KENNEDY QUESTIONS: CHAPTER 1: INTRODUCTION TO COMMUNICATION SYSTEM 1. In a communication system, noise is most likely to affect the signal

- at the transmitter - in the channel - in the information source - at the destination

2. Indicate the false statement. Fourier analysis shows that the saw tooth wave consist of

- fundamental and sub harmonic sine waves - a fundamental sine wave and an infinite number of harmonics - fundamental and harmonic sine waves whose amplitude decreases with

the harmonic number - sinusoidal voltages, some of which are small enough to ignore in

practice 3. Indicate the false statement. Modulation is used to

- reduce the bandwidth used - separate the differing transmissions - ensure that intelligence may be transmitted over long distances - allow the use of practicable antennas

CHAPTER 2: NOISE 4. One of the following types of noise becomes a great importance at high frequencies. Is the

- shot noise - random noise - impulse noise - transit-time noise

5. Indicate the false statement

- HF mixers are generally noisier than HF amplifiers - Impulse noised voltage is independent of the bandwidth - Thermal noise is independent of the frequency at which it is measured - Industrial noise is usually of the impulse type

6. The value of the resistor creating thermal noise is doubled. The noise power generated is therefore

- halved - quadrupled - double - unchanged

7. One of the following is not useful quantity for comparing the noise performance of the receiver

- Input noise voltage

- Equivalent noise resistance - Noise temperature - Noise figure

8. Indicate the noise whose source is in a category different from that of the other three.

- solar noise - cosmic noise - atmospheric noise - galactic noise

9. Indicate the false statement. The square of the thermal noise voltage generated by a resistor is proportional to

- its resistance - its temperature - Boltzmann’s Constant - the bandwidth over which it is measured

CHAPTER 3: AMPLITUDE MODULATION 10. If the plate supply voltage for a plate modulated class C amplifier is V, the maximum plate-cathode voltage could be almost as high as

- 4V - 3V - 2V - V

11. In a low-level AM system, amplifiers following the modulated stage must be

- linear devices - harmonic devices - class C amplifiers - non linear devices

12. If the carrier of a 100 percent modulated AM wave is suppressed, the percentage power saving will be

- 50 - 150 - 100 - 66.66

13. Leak type bias is used in a plate modulated class C amplifier to

- prevent tuned circuit damping - prevent excessive grid current - prevent over modulation - increase the bandwidth

14. The output stage of a television transmitter is most likely to be a

- plate modulated class C amplifier - grid modulated class C amplifier

- screen modulated class C amplifier - grid modulated class A amplifier

15. The modulation index of an AM is changed from 0 to 1. The transmitted power is

- unchanged - halved - doubled - increase by 50percent

16. One of the advantages of base modulation over collector modulation of a transistor class C amplifier

- the lower the modulating power required - high power output per transistor - better efficiency - better linearity

17. The carrier is simultaneously modulated by two sine wave with the modulation indices of 0.3 and 0.4; the total modulation index - is 1 - cannot be calculated unless the phase relations are known - is 0.5 - is 0.7 18. Amplitude modulation is used for broadcasting because

- it is more noise immune than other modulation system - compared with other system it requires less transmitting power - it use avoids receiver complexity - no other modulation system can provide the necessary bandwidth for

high fidelity CHAPTER 4: SINGLE SIDE BAND TECHNIQUES 19. Indicate the false statement regarding the advantages of SSB over double sideband, full carrier AM.

- more channel spaced available - Transmitter circuits must be more stable. Giving better reception. - The signal is more noise-resistant. - Much less power is required for the same signal strength

20. When the modulation index of an AM wave is doubled, the antenna current is also doubled, the AM system being used is

- Single sideband, full carrier (H3E) - Vestigial sideband (C3F) - Single-sideband, suppressed carrier (J3E) - Double sideband, full carrier (A3E)

21. Indicate which one of the following advantages of the phase cancellation method of obtaining SSB over the filter method is false:

- switching from one sideband to the other is simpler

- It is possible to generate SSB at any frequency - SSB with lower audio frequencies present can be generated - There are more balance modulator, therefore the carrier is suppressed

better 22. The most common used filters in SSB generation are

- mechanical - RC - LC - Low-pass

23. In an SSB transmitter, one is most likely to find a

- class C audio amplifier - tuned modulator - class B RF amplifier - class A RF output amplifier

24. Indicate in which of the following only one sideband is transmitted:

- H3E - A3E - B8E - C3F

25. One of the following cannot be used to remove unwanted sideband in SSB. This is the

- Filter system - Phase shift method - Third method - Balance modulator

26. R3E modulation is sometimes used to

- allow the receiver to have frequency synthesizer - simply the frequency stability problem reception - reduce the power that must be transmitted - reduce the bandwidth required for transmission

27. To provide two or more voice circuits with the same carrier, it is necessary to use

- ISB - Carrier reinsertion - SSB with pilot carrier - Lincompex

28. Vestigial sideband modulation (C3F) is normally used for

- HF point to point communications - Monoaural broadcasting - TV broadcasting - Stereo broadcasting

CHAPTER 5: FREQUENCY MODULATION 29. In the stabilized reactance modulator AFC system,

- the discriminator must have a fast time constant to prevent demodulation

- the higher the discriminator frequency, the better the oscillator frequency stability

- the discriminator frequency must not be too low, or the system will fail - phase modulation is converted into FM by the equalizer circuit

30. In the spectrum of a frequency modulated wave

- the carrier frequency disappears when the modulation index is large - the amplitude of any sideband depends on the modulation index - the total number of sidebands depend on the modulation index - the carrier frequency cannot disappear

31. The difference between the phase and frequency modulation

- is purely theoretical because they are the same in practice - is too great to make the two system compatible - lies in the poorer audio response of the phase modulation - lies in the different definitions of the modulation index

32. Indicate the false statement regarding the Armstrong modulation system,

- the system is basically phase and not frequency, modulation. - AFC is not needed, as the crystal oscillator is used. - Frequency multiplications must be used - Equalization is unnecessary.

