NEETS 10 Wave Propagation Flashcards
What is propagation?
Propagation means spreading out.
How is a wave defined as it applies to wave
propagation?
A wave is a disturbance which moves through a medium
What is wave motion?
A means of transferring energy from one place to
another.
What are some examples of wave motion?
Sound waves, light waves, radio waves, heat waves, water waves
What type of wave motion is represented by the motion
of water?
Transverse waves
What are some examples of transverse waves?
Radio waves, light waves, and heat waves
What example of a longitudinal wave was given in the text?
A sound wave.
What are the three requirements for a wave to be propagated?
A source, medium, and detector (receiver).
What is a cycle?
A sequence of events, such as the positive and negative
alternation of electrical current
What is wavelength (λ)?
The space occupied by one cycle of a radio wave at any
given instant.
What is the law of reflection
The law of reflection states: The angle of incidence is
equal to the angle of reflection.
When a wave is reflected from a surface, energy is
transferred. When is the transfer of energy greatest?
When the incident wave is nearly parallel with the
surface
When is the transfer of energy minimum?
When the incident wave is perpendicular to the surface.
Also a dull (or black) surface reflects very little regardless of the angle
A refracted wave occurs when a wave passes from one medium into another medium. What determines the angle of refraction?
The density of the two mediums, and the velocity of the waves
The apparent change in frequency or pitch because
of motion is explained by what effect?
The Doppler Effect
What term describes sounds capable of being heard
by the human ear?
Sonics
Are all sounds audible to the human ear? Why?
Sonics.
Actual Answer:
No, the ave human ear cannot hear all sounds in the infrasonic and ultrasonic regions
Sound waves transmitted from a source are sometimes weak when they reach the detector. What instrument is needed to boost the weak signal?
An Amplifier
What are the three basic requirements for sound?
A source, medium, and detector (receiver).
What are the two general groups of sound?
Noise and tones
What are the three basic characteristics of sound
Pitch, intensity, and quality
What is the normal audible range of the human ear
20 Hz to 20 kHz.
What is intensity as it pertains to sound?
The amount of energy transmitted from a source.
What characteristic of sound enables a person to
distinguish one musical instrument from another, if
they are all playing the same note?
Quality.
How does density and temperature affect the velocity of sound?
Velocity increases as density decreases and temperature increases.
What term is used in describing the science of sound?
Acoustics.
A sound wave that is reflected back toward the source is known as what type of sound?
Echo
What is the term for multiple reflections of sound waves?
Reverberation.
A cavity that vibrates at its natural frequency
produces a louder sound than at other frequencies.
What term is used to describe this phenomenon?
Resonance
What do we call a disturbance that distracts or
distorts the quality of sound?
Noise
What are three means of producing light?
Mechanical, electrical, and chemical.
What is the smallest unit of radiant energy?
A photon.
What unit is used to measure the different wavelengths of light?
Angstrom
What are the three primary colors of light?
Red, green and blue.
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.
Actual Answer:
Sea Water
What is the primary difference between the radio horizon and the natural horizon?
Sea Water
Actual Answer:
Radio horizon is about 1/3 farther
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.
What connecting link is used to transfer energy from
a radio transmitter to its antenna located on the mast
of a ship?
Transmission line.
What term is used for the end of the transmission line
that is connected to a transmitter?
Input end, generator end, transmitter end, sending end,
and source.
What term is used for the end of the transmission line
that is connected to an antenna?
Output end, receiving end, load end and sink.
List the five types of transmission lines in use
today.
Parallel two-wire, twisted pair, shielded pair, coaxial
line and waveguide.
Name two of the three described uses of a two-wire
open line.
Power lines, rural telephone lines, and telegraph lines.
What are the two primary disadvantages of a two-wire
open line?
High radiation losses and noise pickup.
What type of transmission line is often used to
connect a television set to its antenna?
Twin lead.
What is the primary advantage of the shielded pair?
