NEETS 11 : MICROWAVE PRINCIPLES Flashcards
(155 cards)
1
Q
What is the region of the frequency spectrum from 1000
MHz to 100,000 MHz called?
A
Microwave region
2
Q
Microwave theory is based upon what concept
A
Electromagnetic field theory
3
Q
Why are coaxial lines more efficient at microwave
frequencies than two-wire transmission lines?
A
The electromagnetic fields are completely confined
4
Q
What kind of material must be used in the construction
of waveguides?
A
Conductive material
5
Q
The large surface area of a waveguide greatly reduces
what type of loss that is common in two-wire and coaxial
lines?
A
Copper loss
6
Q
What causes the current-carrying area at the center
conductor of a coaxial line to be restricted to a small layer
at the surface?
A
Skin effect
7
Q
What is used as a dielectric in waveguides?
A
Air
8
Q
What is the primary lower-frequency limitation of
waveguides?
A
Physical size
9
Q
At very high frequencies, what characteristics are
displayed by ordinary insulators?
A
The characteristics of the dielectric of a capacitor
10
Q
What type of insulator works well at very high
frequencies?
A
A shorted quarter-wave section called a metallic insulator
11
Q
The frequency range of a waveguide is determined by what
dimensison?
A
The “a” dimension
12
Q
What happens to the bus bar dimensions of the waveguide
when the frequency is increased?
A
The bus bar becomes wider
13
Q
When the frequency is decreased so that two
quarter-wavelengths are longer than the “a” (wide)
dimension of the waveguide, what will happen?
A
Energy will no longer pass through the waveguide.
14
Q
What interaction causes energy to travel down a
waveguide?
A
The interaction of the electric and magnetic fields
15
Q
What is indicated by the number of arrows (closeness
of spacing) used to represent an electric field?
A
The relative strength of the field
16
Q
What primary condition must magnetic lines of force meet
in order to exist?
A
Magnetic lines of force must form a continuous closed loop
17
Q
What happens to the H lines between the conductors of
a coil when the conductors are close together?
A
The H lines cancel
18
Q
For an electric field to exist at the surface of a
conductor, the field must have what angular relationship
to the conductor?
A
The field must be perpendicular to the conductors
19
Q
When a wavefront is radiated into a waveguide, what
happens to the portions of the wavefront that do not satisfy
the boundary conditions?
A
Decrease to zero
20
Q
Assuming the wall of a waveguide is perfectly flat, what
is the angular relationship between the angle of incidence
and the angle of reflection?
A
The angles are equal
21
Q
What is the frequency called that produces angles of
incidence and reflection that are perpendicular to the
waveguide walls?
A
Cutoff frequency
22
Q
Compared to the velocity of propagation of waves in air,
what is the velocity of propagation of waves in waveguides?
A
Slower
23
Q
What term is used to identify the forward progress
velocity of wavefronts in a waveguide?
A
Group velocity
24
Q
What term is used to identify each of the many field
configurations that can exist in waveguides?
A
Mode of operation
25
What field configuration is easiest to produce in a
| given waveguide?
Mode of operation
26
How is the cutoff wavelength of a circular waveguide
| figured?
1.71 times the diameter
27
The field arrangements in waveguides are divided into
what two categories to describe the various modes of
operation?
Transverse electric (TE) and transverse magnetic (TM)
28
The electric field is perpendicular to the "a" dimension
| of a waveguide in what mode?
TE
29
The number of half-wave patterns in the "b" dimension
of rectangular waveguides is indicated by which of the two
descriptive subscripts?
Second
30
Which subscript, in circular waveguide classification,
indicates the number of full-wave patterns around the
circumference?
First
31
What determines the frequency, bandwidth, and
| power-handling capability of a waveguide probe?
Size and shape
32
Loose or inefficient coupling of energy into or out of
| a waveguide can be accomplished by the use of what method?
Slots and apertures
33
What is the result of an impedance mismatch in a
| waveguide?
Standing waves that cause power losses, a reduction in
power-handling capability, and an increase in frequency and
sensitivity
34
What is used to construct irises?
Metal plates
35
An iris placed along the "b" dimension wall produces
| what kind of reactance?
Inductive
36
How will an iris that has portions along both the "a"
| and "b" dimension walls act at the resonant frequency?