33. An FM signal with a modulation index Mf is passed through the frequency tripler. The wave in the output of the Tripler will have a modulation index of

- mf/3 - mf - 3mf - 9mf

34. An FM signal with a deviation (δ) is passed through a mixer, and has its frequency reduced fivefold. The deviation in the output of the mixer is

- 5 δ - Indeterminate - δ / 5 - δ

35. A pre-emphasis circuit provides extra noise immunity by - boosting the bass frequency -amplifying the high audio frequencies -preamplifying the whole audio band -converting the phase modulation to FM

36. Since noise phase modulates the FM wave, as the noise sideband frequency approaches the carrier frequency, the noise amplitude

- remains constant - is decreased - is increased - is equalized

37. When the modulating frequency is doubled, the modulation index is halved, and the modulating voltage remains constant. The modulation system is

- amplitude modulation - phase modulation - frequency modulation - any one of the three

38. Indicate which one of the following is not an advantage of FM over AM:

- better noise immunity is provided - Lower bandwidth is required - The transmitted power is more useful - Less modulating power is required

39. One of the following is an in direct way of generating FM. This is the

- reactance FET modulator - varactor diode - Armstrong modulator - Reactance bipolar transistor modulator

40. In an FM stereo multiplex transmission, the

- sum signal modulates the 19KHz sub carrier - difference signal modulates the, 19KHz sub carrier - difference signal modulates the, 38KHz sub carrier - difference signal modulates the 67KHz sub carrier

CHAPTER 6: RADIO RECEIVERS 41. Indicate which of the following statement about the advantages of the phase discriminator over the slope detector is false:

- much easier alignment - better linearity - great limiting - fewer tuned circuits

42. Show which of the following statement about the amplitude limiter is untrue:

- the circuit is always biased in class C, by the virtue of the leak type bias

- when the input increases past the threshold of limiting, the gain decreases to keep the output constant

- the output must be tuned - leak type biased must be used

43. In a radio receiver with simple AGC

- an increase in signal strength produces more AGC - the audio stage gain is normally controlled by the AGC - the faster the AGC time constant, the more accurate the output - the highest AGC voltage is produced between stations

44. In broadcast super heterodyne receiver, the

- local oscillator operates below the signal frequency - mixer input must be tuned to the signal frequency - local oscillator frequency is normally double the IF - RF amplifier normally works at 455KHz above the carrier frequency

45. To prevent overloading of the last IF amplifier in the receiver, one should use

- squelch - variable sensitivity - variable selectivity - double conversion

46. A super heterodyne receiver with an IF of 450KHz is tuned to a signal at 1200KHz. The image frequency is - 750KHz - 900KHz - 1650KHz - 2100KHz 47. In a radio detector

- the linearity is worse than in phase discriminator - stabilization against signal strength variations is provided - the output is twice that obtainable from a similar phase discriminator - the circuit is the same as in a discriminator, except that the diodes are

reversed 48. The typical squelch circuit

- cuts off an audio amplifier when the carrier is absent - eliminates the RF interference when the signal is weak - cuts off an IF amplifier when the AGC is maximum - cuts off an IF amplifier when the AGC is minimum

49. Indicate the false statement in connection with the communications receivers.

- The noise limiter cuts off the receiver’s output during a noise pulse. - A product demodulator could be used for the reception of morse code - Double conversion is used to improve image rejection - Variable sensitivity is used to eliminate selective fading

50. The controlled oscillator synthesizer is sometimes preferred to the direct one because

- it is simpler piece of equipment

- its frequency stability is better - it does not require crystal oscillators - it is relatively free of spurious frequencies

51. The frequency generated by each decade in a direct frequency synthesizer is much higher than the frequency shown; this is done to

- reduce the spurious frequency problem - increase the frequency stability - reduce the number of decades - reduce the number of crystal required

52. Indicated which of the following circuits could not demodulate SSB.

- balance modulator - product detector - BFO - Phase discriminator

53. If an FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

- improving the effectiveness of the AGC - reducing the effect of negative peak clipping - reducing the effect of noise at low modulation depths - improving the selectivity of the receiver

54. Indicate the false statement. The super heterodyne receiver replaced the TRF receiver because the latter suffered from

- gain variation over the frequency coverage range - insufficient gain and sensitivity - inadequate selectivity at high frequencies - instability

55. The image frequency of a super heterodyne receiver

- is created within he receiver itself - is due to insufficient adjacent channel rejection - is not rejected by the IF tuned circuits - is independent of the frequency to which the receiver is tuned

