of simultaneous use of certain combinations of antennas, frequencies, and cargo handling equipment are used.
Figure 3-35 shows typical RF radiation hazard warning signs.
Figure 3-35. - Typical RF radiation hazard warning signs.
Most studies on the subject of radiation hazards (RADHAZ) have emphasized the impact of electromagnetic radiation on man. Man is singled out because of the biological, thermal, and neurological effects that occur in human organs and other biological tissues. Certain organs of the body are considered to be more susceptible than others to the effects of electromagnetic radiation. Presently available information and experience indicate that the eyes and testes are the most vulnerable body organs. The overwhelming danger to date appears to be the hazard from thermal effects, which are a function of intensity of radiation and frequency. This is particularly true in the range of 1 to 3 gigahertz. Thermal effects appear to taper off in severity outside this range.
When the body is irradiated by energy from a point source, the total body surface is usually not exposed. The larger the area exposed and the larger the radiation power density, the higher the body temperature rise and the greater the hazard. Microwave radiation from a radar source will "cook" you internally, just as a microwave oven cooks a chicken.
An injury of great concern is that to the lens of the eye. Exposure of the lens to high-intensity microwaves may cause cataracts. Current medical evidence indicates that a significant temperature elevation of the lens is required for cataract formation. If exposure is limited to 10 milliwatts per centimeter squared, the lens temperature is not elevated to levels at which cataracts occur.
In addition to thermal effects, nonionizing radiation is known to produce nonthermal effects. An association of a biological hazard with the nonthermal effects has not been demonstrated.
A peculiar effect experienced by some personnel is the sensation of sound when they are exposed to pulsed microwave fields. This occurs at levels below stated hazard limits and is not, by itself, considered dangerous.
Q38. Electromagnetic radiation is hazardous to personnel in what two ways?
Q39. What is the most useful and widespread technique to reduce RF burn hazards?
Now that you have completed this chapter, a short review of what you have learned is in order. The following summary will refresh your memory of basic systems equipment, its principles, terms, and typical circuitry required for you to understand this concept.
A RADIO SET CONTROL UNIT is used to remotely control certain transmitter and receiver functions.
TRANSMITTER TRANSFER SWITCHBOARDS selectively transfer remote control station functions and signals to transmitters.
RECEIVER TRANSFER SWITCHBOARDS transfer receiver audio outputs to remote control station audio circuits.
A TRANSMITTER generates an RF carrier, modulates it with intelligence, amplifies it, and applies it to an antenna.
An ANTENNA COUPLER is a device used for impedance matching between an antenna and a transmitter or receiver.
A RECEIVER receives electromagnetic energy (RF) and may convert it to a visible or audible form.
MULTICOUPLERS patch several receivers or transmitters to one antenna. They also filter out harmonics and spurious responses, and provide impedance matching.
MARKING is when a circuit is closed and current flows in teletypewriter operation. SPACING is when a circuit is open and no current flows in teletypewriter operation. INTELLIGENCE is any signal that conveys information (voice, teletypewriter, facsimile). A START unit is the first unit of a teletypewriter signal. It is always a space.
A STOP unit is the last unit of a teletypewriter signal. It is always a mark.
A TRANSITION is the time it takes to shift from a mark to a space condition or from a space to a mark condition.
A CODE in teletypewriter operation is a combination of mark and space conditions representing symbols, figures, or letters.
NONSYNCHRONOUS teletypewriter operation is when both transmitter and receiver do not operate continuously.
SYNCHRONOUS teletypewriter operation is when both transmitter and receiver operate continuously.
WORDS-PER-MINUTE is an approximate rate of speed. It means the number of five letter words with a space between them that can be transmitted or received in a one-minute period.
BAUD is a measurement of speed based on the number of code elements or units per second.
BITS-PER-SECOND is an acronym of the words binary digit. One bit is equal to one signal unit or element.
NEUTRAL teletypewriter operation is where current flow represents a mark and no flow represents a space.
POLAR teletypewriter operation is where current flow of one polarity represents a mark and current of the opposite polarity is a space.
RUNNING OPEN is the teletypewriter condition where the type hammer constantly strikes the type box but does not print or move across the page.
A KEYER is a device that changes dc pulses to mark and space modulation for teletypewriter transmissions.
A CONVERTER changes an audio signal back to dc pulses during teletypewriter reception.
