NEETS v11 Microwave Q A

8/3/2019 NEETS v11 Microwave Q A

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WAVEGUIDE THEORY AND APPLICATION

Q-1. What is the region of the frequency spectrum from 1000 MHz to 100,000 MHzcalled?A-1. Microwave region.

Q-2. Microwave theory is based upon what conceptA-2. Electromagnetic field theory.

Q-3. Why are coaxial lines more efficient at microwave frequencies than two-wiretransmission lines?A-3. The electromagnetic fields are completely confined.

Q-4. What kind of material must be used in the construction of waveguides?A-4. Conductive material.

Q-5. The large surface area of a waveguide greatly reduces what type of loss that iscommon in two-wire and coaxial lines?A-5. Copper loss.

Q-6. What causes the current-carrying area at the center conductor of a coaxial line tobe restricted to a small layer at the surface?A-6. Skin effect.

Q-7. What is used as a dielectric in waveguides?A-7. Air.

Q-8. What is the primary lower-frequency limitation of waveguides?A-8. Physical size.

Q-9. At very high frequencies, what characteristics are displayed by ordinary insulators?A-9. The characteristics of the dielectric of a capacitor.

Q-10. What type of insulator works well at very high frequencies?A-10. A shorted quarter-wave section called a metallic insulator.

Q-11. The frequency range of a waveguide is determined by what dimensison?A-11. The "a" dimension.

Q-12. What happens to the bus bar dimensions of the waveguide when the frequency isincreased?A-12. The bus bar becomes wider.

Q-13. When the frequency is decreased so that two quarter-wavelengths are longer thanthe "a" (wide) dimension of the waveguide, what will happen?A-13. Energy will no longer pass through the waveguide.

Q-14. What interaction causes energy to travel down a waveguide?A-14. The interaction of the electric and magnetic fields.


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Q-15. What is indicated by the number of arrows (closeness of spacing) used torepresent an electric field?A-15. The relative strength of the field.

Q-16. What primary condition must magnetic lines of force meet in order to exist?A-16. Magnetic lines of force must form a continuous closed loop.

Q-17. What happens to the H lines between the conductors of a coil when theconductors are close together?A-17. The H lines cancel.

Q-18. For an electric field to exist at the surface of a conductor, the field must have whatangular relationship to the conductor?A-18. The field must be perpendicular to the conductors.

Q-19. When a wavefront is radiated into a waveguide, what happens to the portions ofthe wavefront that do not satisfy the boundary conditions?A-19. Decrease to zero.

Q-20. Assuming the wall of a waveguide is perfectly flat, what is the angular relationshipbetween the angle of incidence and the angle of reflection?A-20. The angles are equal.

Q-21. What is the frequency called that produces angles of incidence and reflection thatare perpendicular to the waveguide walls?A-21. Cutoff frequency.

Q-22. Compared to the velocity of propagation of waves in air, what is the velocity ofpropagation of waves in waveguides?A-22. Slower.

Q-23. What term is used to identify the forward progress velocity of wavefronts in awaveguide?A-23. Group velocity.

Q-24. What term is used to identify each of the many field configurations that can exist inwaveguides?A-24. Mode of operation.

Q-25. What field configuration is easiest to produce in a given waveguide?A-25. Dominant mode.

Q-26. How is the cutoff wavelength of a circular waveguide figured?A-26. 1.71 times the diameter.

Q-27. The field arrangements in waveguides are divided into what two categories todescribe the various modes of operation?A-27. Transverse electric (TE) and transverse magnetic (TM).

Q-28. The electric field is perpendicular to the "a" dimension of a waveguide in whatmode?


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A-28. TE.

Q-29. The number of half-wave patterns in the "b" dimension of rectangular waveguidesis indicated by which of the two descriptive subscripts?A-29. Second.

Q-30. Which subscript, in circular waveguide classification, indicates the number of full-wave patterns around the circumference?A-30. First.

Q-31. What determines the frequency, bandwidth, and power-handling capability of awaveguide probe?A-31. Size and shape.

Q-32. Loose or inefficient coupling of energy into or out of a waveguide can beaccomplished by the use of what method?A-32. Slots and apertures.

Q-33. What is the result of an impedance mismatch in a waveguide?A-33. Standing waves that cause power losses, a reduction in power-handling capability,and an increase in frequency and sensitivity.

Q-34. What is used to construct irises?A-34. Metal plates.

Q-35. An iris placed along the "b" dimension wall produces what kind of reactance?A-35. Inductive.

Q-36. How will an iris that has portions along both the "a" and "b" dimension walls act atthe resonant frequency?

A-36. As a shunt resistance.

Q-37. What device is used to produce a gradual change in impedance at the end of awaveguide?A-37. Horn.

