NEETS 11 CH 1,2 Flashcards
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?
Dominant mode
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 joint?
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?
-type and H-type. 1-68
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 effec
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.2-64
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 heli
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.2-66
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.