56. One of the main functions of the RF amplifier in a super heterodyne receiver is to

- provide improved tracking - permit better adjacent-channel rejection - increase the tuning range of the receiver - improve the rejection of the image frequency

57. A receiver has poor IF selectivity. It will therefore also have poor - blocking - double spotting - diversity reception - sensitivity

58. Three point tracking is achieved with - variable selectivity - the padder capacitor - double spotting - double conversion

59. The local oscillator of a broadcast receiver is tuned to a frequency higher that the incoming frequency

- to help the image frequency rejection - to permit easier tracking - because otherwise an intermediate frequency could not be produced - to allow adequate frequency coverage without switching

60. If the intermediate frequency is very high (indicate the false statement)

- image frequency rejection is very good - the local oscillator need to be extremely stable - the selectivity will be poor - tracking will be improved

61. A low ratio of the AC to the DC load impedance of a diode detector results in - diagonal clipping - poor AGC operation - negative peak clipping - poor AF response 62. On of the following cannot be used to demodulate SSB.

- product detector - diode balance modulator - bipolar transistor balance modulator - complete phase shift generator

63. Indicate the false statement. Nothing that no carrier is transmitted with J3E, we see that

- the receiver cannot use a phase comparator for AFC - adjacent channel rejection is more difficult - production of AGC is rather complicated process - the transmission is not compatible with A3E

64. When the receiver has good blocking performance, this means that

- it does not suffer from double spotting - its image frequency rejection is poor - its is unaffected by AGC derived from nearby transmission - its detector suffers from burnout

65. An AM receiver uses a diode detector to demodulation. This enables it satisfactorily to receive

- single sideband, suppressed carrier - single sideband, reduced carrier - ISB

- Single sideband, full carrier CHAPTER 7: TRANSMISSION LINES 66. Indicate the false statement. The SWR on a transmission line infinity; the line is terminated in

- a short circuit - a complex impedance - an open circuit - a pure resistance

67. A (75 - j50) load is connected to a coaxial transmission line of Zo = 75, at 10GHz. The best method of matching consists in connecting - a short circuited stub at the load - an inductance at the load - a capacitance at some specific distance from the load - a short circuited stub at some specific distance from the load 68. The velocity factor of a transmission line

- depends on the dielectric constant of the material used - increase the velocity along the transmission line - is governed by the skin effect - is higher for a solid dielectric than for air

69. Impedance inversion may be obtained with

- a short circuited stub - an open circuited stub - a quarter wave line - a half wave line

70. Short circuited stubs are preferred to open circuited stub because the latter are

- more difficult to make and connect - made of transmission line with a different characteristic impedance - liable to radiate - incapable of giving a full range of reactances

71. For a transmission line load matching over a range of frequencies, it is best to use a

- balun - broadband directional coupler - double stub - single stud of adjustable position

72. The main disadvantage of the two hole directional couple is

- low directional coupling - poor directivity - high SWR - narrow bandwidth

73. To couple a coaxial line to a parallel wire, it is best to use a - slotted line - balun - directional coupler - quarter wave transformer

CHAPTER 8: RADIATION AND PROPAGATION OF WAVES 74. Indicated which one of the following terms applies to troposcatter propagation:

- SIDs - Fading - Atmospheric storms - Faraday Rotation

75. VLF waves are used for some types of services because

- of the low powers required - the transmitting antennas are of convenient size - they are very reliable - they penetrate the ionosphere easily

76. Indicate which of the following frequencies cannot be used for reliable beyond the horizon terrestrial communication without repeaters

- 20KHz - 15MHz - 900MHz - 12GHz

77. High frequency waves are

- absorbed by the F2 layer - reflected by the D layer - capable of use for long distance communication on the moon - affected by the solar cycle

78. Distance near the skip distance should be used for sky-wave propagation

- to avoid tilting - to prevent sky wave and upper ray interference - to avoid the Faraday effect - so as not to exceed the critical frequency

79. A ship to ship communication system is plagued by fading. The best solution seems to be the use of - a more directional antennas - broadband antennas - frequency diversity - space diversity 80. A range of microwave frequency more easily passed by the atmostphere than the other is called a - window

- critical frequency - gyro frequency range - Resonance in the atmosphere 81. Frequencies in the UHF range normally propagate by means of

- ground waves - sky waves - surface waves - space waves

82. Tropospheric scatter is used with frequencies in the following range

- HF - VHF - UHF - VLF

83. The gound wave eventually disappears, as one moves away from the transmitter, because of

- interference from the sky wave - loss of line of sight conditions - maximum single hop distance limitations - tilting

84. In electromagnetic waves, polarization

- is caused by reflection - is due to the transverse nature of the waves - results from the longitudinal nature of waves - is always vertical in an isotropic antenna

85. As electromagnetic waves travel in free space, one one the following can happen to them:

- absorption - attenuation - refraction - reflection

86. The absorption of radio waves by the atmosphere depends on

- their frequency - their distance from the transmitter - the polarization of the waves - the polarization of the atmosphere

87. Electromagnetic waves are refracted when they

- pass into a medium of different dielectric constant - are polarized at right angles to the direction of propagation - encounter a perfectly conducting surface - pass through small slot in a conducting plane

88. Diffraction of electromagnetic wave

- is caused by reflections from the ground

- arises only with the spherical wavefronts - will occur when the waves pass through a large slot - may occur around the edge of a sharp obstacle