AUDIO FREQUENCY TONE SHIFT systems use amplitude modulation to change dc mark and space impulses into audio impulses.
RADIO FREQUENCY CARRIER SHIFT systems use a keyer to shift a radio frequency signal above or below an assigned frequency. These shifts correspond to marks and spaces.
A TELETYPEWRITER is a machine that can transmit and or receive letters, numbers, or symbols. It may have a keyboard similar to a typewriter.
A PERFORATOR is a device that stores a teletypewriter message on a paper tape by punching Baudot coded messages into it.
TRANSMITTER DISTRIBUTOR is a device that reads Baudot code from paper tape and allows a message to be sent or a message to be printed on a page printer.
A REPERFORATOR stores an incoming TTY signal on paper tape.
A PAGE PRINTER prints teletypewriter characters one at a time in a full-page format. This is usually a high-speed printer.
RED is the reference color of equipment that passes classified information. It normally refers to patch panels.
BLACK is the reference color of equipment that passes unclassified information. It normally refers to patch panels.
A PATCH PANEL is used to tie a receiver or transmitter to its associated equipment.
A COMPARATOR compares incoming signals and selects the strongest to be fed to a teletypewriter through a patch panel. This is used in diversity operation
A LISSAJOUS PATTERN is a combined, simultaneous display of the amplitude and phase relationships of two input signals on a CRT.
A TONE-TERMINAL set converts TTY dc pulses into audio tones for modulation of a transmitter in audio-frequency tone shift transmissions.
MULTIPLEXING is the process of transmitting a number of intelligence signals simultaneously over a single RF carrier.
TIME-DIVISION multiplexing is the process that periodically samples several intelligence signals. This can be a received signal or one to be transmitted.
FREQUENCY-DIVISION multiplexing transmits and receives the full 360 degrees of each sine wave.
FACSIMILE is the method for transmitting and receiving still images. These images can be maps, photographs, and handwritten or printed text.
SCANNING is the process of subdividing a picture in an orderly manner into segments. This is used in facsimile transmission.
FRAMING is the process of synchronizing a facsimile receiver to a transmitter. This allows proper picture reproduction.
TEMPEST is a term normally used to describe compromising emanations. These emanations are unintentionally radiated signals that could disclose classified information.
ELECTROMAGNETIC INTERFERENCE is a term used to describe the degradation of a receiver or system by externally produced RF energy.
ANSWERS TO QUESTIONS Q1. THROUGH Q39.
A1. To convert energy electrical/acoustic to acoustic/electrical and to key/unkey a transmitter. Also it mutes a receiver when transmitting.
A2. Transferring remote control functions and signals to transmitters.
A3. Transfers receiver audio outputs to remote control stations.
A4. 800 watts.
A5. Automatic, semiautomatic, and manual.
A6. It matches the impedance of an antenna to that of a transmission line at any desired frequency.
A7. To aid in heat transfer and prevent corona and arcing.
A8. LSB, USB, ISB, AM, CW, FSK.
A10. To connect an antenna/transmission line to a receiver/transmitter.
A11. Patching and filtering and permits the multiple use of receivers and/or transmitters on a single antenna.
A12. Space and mark.
A13. Intelligence (5), start (1), stop (1).
A14. Shift signals.
A15. Synchronous and nonsynchronous.
A16. A unit of modulation rate.
A17. Binary digit.
A18. Neutral and polar.
A19. Converts dc to corresponding mark and space modulation.
A20. Converts the audio signal to dc pulses.
A21. Uses AM to change dc to audio.
A22. A keyer provides RF excitation, which can be shifted above or below the assigned frequency.
A23. 60, 75, or 100 wpm.
A24. Page-size copy paper and perforated tape.
A25. It handles classified information.
A26. To code or decode messages.
A27. The comparator compares the signal strengths from the receivers and the converter converts the frequency-shift RF signal into a TTY set dc loop control signal.
A28. It converts dc to audio or vice versa.
A29. Time-division and frequency-division.
A30. It allows simultaneous transmission of multiple signals on a single transmission path.
A32. Compromising emanations.
A33. Reliability, security, and speed.
A35. To ensure continuous, optimum performance of communications systems.
A36. Electromagnetic interference.
A37. Conduction and radiation.
A38. RF burns and biological, thermal, and neurological effects.
A39. Proper bonding and grounding.
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction 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
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to 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