The conductors are balanced to ground.
What are the two types of coaxial lines in use today?
Air coaxial (rigid) and solid coaxial (flexible).
What is the chief advantage of the air coaxial line?
The ability to minimize radiation losses.
List the three disadvantages of the air coaxial line.
Expensive to construct, must be kept dry, and high
frequency losses limit the practical length of
the line.
List the two common types of waveguides in use today.
Cylindrical and rectangular.
What are the three types of line losses associated
with transmission lines?
Copper, dielectric, and radiation.
Losses caused by skin effect and the I 2 R (power)
loss are classified as what type of loss?
Copper losses.
What types of losses cause the dielectric material
between the conductors to be heated?
Dielectric losses.
What must the physical length of a transmission line
be if it will be operated at 15,000,000 Hz?
20 meters
What are two of the three physical factors that
determine the values of capacitance and inductance of a transmission line?
(1) Type of line used,
(2) dielectric in the line, and
(3) length of line.
A transmission line is said to have distributed
constants of inductance, capacitance, and resistance
along the line. What units of measurement are used to
express these constants?
Inductance is expressed in microhenrys per unit length,
capacitance is expressed in picofarads per unit length,
and resistance is expressed in ohms per unit length
Describe the leakage current in a transmission line
and in what unit it is expressed.
The small amount of current that flows through the
dielectric between two wires of a transmission line and is expressed in micromhos per unit length
All the power sent down a transmission line from a
transmitter can be transferred to an antenna under what optimum conditions?
When the characteristic impedance of the transmission
line and the load impedance are equal.
What symbol is used to designate the characteristic
impedance of a line, and what two variables does it
compare?
Z0 and it is the ratio of E to I at every point along the line.
What is the range of the characteristic impedance
of lines used in actual practice?
Between 50 and 600 ohms.
Two types of waves are formed on a transmission line. What names are given to these waves?
Incident waves from generator to load. Reflected waves from load back to generator.
In figure 3-27, which waveforms on the left have a
resultant wave of zero, and what is indicated by these
waves?
2 and 6 have zero resultant wave and they indicate that
the incident and reflected waves are 180
degrees out of phase at all parts
On an open-ended transmission line, the voltage is always zero at what distance from each end of the line?
One-fourth the distance from each end of the line.
A nonresonant line is a line that has no standing
waves of current and voltage on it and is considered to be flat. Why is this true?
The load impedance of such a line is equal to Z0.
On an open line, the voltage and impedance are maximum at what points on the line?
Even quarter-wave points(1/2λ,1λ,3/2λ, etc)
At what point on an open-circuited rf line do voltage
peaks occur?
At 1/2 wavelength from the end and at every 1/2
wavelength along the line.
What is the square of the voltage standing-wave
ratio called?
Power standing-wave ratio (pswr).
What does vswr measure?
The existence of voltage variations on a line
What are the two basic classifications of antennas?
Half-wave (Hertz) and quarter-wave (Marconi).
What are the three parts of a complete antenna system?
Coupling device, feeder, and antenna.
What three factors determine the type, size, and shape of an antenna?
Frequency of operation of the transmitter, amount of
power to be radiated, and general direction of the
receiving set
If a wave travels exactly the length of an antenna
from one end to the other and back during the period of 1 cycle, what is the length of the antenna?
One-half the wavelength.
What is the term used to identify the points of high
current and high voltage on an antenna?
Current and voltage loops.
What is the term used to identify the points of
minimum current and minimum voltage on an antenna?
Current and voltage nodes.
The various properties of a transmitting antenna can
apply equally to the same antenna when it is used as a
receiving antenna. What term is used for this property?
Reciprocity of antennas.
The direction of what field is used to designate the
polarization of a wave?
Electric (E) field.
If a wave’s electric lines of force rotate through
360 degrees with every cycle of rf energy, what is
the polarization of this wave?
Circular polarization.