As a shunt resistance
37
What device is used to produce a gradual change in
| impedance at the end of a waveguide?
Horn
38
When a waveguide is terminated in a resistive load, the
| load must be matched to what property of the waveguide?
Characteristic impedance
39
What is the primary purpose of a dummy load?
Absorb all energy without producing standing waves
40
The energy dissipated by a resistive load is most often
| in what form?
Heat
41
What is the result of an abrupt change in the size,
| shape, or dielectric of a waveguide?
Reflections
42
A waveguide bend must have what minimum radius?
Greater than 2 wavelengths
43
What is the most common type of waveguide join?
Choke joint
44
What is the most likely cause of losses in waveguide
| systems?
Improperly connected joints or damaged inner surface
45
What is the primary purpose of a directional coupler?
Sampling energy within a waveguide
46
How far apart are the two holes in a simple directional
| coupler?
1/4 wavelength
47
What is the purpose of the absorbent material in a directional coupler?
Absorb the energy not directed at the pick-up probe and a
| portion of the overall energy
48
In a directional coupler that is designed to sample the
incident energy, what happens to the two portions of the
wavefront when they arrive at the pickup probe?
The wavefront portions add
49
What happens to reflected energy that enters a
directional coupler that is designed to sample incident
energy?
The reflected energy adds at the absorbent material and is
| absorbed
50
What two variables determine the primary frequency of
| a resonant cavity?
Size and shape of the cavity
51
Energy can be inserted or removed from a cavity by what
| three methods?
Probes, loops, and slots
52
Inductive tuning of a resonant cavity is accomplished
| by placing a nonmagnetic slug in what area?
The area of maximum H lines
53
What are the two basic types of T junctions?
E-type and H-type
54
Why is the H-type T junction so named?
The junction arm extends in a direction parallel to the H
| lines in the main waveguide
55
The magic-T is composed of what two basic types of T
| junctions?
E-type and H-type
56
What are the primary disadvantages of the magic-T?
Low power-handling capability and power losses
57
What type of junctions are formed where the arms of a
| hybrid ring meet the main ring?
Basic E-type junctions
58
Hybrid rings are used primarily for what purpose?
High-power duplexes
59
Ferrite devices are useful in microwave applications
| because they possess what properties?
Magnetic properties and high resistance
60
Which of the two types of electron motion (orbital
movement and electron spin) is more important in the
explanation of magnetism?
Electron spin
61
The interaction between an external field and the
| binding force of an atom causes electrons to do what?
Wobble at a natural resonant frequency.
62
The resonant frequency of electron wobble can be changed
| by variation of what force?
The applied magnetic field
63
Rotating the plane of polarization of a wavefront by
| passing it through a ferrite device is called what?
Faraday rotation
64
What happens to the impedance of interelectrode
| capacitance as frequency increases?
Impedance decreases
65
What undesirable effect is caused by the inductance of
| the cathode lead?
Degenerative feedback
66
How does transit time affect the relationship of the grid
| voltage and the plate current at high frequencies?
Transit time causes the grid voltage and plate current to
| be out of phase
67
Moving tube electrodes apart to decrease interelectrode
capacitance causes an increase in the effect of what
property?
Transit time
68
The kinetic energy of an electron is directly
| proportional to what property?
Velocity
69
What will be the effect upon an electron traveling in
the opposite direction to the lines of force in an
electrostatic field?
The electron will be accelerated
70
How is a beam of electrons velocity-modulated?
By alternately speeding up or slowing down the electrons
71
What portion of an electron gun causes the electrons to
| accelerate or decelerate?
The buncher grids
72
What is the effect upon an electron that enters the
buncher gap when the potential across the grids is at 0
volts?
There is no effect
73
What determines the placement of the catcher cavity?
The frequency period of the buncher grid signal
74
What is the basic principle of operation of a klystron?
Velocity modulation
75
The electrons in the beam of a klystron are speeded up
| by a high dc potential applied to what elements?
The accelerator grid and the buncher grids
76
The two-cavity klystron uses what cavity as an output
| cavity?
The catcher cavity
77
A two-cavity klystron without a feedback path will
| operate as what type of circuit?
