NEETS 11 : MICROWAVE PRINCIPLES Flashcards
What is the region of the frequency spectrum from 1000
MHz to 100,000 MHz called?
Microwave region
Microwave theory is based upon what concept
Electromagnetic field theory
Why are coaxial lines more efficient at microwave
frequencies than two-wire transmission lines?
The electromagnetic fields are completely confined
What kind of material must be used in the construction
of waveguides?
Conductive material
The large surface area of a waveguide greatly reduces
what type of loss that is common in two-wire and coaxial
lines?
Copper loss
What causes the current-carrying area at the center
conductor of a coaxial line to be restricted to a small layer
at the surface?
Skin effect
What is used as a dielectric in waveguides?
Air
What is the primary lower-frequency limitation of
waveguides?
Physical size
At very high frequencies, what characteristics are
displayed by ordinary insulators?
The characteristics of the dielectric of a capacitor
What type of insulator works well at very high
frequencies?
A shorted quarter-wave section called a metallic insulator
The frequency range of a waveguide is determined by what
dimensison?
The “a” dimension
What happens to the bus bar dimensions of the waveguide
when the frequency is increased?
The bus bar becomes wider
When the frequency is decreased so that two
quarter-wavelengths are longer than the “a” (wide)
dimension of the waveguide, what will happen?
Energy will no longer pass through the waveguide.
What interaction causes energy to travel down a
waveguide?
The interaction of the electric and magnetic fields
What is indicated by the number of arrows (closeness
of spacing) used to represent an electric field?
The relative strength of the field
What primary condition must magnetic lines of force meet
in order to exist?
Magnetic lines of force must form a continuous closed loop
What happens to the H lines between the conductors of
a coil when the conductors are close together?
The H lines cancel
For an electric field to exist at the surface of a
conductor, the field must have what angular relationship
to the conductor?
The field must be perpendicular to the conductors
When a wavefront is radiated into a waveguide, what
happens to the portions of the wavefront that do not satisfy
the boundary conditions?
Decrease to zero
Assuming the wall of a waveguide is perfectly flat, what
is the angular relationship between the angle of incidence
and the angle of reflection?
The angles are equal
What is the frequency called that produces angles of
incidence and reflection that are perpendicular to the
waveguide walls?
Cutoff frequency
Compared to the velocity of propagation of waves in air,
what is the velocity of propagation of waves in waveguides?
Slower
What term is used to identify the forward progress
velocity of wavefronts in a waveguide?
Group velocity
What term is used to identify each of the many field
configurations that can exist in waveguides?
Mode of operation
What field configuration is easiest to produce in a
given waveguide?
Mode of operation
How is the cutoff wavelength of a circular waveguide
figured?
1.71 times the diameter
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)
The electric field is perpendicular to the “a” dimension
of a waveguide in what mode?
TE
The number of half-wave patterns in the “b” dimension
of rectangular waveguides is indicated by which of the two
descriptive subscripts?
Second
Which subscript, in circular waveguide classification,
indicates the number of full-wave patterns around the
circumference?
First
What determines the frequency, bandwidth, and
power-handling capability of a waveguide probe?
Size and shape
Loose or inefficient coupling of energy into or out of
a waveguide can be accomplished by the use of what method?
Slots and apertures
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
What is used to construct irises?
Metal plates
An iris placed along the “b” dimension wall produces
what kind of reactance?
Inductive
How will an iris that has portions along both the “a”
and “b” dimension walls act at the resonant frequency?
As a shunt resistance
What device is used to produce a gradual change in
impedance at the end of a waveguide?
Horn
When a waveguide is terminated in a resistive load, the
load must be matched to what property of the waveguide?
Characteristic impedance
What is the primary purpose of a dummy load?
Absorb all energy without producing standing waves
The energy dissipated by a resistive load is most often
in what form?
Heat
What is the result of an abrupt change in the size,
shape, or dielectric of a waveguide?
Reflections
A waveguide bend must have what minimum radius?