Q-38. When a waveguide is terminated in a resistive load, the load must be matched towhat property of the waveguide?A-38. Characteristic impedance.

Q-39. What is the primary purpose of a dummy load?A-39. Absorb all energy without producing standing waves.

Q-40. The energy dissipated by a resistive load is most often in what form?A-40. Heat.

Q-41. What is the result of an abrupt change in the size, shape, or dielectric of awaveguide?A-41. Reflections.

Q-42. A waveguide bend must have what minimum radius?


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A-42. Greater than 2 wavelengths.

Q-43. What is the most common type of waveguide joint?A-43. Choke joint.

Q-44. What is the most likely cause of losses in waveguide systems?

A-44. Improperly connected joints or damaged inner surface.

Q-45. What is the primary purpose of a directional coupler?A-45. Sampling energy within a waveguide.

Q-46. How far apart are the two holes in a simple directional coupler?A-46. 1/4 wavelength.

Q-47. What is the purpose of the absorbent material in a directional coupler?A-47. Absorb the energy not directed at the pick-up probe and a portion of the overallenergy.

Q-48. In a directional coupler that is designed to sample the incident energy, whathappens to the two portions of the wavefront when they arrive at the pickup probe?A-48. The wavefront portions add.

Q-49. What happens to reflected energy that enters a directional coupler that isdesigned to sample incident energy?A-49. The reflected energy adds at the absorbent material and is absorbed.

Q-50. What two variables determine the primary frequency of a resonant cavity?A-50. Size and shape of the cavity.

Q-51. Energy can be inserted or removed from a cavity by what three methods?

A-51. Probes, loops, and slots.

Q-52. Inductive tuning of a resonant cavity is accomplished by placing a nonmagneticslug in what area?A-52. The area of maximum H lines.

Q-53. What are the two basic types of T junctions?A-53. E-type and H-type.

Q-54. Why is the H-type T junction so named?A-54. The junction arm extends in a direction parallel to the H lines in the mainwaveguide.

Q-55. The magic-T is composed of what two basic types of T junctions?A-55. E-type and H-type.

Q-56. What are the primary disadvantages of the magic-T?A-56. Low power-handling capability and power losses.

Q-57. What type of junctions are formed where the arms of a hybrid ring meet the mainring?


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A-57. Basic E-type junctions.

Q-58. Hybrid rings are used primarily for what purpose?A-58. High-power duplexes.

Q-59. Ferrite devices are useful in microwave applications because they possess what

properties?A-59. Magnetic properties and high resistance.

Q-60. Which of the two types of electron motion (orbital movement and electron spin) ismore important in the explanation of magnetism?A-60. Electron spin.

Q-61. The interaction between an external field and the binding force of an atom causeselectrons to do what?A-61. Wobble at a natural resonant frequency.

Q-62. The resonant frequency of electron wobble can be changed by variation of what

force?A-62. The applied magnetic field.

Q-63. Rotating the plane of polarization of a wavefront by passing it through a ferritedevice is called what?A-63. Faraday rotation.

MICROWAVE COMPONENTS AND CIRCUITS

Q-1. What happens to the impedance of interelectrode capacitance as frequencyincreases?A-1. Impedance decreases.

Q-2. What undesirable effect is caused by the inductance of the cathode lead?A-2. Degenerative feedback.

Q-3. How does transit time affect the relationship of the grid voltage and the platecurrent at high frequencies?A-3. Transit time causes the grid voltage and plate current to be out of phase.

Q-4. Moving tube electrodes apart to decrease interelectrode capacitance causes anincrease in the effect of what property?A-4. Transit time.

Q-5. The kinetic energy of an electron is directly proportional to what property?A-5. Velocity.

Q-6. What will be the effect upon an electron traveling in the opposite direction to thelines of force in an electrostatic field?A-6. The electron will be accelerated.

Q-7. How is a beam of electrons velocity-modulated?A-7. By alternately speeding up or slowing down the electrons.


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Q-8. What portion of an electron gun causes the electrons to accelerate or decelerate?A-8. The buncher grids.

Q-9. What is the effect upon an electron that enters the buncher gap when the potentialacross the grids is at 0 volts?

A-9. There is no effect.

Q-10. What determines the placement of the catcher cavity?A-10. The frequency period of the buncher grid signal.

Q-11. What is the basic principle of operation of a klystron?A-11. Velocity modulation.

Q-12. The electrons in the beam of a klystron are speeded up by a high dc potentialapplied to what elements?A-12. The accelerator grid and the buncher grids.

Q-13. The two-cavity klystron uses what cavity as an output cavity?A-13. The catcher cavity.

Q-14. A two-cavity klystron without a feedback path will operate as what type of circuit?A-14. Amplifier.

Q-15. What can be added to the basic two-cavity klystron to increase the amount ofvelocity modulation and the power output?A-15. Intermediate cavities between the input and output cavities.