Amplifier
78
What can be added to the basic two-cavity klystron to
increase the amount of velocity modulation and the power
output?
Intermediate cavities between the input and output
| cavities
79
How is the electron beam of a three-cavity klystron
| accelerated toward the drift tube?
A large negative pulse ?is applied to the cathode
80
Which cavity of a three-cavity klystron causes most of
| the velocity modulation
The middle cavity
81
In a multicavity klystron, tuning all the cavities to
the same frequency has what effect on the bandwidth of the
tube?
The bandwidth decreases
82
The cavities of a multicavity klystron are tuned to
| slightly different frequencies in what method of tuning?
Stagger tuning
83
What element of the reflex klystron replaces the output
| cavity of a normal klystron?
The reflector or repeller
84
When the repealer potential is constant, what property
of the electron determines how long it will remain in the
drift space of the reflex klystron?
Velocity
85
The constant-speed electrons of an electron bunch in
a reflex klystron must remain in the repeller field for what
minimum time?
Three-quarter cycle
86
If the constant-speed electrons in a reflex klystron
remain in the repeller field for 1 3/4 cycles, what is the
mode of operation?
Mode 2
87
Debunching of the electron bunches in the higher modes
of a reflex klystron has what effect on
output power?
Power is reduced
88
What limits the tuning range around the center frequency
| of a reflex klystron in a particular mode of operation?
The half-power points of the mode
89
What is the primary use of the twt?
Voltage amplification
90
The magnet surrounding the body of a twt serves what
| purpose?
Used to focus the electrons into a tight beam
91
How are the input and output directional couplers in
| a twt connected to the helix?
The directional couplers are not physically connected to
| the helix
92
What relationship must exist between the electron beam
and the traveling wave for bunching to occur in the electron
beam of a twt?
The traveling wave must have a forward velocity equal to
| or less than the speed of the electrons in the beam.
93
What structure in the twt delays the forward progress
| of the traveling wave?
The helix
94
The folded waveguide in a bwo serves the same purpose
| as what component in a twt?
Helix
95
What serves as a grid in a magnetron?
A magnetic field
96
A cylindrical copper block with resonant cavities
around the circumference is used as what component of a
magnetron?
Anode or plate
97
What controls the output frequency of a magnetron?
The resonant cavities
98
What element in the magnetron causes the curved path
| of electron flow?
The permanent magnet
99
What is the term used to identify the amount of field
strength required to cause the electrons to just miss the
plate and return to the filament in a circular orbit?
The critical value of field strength
100
A magnetron will produce oscillations when the
| electrons follow what type of path?
Circular
101
What is the primary difference in construction between
| the basic magnetron and the negativeresistance magnetron?
The negative-resistance magnetron has a split plate
102
What starts the oscillations in a negative-resistance
| magnetron?
The application of the proper magnetic field
103
Why is the negative-resistance magnetron often operated
| with reduced filament voltage?
To reduce the effects of filament bombardment
104
What type of electron-resonance anode block does not
| require strapping?
Rising-sun block
105
Without strapping, the resonant cavities of a
| hole-and-slot anode are connected in what manner?
Series
106
What are the electrons called that give up energy to
| the ac field in a magnetron?
Working electrons
107
Why is the pi mode the most commonly used magnetron mode
| of operation?
Greater power output
108
What two methods are used to couple energy into and out
| of magnetrons?
Loops and slots
109
Magnetron tuning by altering the surface-to-volume
ratio of the hole portion of a hole-and-slot cavity is what
type of tuning?
Inductive
110
Capacitive tuning by inserting a ring into the cavity
slot of a magnetron is accomplished by what type of tuning
mechanism?
A cookie-cutter tuner
111
Name the procedure used to reduce excessive arcing in
| a magnetron?
Baking in
112
What causes the negative-resistance property of tunnel
| diodes?
The tunneling action
113
What determines the frequency of a tunnel-diode
| oscillator?
The tuned circuit or cavity frequency
114
Why is the tunnel diode loosely coupled to the cavity
| in a tunnel-diode oscillator?
To increase the stability
115
What is the purpose of the circulator in a tunnel-diode
| amplifier?
Prevent feedback to the tuned input circuit
116
What limits the usefulness of high-gain, tunnel-diode
| frequency converters?