Greater than 2 wavelengths
What is the most common type of waveguide join?
Choke joint
What is the most likely cause of losses in waveguide
systems?
Improperly connected joints or damaged inner surface
What is the primary purpose of a directional coupler?
Sampling energy within a waveguide
How far apart are the two holes in a simple directional
coupler?
1/4 wavelength
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
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
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
What two variables determine the primary frequency of
a resonant cavity?
Size and shape of the cavity
Energy can be inserted or removed from a cavity by what
three methods?
Probes, loops, and slots
Inductive tuning of a resonant cavity is accomplished
by placing a nonmagnetic slug in what area?
The area of maximum H lines
What are the two basic types of T junctions?
E-type and H-type
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
The magic-T is composed of what two basic types of T
junctions?
E-type and H-type
What are the primary disadvantages of the magic-T?
Low power-handling capability and power losses
What type of junctions are formed where the arms of a
hybrid ring meet the main ring?
Basic E-type junctions
Hybrid rings are used primarily for what purpose?
High-power duplexes
Ferrite devices are useful in microwave applications
because they possess what properties?
Magnetic properties and high resistance
Which of the two types of electron motion (orbital
movement and electron spin) is more important in the
explanation of magnetism?
Electron spin
The interaction between an external field and the
binding force of an atom causes electrons to do what?
Wobble at a natural resonant frequency.
The resonant frequency of electron wobble can be changed
by variation of what force?
The applied magnetic field
Rotating the plane of polarization of a wavefront by
passing it through a ferrite device is called what?
Faraday rotation
What happens to the impedance of interelectrode
capacitance as frequency increases?
Impedance decreases
What undesirable effect is caused by the inductance of
the cathode lead?
Degenerative feedback
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
Moving tube electrodes apart to decrease interelectrode
capacitance causes an increase in the effect of what
property?
Transit time
The kinetic energy of an electron is directly
proportional to what property?
Velocity
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
How is a beam of electrons velocity-modulated?
By alternately speeding up or slowing down the electrons
What portion of an electron gun causes the electrons to
accelerate or decelerate?
The buncher grids
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
What determines the placement of the catcher cavity?
The frequency period of the buncher grid signal
What is the basic principle of operation of a klystron?
Velocity modulation
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
The two-cavity klystron uses what cavity as an output
cavity?
The catcher cavity
A two-cavity klystron without a feedback path will
operate as what type of circuit?
Amplifier
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
How is the electron beam of a three-cavity klystron
accelerated toward the drift tube?
A large negative pulse ?is applied to the cathode
Which cavity of a three-cavity klystron causes most of
the velocity modulation
The middle cavity
In a multicavity klystron, tuning all the cavities to
the same frequency has what effect on the bandwidth of the
tube?
The bandwidth decreases
The cavities of a multicavity klystron are tuned to
slightly different frequencies in what method of tuning?
Stagger tuning
What element of the reflex klystron replaces the output
cavity of a normal klystron?
The reflector or repeller
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
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
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
Debunching of the electron bunches in the higher modes
of a reflex klystron has what effect on
output power?
Power is reduced
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
What is the primary use of the twt?
Voltage amplification
The magnet surrounding the body of a twt serves what
purpose?
Used to focus the electrons into a tight beam
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
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.
What structure in the twt delays the forward progress
of the traveling wave?
The helix
The folded waveguide in a bwo serves the same purpose
as what component in a twt?
Helix
What serves as a grid in a magnetron?
A magnetic field
A cylindrical copper block with resonant cavities
around the circumference is used as what component of a
magnetron?
Anode or plate
What controls the output frequency of a magnetron?
The resonant cavities
What element in the magnetron causes the curved path
of electron flow?
The permanent magnet
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
A magnetron will produce oscillations when the
electrons follow what type of path?
Circular
What is the primary difference in construction between
the basic magnetron and the negativeresistance magnetron?
The negative-resistance magnetron has a split plate
What starts the oscillations in a negative-resistance
magnetron?