Q-16. How is the electron beam of a three-cavity klystron accelerated toward the drifttube?

A-16. A large negative pulse is applied to the cathode.

Q-17. Which cavity of a three-cavity klystron causes most of the velocity modulation?A-17. The middle cavity.

Q-18. In a multicavity klystron, tuning all the cavities to the same frequency has whateffect on the bandwidth of the tube?A-18. The bandwidth decreases.

Q-19. The cavities of a multicavity klystron are tuned to slightly different frequencies inwhat method of tuning?A-19. Stagger tuning.

Q-20. What element of the reflex klystron replaces the output cavity of a normalklystron?A-20. The reflector or repeller.

Q-21. When the repealer potential is constant, what property of the electron determineshow long it will remain in the drift space of the reflex klystron?A-21. Velocity.


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Q-22. The constant-speed electrons of an electron bunch in a reflex klystron mustremain in the repeller field for what minimum time?A-22. Three-quarter cycle.

Q-23. If the constant-speed electrons in a reflex klystron remain in the repeller field for 13/4 cycles, what is the mode of operation?

A-23. Mode 2.

Q-24. Debunching of the electron bunches in the higher modes of a reflex klystron haswhat effect on output power?A-24. Power is reduced.

Q-25. What limits the tuning range around the center frequency of a reflex klystron in aparticular mode of operation?A-25. The half-power points of the mode.

Q-26. What is the primary use of the twt?A-26. Voltage amplification.

Q-27. The magnet surrounding the body of a twt serves what purpose?A-27. Used to focus the electrons into a tight beam.

Q-28. How are the input and output directional couplers in a twt connected to the helix?A-28. The directional couplers are not physically connected to the helix.

Q-29. What relationship must exist between the electron beam and the traveling wavefor bunching to occur in the electron beam of a twt?A-29. The traveling wave must have a forward velocity equal to or less than the speed ofthe electrons in the beam.

Q-30. What structure in the twt delays the forward progress of the traveling wave?A-30. The helix.

Q-31. The folded waveguide in a bwo serves the same purpose as what component in atwt?A-31. Helix.

Q-32. What serves as a grid in a magnetron?A-32. A magnetic field.

Q-33. A cylindrical copper block with resonant cavities around the circumference is usedas what component of a magnetron?

A-33. Anode or plate.

Q-34. What controls the output frequency of a magnetron?A-34. The resonant cavities.

Q-35. What element in the magnetron causes the curved path of electron flow?A-35. The permanent magnet.


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Q-36. What is the term used to identify the amount of field strength required to cause theelectrons to just miss the plate and return to the filament in a circular orbit?A-36. The critical value of field strength.

Q-37. A magnetron will produce oscillations when the electrons follow what type of path?A-37. Circular.

Q-38. What is the primary difference in construction between the basic magnetron andthe negative resistance magnetron?A-38. The negative-resistance magnetron has a split plate.

Q-39. What starts the oscillations in a negative-resistance magnetron?A-39. The application of the proper magnetic field.

Q-40. Why is the negative-resistance magnetron often operated with reduced filamentvoltage?A-40. To reduce the effects of filament bombardment.

Q-41. What type of electron-resonance anode block does not require strapping?A-41. Rising-sun block.

Q-42. Without strapping, the resonant cavities of a hole-and-slot anode are connected inwhat manner?A-42. Series.

Q-43. What are the electrons called that give up energy to the ac field in a magnetron?A-43. Working electrons.

Q-44. Why is the pi mode the most commonly used magnetron mode of operation?A-44. Greater power output.

Q-45. What two methods are used to couple energy into and out of magnetrons?A-45. Loops and slots.

Q-46. Magnetron tuning by altering the surface-to-volume ratio of the hole portion of ahole-and-slot cavity is what type of tuning?A-46. Inductive.

Q-47. Capacitive tuning by inserting a ring into the cavity slot of a magnetron isaccomplished by what type of tuning mechanism?A-47. A cookie-cutter tuner.

Q-48. Name the procedure used to reduce excessive arcing in a magnetron?A-48. Baking in.

Q-49. What causes the negative-resistance property of tunnel diodes?A-49. The tunneling action.

Q-50. What determines the frequency of a tunnel-diode oscillator?A-50. The tuned circuit or cavity frequency.


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Q-51. Why is the tunnel diode loosely coupled to the cavity in a tunnel-diode oscillator?A-51. To increase the stability.

Q-52. What is the purpose of the circulator in a tunnel-diode amplifier?A-52. Prevent feedback to the tuned input circuit.

Q-53. What limits the usefulness of high-gain, tunnel-diode frequency converters?A-53. Stability problems.

Q-54. The varactor is a pn junction that acts as what type of electronic device?A-54. Variable capacitor.