89. When microwave signals follow the curvature of the earth, this I known as

- the faraday effect - ducting - troposheric scatter - ionospheric reflections

90. Helical antennas are often used for satellite tracking at VHF because of

- troposcatter - supperefraction - ionospheric refraction - the farady effect

CHAPTER 9: ANTENNAS 91. An ungrounded antenna near the ground

- acts a single antenna of twice the height - is unlikely need an earth mat - acts as an antenna array - must be horizontally polarized

92. One of the following consist of non resonant antenna

- rhombic antenna - folded dipole - end fire array - broadside array

93. One of the following is very useful as a multiband HF receiving antenna. This is the

- conical horn - folded dipole - log-periodic - square loop

94. Which of the following antenna is best excited from a waveguide?

- bioconical - horn - helical - discone

95. Indicate which of the following reasons for using a counter poise with antennas is false

- impossibility of a good ground connection - protection of personnel working underneath - provision of an earth for the antenna - rockiness of the ground itself

96. One of the following is not a reason for the use of an antenna coupler: - to make the antenna look resistive - to provide the output amplifier with the correct load impedance - to discriminate against harmonics - to prevent reradation of the local oscillator

97. Indicate the antenna that is not wideband:

- discone - folded dipole - helical - Marconi

98. Indicate which of the following reasons for use of an earth mat with antennas is false:

- impossibility of a good ground connection - provision of an earth for the antenna - protection of personnel working underneath - improvement of radiation pattern of the antenna

99. Which one of the following terms does not apply the yagi uda array?

- good bandwidth - parasitic elements - folded dipole - high gain

100. An antenna that is circularly polarized is the

- helical - small circular loop - parabolic reflector - yagi – uda

101. The standard reference antenna for the directive gain is the

- infinitesimal dipole - isotropic antenna - elementary doublet - half wave dipole

102. Top loading is sometimes used with an antenna in order to increase its

- effective height - bandwidth - beam width - input capacitance

103. Cassegrain feed is used with a parabolic reflector to

- increase the gain of the system - increase the beam width of the system - reduce the size of the main reflector - allow the feed to be placed at a convenient point

104. Zoning is used with a dielectric antenna in order to

- reduce the bulk of the lens - increase the bandwidth of the lens - permit pin point focusing - correct the curvature of the wave front from a horn that is too short

105. Helical antenna is used for satellite tracking because of its

- circular polarization - maneuverability - broad bandwidth - good front to back ratio

106. The discone antenna is

- a useful direction finding antenna - used as a radar receiving antenna - circularly polarized like other circular antennas - useful as a UHF receiving antenna

107. One of the following is not an omni directional antenna

- half wave dipole - log periodic - discone - Marconi

CHAPTER 10: WAVEGUIDES, RESONATORS and COMPONENTS 108. When electromagnetic waves are propagated in a wave guide

- they travel along the broader walls of the guide - they are reflected from the walls but do not travel along them - they travel through the dielectric without touching the walls - they travel along all four walls of the waveguide

109. Waveguides are used mainly for microwave signals because

- the depend on the straight line propagation which applies to microwaves only

- losses would be to heavy at lower frequencies - there are no generators powerful enough to excite them at lower

frequencies - they would be too bulky at lower frequencies

110. The wavelength of a wave in a waveguide

- is greater than in free space - depends only on the wave guide dimensions and the free space

wavelength - is inversely proportional to the phase velocity - is directly proportional to the group velocity

111. The main difference between the operation of transmission lines and wave guides is that

- the latter are not distributed, like transmission lines

- the former can use stubs and quarter wave transformers, unlike the latter

- transmission lines is use the principal mode of propagation, and therefore do not suffer from low frequency cut off

- terms such as impedance matching and standing wave ration cannot be applied to waveguides

112. Compared with the equivalent transmission line, 3GHz waveguides (indicate the false statement)

- are less lossy - can carry high powers - are less bulky - have lower attenuation

113. When a particular mode is excited in a waveguide, there appears an extra electric component, in the direction of propagation. The resulting mode is

- transverse electric - transverse magnetic - longitudinal - transverse electromagnetic

114. When electromagnetic waves are reflected at an angle from a wall, their wave length along the wall is

- the same as in free space - the same as the wavelength perpendicular to the wall - shortened because of the Doppler effect - greater than in the actual direction of propagation

115. As a result of reflections from a plane conducting wall, electromagnetic waves require an apparent velocity greater than the velocity of light in space. This is called

- velocity of propagation - normal velocity - group velocity - phase velocity

116. Indicate the false statement. When the free space wavelength of a signal equals the cutoff waveguide of the guide

- the group velocity of the signal becomes zero - the phase velocity of the signal becomes infinite - the characteristic impedance of the guide becomes infinite - the wavelength within the wave guide becomes infinite

117. A signal propagated in a waveguide has a full wave of electric intensity change between the two further walls, and no component of the electric field in the direction of propagation. The mode is - TE 1,1 - TE 1,0 - TM 2,2

- TE 2,0 118. The dominant mode of propagation is preferred with rectangular wave guides because (false)

- it leads to the smallest waveguide dimensions - the resulting impedance can be matched directly to coaxial lines - its is easier to excite than the other modes - propagation of it without any spurious generation can be ensured