The application of the proper magnetic field
Why is the negative-resistance magnetron often operated
with reduced filament voltage?
To reduce the effects of filament bombardment
What type of electron-resonance anode block does not
require strapping?
Rising-sun block
Without strapping, the resonant cavities of a
hole-and-slot anode are connected in what manner?
Series
What are the electrons called that give up energy to
the ac field in a magnetron?
Working electrons
Why is the pi mode the most commonly used magnetron mode
of operation?
Greater power output
What two methods are used to couple energy into and out
of magnetrons?
Loops and slots
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
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
Name the procedure used to reduce excessive arcing in
a magnetron?
Baking in
What causes the negative-resistance property of tunnel
diodes?
The tunneling action
What determines the frequency of a tunnel-diode
oscillator?
The tuned circuit or cavity frequency
Why is the tunnel diode loosely coupled to the cavity
in a tunnel-diode oscillator?
To increase the stability
What is the purpose of the circulator in a tunnel-diode
amplifier?
Prevent feedback to the tuned input circuit
What limits the usefulness of high-gain, tunnel-diode
frequency converters?
Stability problems
The varactor is a pn junction that acts as what type
of electronic device?
Variable capacitor
The underlying principle of operation of the parametric
amplifier is based on what property?
Reactance
What is the most important feature of the parametric
amplifier?
The low-noise characteristic
How is amplification achieved in the circuit shown in
figure 2-43?
By varying the amount of capacitance in the circuit
What is the purpose of the pump in a parametric
amplifier?
Supplies the electrical energy required to vary the
capacitance
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
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.
In a nondegenerative parametric amplifier the
difference between the input frequency and the pump
frequency is called what?
Idler- or lower-sideband frequency
What is the output frequency of an upper-sideband
parametric-frequency converter?
The sum of the input frequency and the pump frequency
What is the primary advantage of bulk-effect devices
over normal pn-junction semiconductors?
Larger microwave power outputs
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
The point on the current curve of a gallium-arsenide
semiconductor at which it begins to exhibit negative
resistance is called what?
Threshold
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
What characteristic of a gunn oscillator is inversely
proportional to the transit time of the domain
across the semiconductor?
The frequency
What is the junction arrangement of the original
avalanche transit-time diode?
Pnin
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
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
What is the capacitive reactance across a point-contact
diode as compared to a normal junction diode?
Lower
What are the most important advantages of the Schottky
barrier diode?
Lower forward resistance and low noise
At frequencies above 100 megahertz, the intrinsic (i)
region causes a pin diode to act as what?
Variable resistance
The pin diode is primarily used for what purpose?
A switching device
Microwave antennas and low-frequency antennas are
similar in what ways?
Operating principles and electrical characteristics
What term is used to express the efficiency of an antenna?
Power gain or power ratio
What term is used to express the measurement of the degree
of mismatch between a line and its load?
Standing-wave ratio (swr)
What type of antenna radiates in and receives energy from
all directions at once?
Omnidirectional
What is the term that is used to describe narrowness in
the radiated beam of an antenna?
Antenna directivity
What characteristic allows the same antenna to both
transmit and receive?
Reciprocity
What type of reflector is most often used in directive
antennas?
Parabolic
Microwaves can be focused and reflected in the same way
as what other type of waves?
Light waves
How many major lobes are radiated by a parabolic
reflector?
One
A horizontally truncated paraboloid antenna is used for
what purpose?
Determine elevation
The beam from a horizontally positioned cylindrical
paraboloid is narrow in what plane?
Vertical
What is the purpose of a collimating lens?
Forces the radial segments of a wavefront into parallel
paths.
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.
What type of lens decelerates a portion of a spherical
wavefront?
Delay lens
What is a set of antenna elements called?
Antenna Array
What type of antenna has all elements connected to the
same energy source?
Driven Array
What determines the beam elevation angle of an antenna
that is electronically scanned in elevation?
Frequency or phase of radiated energy
What is the polarization of the energy radiated by a
vertical slot?
Horizontal