Q-55. The underlying principle of operation of the parametric amplifier is based on whatproperty?A-55. Reactance.

Q-56. What is the most important feature of the parametric amplifier?A-56. The low-noise characteristic.

Q-57. How is amplification achieved in the circuit shown in figure 2-43?A-57. By varying the amount of capacitance in the circuit.

Q-58. What is the purpose of the pump in a parametric amplifier?A-58. Supplies the electrical energy required to vary the capacitance.

Q-59. The pump signal frequency must be of what value when compared to the inputsignal of a simple parametric amplifier?A-59. Exactly double the input frequency.

Q-60. What is the primary difference between the pump signal of a simple parametric

amplifier and the pump signal of a nondegenerative parametric amplifier?A-60. The pump signal of a nondegenerative parametric amplifier is higher than twicethe input signal.

Q-61. In a nondegenerative parametric amplifier the difference between the inputfrequency and the pump frequency is called what?A-61. Idler- or lower-sideband frequency.

Q-62. What is the output frequency of an upper-sideband parametric-frequencyconverter?A-62. The sum of the input frequency and the pump frequency.

Q-63. What is the primary advantage of bulk-effect devices over normal pn-junctionsemiconductors?A-63. Larger microwave power outputs.

Q-64. What happens to the electrons of a gallium-arsenide semiconductor when theymove from the normal low-energy conduction band to the high-energy conduction band?A-64. The electrons become immobile.


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Q-65. The point on the current curve of a gallium-arsenide semiconductor at which itbegins to exhibit negative resistance is called what?A-65. Threshold.

Q-66. The domain in a gallium-arsenide semiconductor has what type of electrical fieldwhen compared to the other regions across the body of a semiconductor?

A-66. A field of much greater intensity.

Q-67. What characteristic of a gunn oscillator is inversely proportional to the transit timeof the domain across the semiconductor?A-67. The frequency.

Q-68. What is the junction arrangement of the original avalanche transit-time diode?A-68. Pnin.

Q-69. What causes dc bias energy to be absorbed by avalanche electrons and given upto the microwave field applied to an avalanche transit-time diode?A-69. The negative-resistance property.

Q-70. During the manufacture of a point-contact diode, what is the purpose of passing arelatively large current from the catwhisker to the silicon crystal?A-70. To form a small region of p-type material.

Q-71. What is the capacitive reactance across a point-contact diode as compared to anormal junction diode?A-71. Lower.

Q-72. What are the most important advantages of the Schottky barrier diode?A-72. Lower forward resistance and low noise.

Q-73. At frequencies above 100 megahertz, the intrinsic (i) region causes a pin diode toact as what?A-73. Variable resistance.

Q-74. The pin diode is primarily used for what purpose?A-74. A switching device.

MICROWAVE ANTENNASQ-1. Microwave antennas and low-frequency antennas are similar in what ways?A-1. Operating principles and electrical characteristics.

Q-2. What term is used to express the efficiency of an antenna?A-2. Power gain or power ratio.

Q-3. What term is used to express the measurement of the degree of mismatch betweena line and its load?A-3. Standing-wave ratio (swr).

Q-4. What type of antenna radiates in and receives energy from all directions at once?A-4. Omnidirectional.


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Q-5. What is the term that is used to describe narrowness in the radiated beam of anantenna?A-5. Antenna directivity.

Q-6. What characteristic allows the same antenna to both transmit and receive?A-6. Reciprocity.

Q-7. What type of reflector is most often used in directive antennas?A-7. Parabolic.

Q-8. Microwaves can be focused and reflected in the same way as what other type ofwaves?A-8. Light waves.

Q-9. How many major lobes are radiated by a parabolic reflector?A-9. One.

Q-10. A horizontally truncated paraboloid antenna is used for what purpose?

A-10. Determine elevation.

Q-11. The beam from a horizontally positioned cylindrical paraboloid is narrow in whatplane?A-11. Vertical.

Q-12. What is the purpose of a collimating lens?A-12. Forces the radial segments of a wavefront into parallel paths.

Q-13. How does a waveguide-type lens focus spherical wavefront microwave energy?A-13. Some wavefronts are accelerated so that all wavefronts exit the lens at the sametime.

Q-14. What type of lens decelerates a portion of a spherical wavefront?A-14. Delay lens.

Q-15. What is a set of antenna elements called?A-15. Antenna Array.

Q-16. What type of antenna has all elements connected to the same energy source?A-16. Driven Array.

Q-17. What determines the beam elevation angle of an antenna that is electronicallyscanned in elevation?

A-17. Frequency or phase of radiated energy.

Q-18. What is the polarization of the energy radiated by a vertical slot?A-18. Horizontal.

NEETS v11 Microwave Q A

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