119. A choke flange may be used to couple two waveguides - to help in alignment of the waveguides - because it is simplier than any other join - to compensate for discontinuities at the join - to increase the bandwidth of the system 120. In order to couple two generators to a waveguide system without coupling them to each other, one could not use a

- rat race - E – plane T - Hybrid ring - Magic T

121. Which of the following wavguide tuning components is not easily adjustable?

- screw - stub - iris - plunger

122. A piston attenuator is a - vane attenuator - waveguide below cutoff - mode filter - flap attenuator 123. Cylindrical cavity resonators are not used with klystrons because they have

- a Q that is too low - a shape whose resonant frequency is too difficult to calculate - harmonically related resonant frequencies - too heavy losses

124. A directional coupler with three or more holes is sometimes used in preference to the two hole coupler - because it is more efficient - to increase coupling of the signal - to reduce spurious mode generation - to increase the bandwidth of the system 125. A ferrite is

- a non conductor with magnetic properties - a intermetallic compound with particularly good conductivity - an insulator which heavily attenuates magnetic fields - a microwave semiconductor invented by faraday 126. Manganese ferrite may be used as a (false)

- circulator - isolator - garnet - phase shifter

127. The maximum power that may be handled by a ferrite component is limited by the

- curie temperature - saturation magnetization - line width - gyromagnetic resonance

128. A PIN diode is - a metal semiconductor point contact diode - a microwave mixer diode - often used as a microwave detector - suitable for the use as a microwave switch 129. A duplexer is used - to couple two different antennas to a transmitter with out mutual interference - to allow the one antenna to be used for reception or transmission with out mutual interference - To prevent interference between two antennas when they are connected to a receiver

- to increase the speed of the pulses in pulsed radar 130. For some applications, circular waveguides may be preferred to rectangular ones because of

- smaller cross section needed at any frequency - lower attenuation - freedom from spurious modes - rotation of polarization

131. Indicated which of the following cannot be followed by the word “waveguide”:

- Elliptical - Felixble - Coaxial - Ridged

132. In order to reduce cross sectional dimensions, the waveguide to use is

- circular - ridged

- rectangular - flexible

133. For low attenuation, the best transmission medium is

- flexible waveguide - ridged waveguide - rectangular waveguide - coaxial line

CHAPTER 11: MICROWAVE TUBES AND CIRCUITS 134. A microwave tube amplifier uses an axial magnetic field and a radial electric field. This is the - reflex klystron - coaxial magnetron - travelling wave tube magnetron - CFA 135. One of the following is unlikely to be used as a pulsed device. It is the

- multicavity - BWO - CFA - TWT

136. One of the reasons why vacuum tubes eventually fail at microwave frequencies is that their

- noise figure increases - transit time becomes to short - shunt capacitive reactance becomes too large - series inductive reactance becomes too small

137. Indicate the false statement. Transit time in microwave tubes will be reduced if

- the electrodes are brought closer together - a higher anode current is used - multiple or coaxial leads are used - the anode voltage is made larger

138. The Multicavity klystron

- is not a good low level amplifier because of noise - has a high repeller voltage to ensure a rapid transit time - is not suitable for pulsed operation - needs a long transit time through the buncher cavity to ensure current

modulation 139. Indicate the false statement. Klystron may use intermediate cavities to

- prevent the oscillations that occur in two cavity klystron - increase the bandwidth of the device - improve the power gain - increase the efficiency of the klystron

140. The TWT is sometimes preferred to the multicavity klystron amplifier, because the former

- is more efficient - has a greater bandwidth - has a higher number of modes - produces a higher output power

141. The transit time in the repeller space of a reflex klystron must be n + ¾ cycles to ensure that

- electrons are accelerated by the gap voltages on their return - returning electron give energy to gap oscillations - it is equal to the period of the cavity oscillations - the repeller is not damaged by the striking electrons

142. The cavity magnetron uses strapping to

- prevent mode jumping - prevent cathode back heating - ensure bunching - improve the phase focusing effect

143. Magnetic field is used in the cavity magnetron to

- prevent anode current in absence of oscillations - ensure that the oscillations are pulsed - help in focusing the electron beam, thus preventing spreading - ensure electrons will orbit around the cathode

144. To avoid difficulties with strapping at high frequencies, the type of cavity structure used in the magnetron is the

- hole and slot - slot - vane - rising sun

145. The primary purpose of the helix in a travelling wave tube is to

- prevent the electron beam from spreading in the long tube - reduce the axial velocity of the RF field - ensure broadband operation - reduce noise figure

146. The attenuator is used in the travelling wave tube to

- help bunching - prevent oscillations - prevent saturations - increase gain

147. Periodic permanent magnet focusing is used with TWTs to

- allow pulsed operation - improve electron bunching - avoid the bulk of an electromagnet

- allow coupled cavity operation at highest frequencies 148. The TWT is sometimes preferred to the magnetron as a radar transmitter output tube because it is

- capable of longer duty cycle - more efficient amplifier - more broadband - less noisy

149. Magnetron whose oscillating frequency is electronically adjustable over a wide range is called a

- coaxial magnetron - dither-tuned amplifier - frequency- agile magnetron - VTM

150. Indicate which of the following is not TWT slow rate structure: - periodic permanent magnet - coupled cavity - helix - ring bar