Stability problems
117
The varactor is a pn junction that acts as what type
| of electronic device?
Variable capacitor
118
The underlying principle of operation of the parametric
| amplifier is based on what property?
Reactance
119
What is the most important feature of the parametric
| amplifier?
The low-noise characteristic
120
How is amplification achieved in the circuit shown in
| figure 2-43?
By varying the amount of capacitance in the circuit
121
What is the purpose of the pump in a parametric
| amplifier?
Supplies the electrical energy required to vary the
| capacitance
122
The pump signal frequency must be of what value when
compared to the input signal of a simple parametric
amplifier?
Exactly double the input frequency
123
What is the primary difference between the pump signal
of a simple parametric amplifier and the pump signal of a
nondegenerative parametric amplifier?
The pump signal of a nondegenerative parametric amplifier
| is higher than twice the input signal.
124
In a nondegenerative parametric amplifier the
difference between the input frequency and the pump
frequency is called what?
Idler- or lower-sideband frequency
125
What is the output frequency of an upper-sideband
| parametric-frequency converter?
The sum of the input frequency and the pump frequency
126
What is the primary advantage of bulk-effect devices
| over normal pn-junction semiconductors?
Larger microwave power outputs
127
What happens to the electrons of a gallium-arsenide
semiconductor when they move from the normal low-energy
conduction band to the high-energy conduction band?
The electrons become immobile
128
The point on the current curve of a gallium-arsenide
semiconductor at which it begins to exhibit negative
resistance is called what?
Threshold
129
The domain in a gallium-arsenide semiconductor has what
type of electrical field when compared to the other regions
across the body of a semiconductor?
A field of much greater intensity
130
What characteristic of a gunn oscillator is inversely
proportional to the transit time of the domain
across the semiconductor?
The frequency
131
What is the junction arrangement of the original
| avalanche transit-time diode?
Pnin
132
What causes dc bias energy to be absorbed by avalanche
electrons and given up to the microwave field applied to
an avalanche transit-time diode?
The negative-resistance property
133
During the manufacture of a point-contact diode, what
is the purpose of passing a relatively large current from
the catwhisker to the silicon crystal?
To form a small region of p-type material
134
What is the capacitive reactance across a point-contact
| diode as compared to a normal junction diode?
Lower
135
What are the most important advantages of the Schottky
| barrier diode?
Lower forward resistance and low noise
136
At frequencies above 100 megahertz, the intrinsic (i)
| region causes a pin diode to act as what?
Variable resistance
137
The pin diode is primarily used for what purpose?
A switching device
138
Microwave antennas and low-frequency antennas are
| similar in what ways?
Operating principles and electrical characteristics
139
What term is used to express the efficiency of an antenna?
Power gain or power ratio
140
What term is used to express the measurement of the degree
| of mismatch between a line and its load?
Standing-wave ratio (swr)
141
What type of antenna radiates in and receives energy from
| all directions at once?
Omnidirectional
142
What is the term that is used to describe narrowness in
| the radiated beam of an antenna?
Antenna directivity
143
What characteristic allows the same antenna to both
| transmit and receive?
Reciprocity
144
What type of reflector is most often used in directive
| antennas?
Parabolic
145
Microwaves can be focused and reflected in the same way
| as what other type of waves?
Light waves
146
How many major lobes are radiated by a parabolic
| reflector?
One
147
A horizontally truncated paraboloid antenna is used for
| what purpose?
Determine elevation
148
The beam from a horizontally positioned cylindrical
| paraboloid is narrow in what plane?
Vertical
149
What is the purpose of a collimating lens?
Forces the radial segments of a wavefront into parallel
| paths.
150
How does a waveguide-type lens focus spherical
| wavefront microwave energy?
Some wavefronts are accelerated so that all wavefronts exit
| the lens at the same time.
151
What type of lens decelerates a portion of a spherical
| wavefront?
Delay lens
152
What is a set of antenna elements called?
Antenna Array
153
What type of antenna has all elements connected to the
| same energy source?
Driven Array
154
What determines the beam elevation angle of an antenna
| that is electronically scanned in elevation?
Frequency or phase of radiated energy
155
What is the polarization of the energy radiated by a
| vertical slot?
Horizontal