What type of polarization should be used at medium
and low frequencies?
Vertical polarization.
What is an advantage of using horizontal
polarization at high frequencies?
Less interference is experienced by man-made noise sources.
What type of polarization should be used if an
antenna is mounted on a moving vehicle at frequencies below 50 megahertz?
Vertical polarization.
What is the radiation resistance of a half-wave
antenna in free space?
73 ohms.
A radiating source that radiates energy stronger in
one direction than another is known as what type of
radiator?
Anisotropic radiator
A radiating source that radiates energy equally in
all directions is known as what type of radiator?
Isotropic radiator.
A flashlight is an example of what type of radiator?
Anisotropic radiator.
What terms are often used to describe basic half-wave antennas?
Dipole, doublet and Hertz.
If a basic half-wave antenna is mounted vertically,
what type of radiation pattern will be produced?
Nondirectional.
In which plane will the half-wave antenna be
operating if it is mounted horizontally?
Vertical plane.
Since the radiation pattern of a dipole is similar
to that of a doublet, what will happen to the pattern
if the length of the doublet is increased?
The pattern would flatten.
What is the simplest method of feeding power to the
half-wave antenna?
To connect one end through a capacitor to the final
output stage of the transmitter.
What is the radiation pattern of a quarter-wave
antenna?
A circular radiation pattern in the horizontal plane,
or same as a half wave
Describe the physical arrangement of a ground
screen.
It is composed of a series of conductors arranged in a
radial pattern and buried 1 to 2 feet below
the ground.
What is the difference in the amount of impedance
between a three-wire dipole and a simple center-fed dipole?
Nine times the feed-point impedance.
Which has a wider frequency range, a simple dipole
or a folded dipole?
Folded dipole.
What is the purpose of antenna stubs?
To produce desired phase relationship between connected elements.
What is the primary difference between the major and minor lobes of a radiation pattern?
Major lobes have the greatest amount of radiation
What is the maximum number of elements ordinarily used in a collinear array?
Four.
Why is the number of elements used in a collinear
array limited?
As more elements are added, an unbalanced condition in the system occurs which impairs efficiency
How can the frequency range of a collinear array be
increased?
By increasing the lengths of the elements of the array.
Property of
How is directivity of a collinear array affected
when the number of elements is increased?
Directivity increases.
What is the primary cause of broadside arrays losing
efficiency when not operating at their designed
frequency?
Lower radiation resistance
When more than two elements are used in a broadside array, how are the elements arranged?
Parallel and in the same plane.
As the spacing between elements in a broadside array increases, what is the effect on the major lobes?
They sharpen.
What are some disadvantages of the end-fire array?
Extremely low radiation resistance, confined to one
frequency, and affected by atmospheric conditions.
Where does the major lobe in the end-fire array occur?
Along the major axis
To maintain the required balance of phase
relationships and critical feeding, how must the
end-fire array be constructed?
Symmetrically.
What two factors determine the directivity pattern
of the parasitic array?
What two factors determine the directivity pattern
of the parasitic array?
What two factors determine the directivity pattern
of the parasitic array?
What two factors determine the directivity pattern
of the parasitic array?
The parasitic array can be rotated to receive or
transmit in different directions. What is the name given to such an antenna?
Rotary array
What are the disadvantages of the parasitic array?
Their adjustment is critical and they do not operate
over a wide frequency range.
What is the advantage of adding parasitic elements
to a Yagi array?
Increased gain.
The Yagi antenna is an example of what type of array?
Multielement parasitic array.
To radiate power efficiently, a long-wire antenna
must have what minimum overall length?
One-half wavelength
What is an other name for the Beverage antenna?
Wave antenna.
What is the polarity of the currents that feed the
V antenna?
Opposite.
What is the main disadvantage of the rhombic
antenna?
It requires a large antenna site.
What is the primary reason for the development of
the turnstile antenna?
For omnidirectional vhf communications