151. The glass tube of a TWT may be coated with aquadag to

- help focusing - provide attenuation - improve bunching - increase gain

152. Back ward wave oscillator is based on the

- rising sun magnetron - crossed field amplifier - coaxial magnetron - travelling wave tube

CHAPTER 12: SEMICONDUCTOR MICROWAVE DEVICES AND CIRCUITS 153. Parametric amplifier must be cooled

- because parametric amplification generates a lot of heat - to increase bandwidth - because it cannot operate at room temperature - to improve the noise performance

154. Ruby maser amplifier must be cooled

- because maser amplification generates a lot of heat - to increase bandwidth - because it cannot operate at room temperature - to improve the noise performance

155. Disadvantage of microstrip compared with stripline is that microstrip

- does not readily lend itself to printed circuit techniques - is more likely to radiate - is bulkier - is more expensive and complex to manufacture

156. The transmission system using two ground planes is

- microstrip - elliptical waveguide - parallel wire line - stripline

157. Indicate the false statement. An advantage of stripline over waveguides is its

- smaller bulk - greater bandwidth - higher power handling capability - greater compatibility with solid state devices

158. Indicate the false statement. An advantage of stripline over microstrip is its

- easier integration with semiconductor devices - lower tendency to radiate - higher isolation between adjacent circuits - higher Q

159. Surface acoustic waves propagate in

- gallium arsenide - indium phosphide - stripline - quartz crystal

160. SAW devices may be used as

- transmission media like stripline - filters - UHF amplifiers - Oscillators at millimiter frequencies

161. Indicate the false statement. FETs are preferred to bipolar transistor at the high frequencies because they

- are less noisy - lend themselves more readily to integration - are capable of higher efficiencies - can provide higher gains

162. for best low level noise performance in the X-band, an amplifier should use

- a bipolar transistor - a gunn diode - a step-recovery diode

- an IMPATT diode 163. The biggest advantage of the TRAPATT diode over the IMPATT diode is its

- low noise - higher efficiency - ability to operate at higher frequencies - lesser sensitivity to harmonics

164. Indicate which of the following diodes will produce the highest pulsed power output

- Varactor - Gunn - Schottky barrier - RIMPATT

165. Indicate which of the following diodes does not use negative resistance in its opearation:

- Backward - Gunn - IMPATT - Tunnel

166. One of the following is not used as a microwave mixer or detector.

- crystal diode - schottky barrier diode - backward diode - PIN diode

167. One of the following microwave diodes is suitable for very low power oscillations only:

- tunnel - avalanche - gunn - IMPATT

168. The transferred electron bulk effect occurs in

- germanium - gallium arsenide - silicon - metal semiconductor, junctions

169. The gain bandwidth frequency of a microwave transistor, fT is the frequency at which the

- alpha of the transistor falls by 3dB - beta of the transistor falls by 3dB - power gain of the transistor falls to unity - beta of the transistor falls to unity

170. For microwave transistor to operate at the high frequencies, the (false)

- collector voltage must be large - collector current must be high - base should be thin - emitter area must be large

171. Varactor diode may be useful at microwave frequencies (false)

- for electronic tuning - for frequency multiplication - as an oscillator - as a parametric amplifier

172. If high order frequency multiplication is required from a diode multiplier,

- the resistive cuttoff frequency must be high - a small value of base resistance is required - a step recovery diode must be used - a large range of capacitance variation is needed

173. Parametric amplifier has an input and output frequency of 2.25GHz and is pumped at 4.5GHz. It is a

- travelling wave tube - degenerate amplifier - lower sideband up converter - upper sideband up converter

174. Non degenerate parametric amplifier has an input frequency Fi and a pump frequency Fp. The idler frequency is

- Fi - 2Fi - Fi – Fp - Fp – Fi

175. Travelling wave parametric amplifiers are used to

- provide a greater gain - reduce the number of varactor diodes required - avoid the need for cooling - provide a greater bandwidth

176. A parametric amplifier sometimes uses a circulator to - prevent noise feedback - allow the antenna to be used simultaneously for transmission and reception - separate the signal and idler frequencies - permit more efficient pumping 177. The non degenerate one port parametric amplifier should have a high ratio of pump to signal frequency because this

- permits satisfactory high frequency operation - yields a low noise figure - reduces the pump power required - permits satisfactory low frequency operation

178. Tunnel diode - has a tiny hole through its center to facilitate tunnelling - is a point contact diode with a very high reverse resistance - uses a high doping level to provide a norrow junction - works by quantum tunnelling exhibited by gallium arsenide only

179. A tunnel diode is loosely coupled to its cavity in order to - increase the frequency stability - increase the availability negative resistance - facilitate tunning - allow operation at the highest frequencies 180. The negative resistance in a tunnel diode

- is maximum at the peak point of the characteristic - is available between the peak and valley points - is maximum at the valley point - may be improved by the use of reverse bias

181. The biggest advantage of gallium antimonide over germanium for tunnel diode use is that the former has a

- lower noise - higher ion mobility - larger voltage swing - simpler fabrication process

182. Negative resistance is obtained with a Gunn diode because of

- electron transfer to less mobile energy level - avalanche breakdown with the high voltage current - tunnelling across the junction - electron domains forming at the junction

