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Radio Wave Propagation

NAVEDTRA 14182, NEETS, MODULE 10--INTRODUCTION TO WAVE PROPAGATION, TRANSMISSION LINES, AND ANTENNAS, CHAP 2, 4
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Which two composite fields (composed of E and H fields) are associated with every antenna?
Induction field and radiation field.
What composite field (composed of E and H fields) is found stored in the antenna?
Induction field.
What composite field (composed of E and H fields) is propagated into free space?
Radiation field
What is the term used to describe the basic frequency of a radio wave?
fundamental frequency
What is the term used to describe a whole number multiple of the basic frequency of a radio
wave?
Harmonic frequency or harmonics
It is known that WWV operates on a frequency of 10 megahertz. What is the wavelength of WWV?
30 meters.
A station is known to operate at 60-meters. What is the frequency of the unknown station?
5 megahertz.
If a transmitting antenna is placed close to the ground, how should the antenna be polarized to give the greatest signal strength?
Vertically polarized.
In the right-hand rule for propagation, the thumb points in the direction of the E field and the forefinger points in the direction of the H field. In what direction does the middle finger point?
Direction of wave propagation.
What is one of the major reasons for the fading of radio waves which have been reflected from a
surface?
Shifting in the phase relationships of the wave.
What are the three layers of the atmosphere?
Troposphere, stratosphere, and ionosphere.
Which layer of the atmosphere has relatively little effect on radio waves?
Stratosphere.
What is the determining factor in classifying whether a radio wave is a ground wave or a space wave?
Whether the component of the wave is travelling along the surface or over the surface of the earth.
What is the best type of surface or terrain to use for radio wave transmission?
Radio horizon is about 1/3 farther.
What is the primary difference between the radio horizon and the natural horizon?
Sea water.
What three factors must be considered in the transmission of a surface wave to reduce attenuation?
a) electrical properties of the terrain (b) frequency (c) polarization of the antenna
What causes ionization to occur in the ionosphere?
High energy ultraviolet light waves from the sun.
How are the four distinct layers of the ionosphere designated?
D, E, F1, and F2 layers.
What is the height of the individual layers of the ionosphere?
D layer is 30-55 miles, E layer 55-90 miles, and F layers are 90-240 miles.
What factor determines whether a radio wave is reflected or refracted by the ionosphere?
Thickness of ionized layer.
There is a maximum frequency at which vertically transmitted radio waves can be refracted back
to Earth. What is this maximum frequency called?
Critical frequency.
What three main factors determine the amount of refraction in the ionosphere?
(a) density of ionization of the layer (b) frequency (c) angle at which it enters the layer
What is the skip zone of a radio wave?
A zone of silence between the ground wave and sky wave where there is no reception.
Where does the greatest amount of ionospheric absorption occur in the ionosphere?
Where ionization density is greatest.
What is meant by the term "multipath"?
A term used to describe the multiple pattern a radio wave may follow.
When a wide band of frequencies is transmitted simultaneously, each frequency will vary in the
amount of fading. What is this variable fading called?
Selective fading.
What are the two main sources of emi with which radio waves must compete?
Natural and man-made interference.
Thunderstorms, snowstorms, cosmic sources, the sun, etc., are a few examples of emi sources What type of emi comes from these sources?
Natural
Motors, switches, voltage regulators, generators, etc., are a few examples of emi sources. What
type of emi comes from these sources?
Man-made.
What are three ways of controlling the amount of transmitter-generated emi?
(a) filtering and shielding of the transmitter (b) limiting bandwidth (c) cutting the antenna to the
correct frequency
What are three ways of controlling radiated emi during transmission?
(a) physical separation of the antenna (b) limiting bandwidth of the antenna (c) use of directional
antennas
What are the two general types of variations in the ionosphere?
Regular and irregular variations.
What is the main difference between these two types of variations?
Regular variations can be predicted but irregular variations are unpredictable.
What are the four main classes of regular variation which affect the extent of ionization in the ionosphere?
Daily, seasonal, 11-year, and 27-days variation.
What are the three more common types of irregular variations in the ionosphere?
Sporadic E, sudden disturbances, and ionospheric storms.
What do the letters muf, luf, and fot stand for?
Muf is maximum usable frequency. Luf is lowest usable frequency. Fot is commonly known as
optimum working frequency.
When is muf at its highest and why?
Muf is highest around noon. Ultraviolet light waves from the sun are most intense.
What happens to the radio wave if the luf is too low?
When luf is too low it is absorbed and is too weak for reception.
What are some disadvantages of operating transmitters at or near the luf?
Signal-to-noise ratio is low and the probability of multipath propagation is greater.
What are some disadvantages of operating a transmitter at or near the muf?
Frequent signal fading and dropouts.
What is fot?
Fot is the most practical operating frequency that can be relied on to avoid problems of multipath, absorbtion, and noise.
How do raindrops affect radio waves?
They can cause attenuation by scattering.
How does fog affect radio waves at frequencies above 2 gigahertz?
It can cause attenuation by absorbtion.
How is the term "temperature inversion" used when referring to radio waves?
It is a condition where layers of warm air are formed above layers of cool air.
How does temperature inversion affect radio transmission?
It can cause vhf and uhf transmission to be propagated far beyond normal line-of-sight distances.
In what layer of the atmosphere does virtually all weather phenomena occur?
Troposphere.
Which radio frequency bands use the tropospheric scattering principle for propagation of radio
waves?
Vhf and above.
Where is the tropospheric region that contributes most strongly to tropospheric scatter propagation?
Near the mid-point between the transmitting and receiving antennas, just above the radio horizon.
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