NEETS Module 17 — Radio-Frequency Communications Principles
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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.
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
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.
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
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?
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.
transfer receiver audio outputs to remote control station audio circuits.
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
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
A CODE in teletypewriter operation is a combination of mark and space
conditions representing symbols, figures, or letters.
operation is when both transmitter and receiver do not operate continuously.
teletypewriter operation is when both transmitter and receiver operate continuously.
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.
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.
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.
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
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
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.
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.
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
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.
It matches the impedance of an antenna to that of a transmission line at any desired frequency.
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
A11. Patching and filtering and permits the multiple use of receivers and/or transmitters on a single
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.
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.
It allows simultaneous transmission of multiple signals on a single transmission path.
A32. Compromising emanations.
A33. Reliability, security, and
A35. To ensure continuous, optimum performance of
A36. Electromagnetic interference.
A38. RF burns and biological, thermal, and neurological effects.
A39. Proper bonding and grounding.
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 Power Supplies, Introduction
to Amplifiers, Introduction to Wave-Generation
and Wave-Shaping Circuits, Introduction to
Wave Propagation, Transmission Lines, and Antennas,
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