183. For Gunn diodes, gallium arsenide is preferred to silicon because the former

- has suitable empty energy band, which silicon does not have - has a higher ion mobility - has lower noise at the highest frequencies

184. The biggest disadvantage of IMPATT diode is its

- lower efficiency than that of the other microwave diodes - high noise - inability to provide pulsed operation - lower ability handling ability

185. The magnetic field is used with a ruby maser to

- provide sharp focusing for the electron beam - increase the population inversion - allow room temperature operation - provide frequency adjustment

186. The ruby maser has been preferred to the ammonia maser for microwave amplification, because the former has

- a much greater bandwidth - a better frequency stability - a low noise figure - no need for circulator

187. Parametric amplifiers and masers a re similar to each other in that both (false)

- must have pumping - are extremely low noise amplifiers - must be cooled down to a few kelvins - generally require circulators, since the are one port devices

188. Maser RF amplifier is not really suitable for

- radio astronomy - satellite communications - radar - troposcatter receivers

189. The ruby maser laser differs from the ruby maser in that the former

- does not require pumping - needs no resonator - is an oscillator - produces much lower powers

190. The output from a laser is monochromatic; this means that it is

- infrared - polarized - norrow beam - single frequency

191. For a given average power, the peak output power of a ruby laser may be increased by

- using cooling - using Q spoiling - increasing the magnetic field - dispensing the Farby-Perot resonator

192. Communications laser are used with optical fiber, rather that in open links, to

- ensure that the beams does not spread - prevent atmospheric interference - prevent interference by other lasers - ensure people are not blinded by them

193. Indicate the false statement. The advantage of semiconductor lasers over LEDs include

- monochromatic output - higher power output

- lower cost - ability to be pulsed at higher rates

CHAPTER 13: PULSE COMMUNICATIONS 194. Indicate which of the following is not a binary code

- morse - baudot - CCITT-2 - ARQ

195. To permit the selection of 1 out of 16 equiprobable events, the number of bits required is

- 2 - Log10 16 - 8 - 4

196. Signalling system in which each letter alphabet is represented by a different symbol is not because

- it would be too difficult for an operator to memorize - it is redundant - noise would introduce too many errors - too many pulses per letter are required

197. The Hartley law states that

- the maximum rate of information transmission depends on the channel bandwidth

- the maximum rate of information transmission depends on depth of modulation

- redundancy is essential - only binary codes may be used

198. Indicate the false statement. In order to combat noise,

- the channel bandwidth may be increased - redundancy may be used - the transmitted power may be increased - the signalling rate may be reduced

199. The most common modulation system used for telegraphy is

- frequency shift keying - two tone modulation - pulse code modulation - single tone modulation

200. Pulse width modulation may be generated

- by differentiating pulse position modulation - with a mono stable multivibrator - by integrating the signal - with free running multi vibrator

201. Indicate which of the following system is digital

- Pulse position modulation - Pulse code modulation - Pulse width modulation - Pulse frequency modulation

202. Quantizing noise occurs in

- time division multiplex - frequency division multiplex - pulse code modulation - pulse width modulation

203. The modulation system inherently most noise resistant is

- SSB, suppressed carrier - Frequency modulation - Pulse position modulation - Pulse code modulation

204. In order to reduce quantizing noise, one must

- increase the number of standard amplitudes - sends pulses whose sides are most nearly vertical - use an RF amplifier in the receiver - increase the number of samples per second

205. The Hartley Shannon theorems sets a limit on the

- highest frequency that may be sent over a given channel - maximum capacity of a channel with a given noise level - maximum number of coding levels in a channel with a given noise level - maximum number of quantizing levels in a channel of a given

bandwidth 206. Indicate which of the following pulse modulation system analog.

- PCM - Differential PCM - PWM - Delta

207. Companding is used

- to overcome quantizing noise in PCM - in PCM transmitter, to allow amplitude limiting in the receivers - to protect small signals in PCM from quantizing distortion - in the PCM receivers to overcome impulse noise

208. The biggest disadvantage of PCM is

- its inability to handle analog signals - the high error rate which its quantizing noise introduces - its incompatibility with TM - the large bandwidths what are required for it

CHAPTER 14: DIGITAL COMMUNICATIONS 209. Digital signals

- do not provide a continuous set of values - present values as discrete steps - can utilized decimal or binary systems - all of the above

210. The event which marked the start of the modern computer age was

- design of the ENIAC computer - development of the Hollerith code - development of the transistor - development of the disk drives for data storage

211. The baud rate

- is always equal to the bit transfer rate - is equal to twice the bandwidth of an ideal channel - is not equal to the signalling rate - is equal to one half the bandwidth of an ideal channel

212. The Shannon Hartley Law

- refers to distortion - defines bandwidth - describes signalling rates - refers to noise

213. The code which provides parity check is

- Baudot - ASCII - EBCDIC - CCITT-2

214. Forward error correcting code corrects errors by

- requiring partial retransmission of the signal - requiring retransmission of the entire signal - requiring no part pf the signal to be retransmitted - using parity to correct the errors in all cases

215. Full duplex operation

- requires two pairs of cables - can transfer data in both directions at once - requires modems at both ends of the circuit - all of the above

216. The RS-232 interface

- interconnects data sets and transmission circuits - uses several different connectors - permits custom wiring of signal lines to the connector pins as desired - all of the above

217. Switching systems - improve the efficiency of the data transfer - are not used in data system - require additional lines - are limited to small data networks

CHAPTER 15: BROADBAND COMMUNICATIONS SYSTEM 218. Broadband long distance communications are originally made possible by the advent of

- telegraph cables - repeater amplifiers - HF radio - Geostationary satellites

219. Scheme in which several channels are interleaved and then transmitted together is known as - Frequency division multiplex - Time division multiplex - A group - A super group 220. Basic group B

- occupies the frequency range from 60 to 108 KHz - consist of erect channels only - is formed at the group translating equipment - consist of five super group

221. Time division multiplex

- can be used with PCM only - combines five groups into a super group - stacks 24 channels in adjacent frequency slots - interleaves pulses belonging to different transmissions

222. The number of repeaters along a coaxial cable link depends on

- whether separate tubes are used for the two directions of transmission - the bandwidth of the system - the number of coaxial cables in the tube - the separation of the equalizers

223. Super group pilot is

- applied at each multiplexing bay - used to regulate the gain of individual repeaters - applied at each adjustable equalizer - fed in at a GTE

224. Microwave link repeaters are typically 50km apart

- because of atmospheric attenuation - because of output power limitations - because of earths curvature

- to ensure that the applied dc voltage is not excessive 225. Microwave links are generally preferred to coaxial cable for television transmission because

- the have less overall phase distortion - they are cheaper - of their greater bandwidths - of their relatively immunity to impulse noise

226. Armored submarine cable is used

- to protect the cable at great depths - to prevent inadvertent ploughing in of the cable - for the shallow shore ends of the cable - to prevent insulation breakdown from the high feeds voltages

227. Submarine cable repeater contains among other equipments

- a dc power supply and regulator - filters for two directions of transmission - multiplexing and demultiplexing equipment - pilot inject and pilot extract equipment

228. Geostationary satellite

- is motionless in space (except for its spins) - is not really stationary at all but its orbits the earth within a 24hr

period - appears stationary above the Earth’s magnetic poles - is located at height of 35,800km to ensure global coverage

229. Indicate the correct statement regarding satellite communications.

- if two earth stations do not face a common satellite the should be communicate via double satellite hop

- satellite are allocated so that it is impossible for to earths station not to face the same satellite

- collocated earth station are used for frequency diversity - A satellite earth station must have as many receiver chains as there are

carriers transmitted to it. 230. Satellite used for inter continental communications are known as

- Comsat - Domsat - Marisat - Intelsat

231. Identical telephone number in different parts of a country are distinguished by their

- language digits - access digits - area codes - central office codes

232. Telephone traffic is measured - with echo cancellers - by their relative congestions - in terms of the grade of services - in erlangs

233. In order to separate channels in a TDM receiver, it is necessary to use

- AND gates - Bandpass filter - Differentiation - Integration

234. To separate channels in a FDM receiver, it is necessary to use

- AND gates - Bandpass filter - Differentiation - Integration

235. Higher order TDM levels are obtained by

- dividing pulse widths - using the a-law - using the u-law - forming supermastergroups

236. Losses in optical fiber can be caused by (false)

- impurities - microbending - attenuation in the glass - stepped index operation

237. The 1.55um “window” is not yet in used with fiber optic system because

- the attenuation is higher than at 0.85um - the attenuation is higher than at 1.3um - suitable laser devices have not yet been developed - it does not lend itself to wavelength multiplexing

238. Indicate which of the following is not a submarine cable

- TAT-7 - INTELSAT V - ATLANTIS - CANTAT 2

239. Indicate which of the following is an American Domsat system

- INTELSAT - COMSAT - TELSAT - INMARSAT

CHAPTER 16: RADAR SYSTEM

240. If the peak transmitted power in a radar system is increased by factor of 16, the maximum range will be increased by factor of

- 2 - 4 - 8 - 16

241. If the antenna diameter in a radar system is increased by factor of 4, the maximum range will be increased by factor of

- (2)^1/2 - 2 - 4 - 8

242. If the ratio of the antenna diameter to the wavelength in a radar system is high this will result in (false)

- large maximum range - good target discrimination - difficult target acquisition - increased capture area

243. The radar cross section to a target (false)

- depends on the frequency used - may be reduced by special coating of the target - depends on the aspect of a target, if this is nonspherical - is equal to the actual cross sectional area fro small targets

244. Flat topped rectangular pulses must be transmitted in radar to (false)

- allow good minimum range - make the returned echoes easier to distinguish from noise - prevent frequency changes in the magnetron - allow accurate range measurements

245. High PRF will (false)

- make the returned echoes easier to distinguish from noise - make target tracing easier with conical scanning - increase maximum range - have no effect on the range resolution

246. The IF bandwidth of a radar receiver is inversely proportional to the

- pulse width - pulse repetition frequency - pulse interval - square root of the peak transmitted power

247. If a return echo arrives after the allocated pulse interval,

- it will interfere with the operation of the transmitter - the receiver might be overloaded - it will not be received - the target will appear closer than it really is

248. After a target has been acquired, the best scanning system for tracking is

- nodding - spiral - conical - helical

249. If the target cross section is changing, the best system for accurate tracking is

- lobe switching - sequential lobbing - conical scanning - mono pulse

250. The biggest disadvantage of CW Doppler radar is that

- it does not give the target velocity - the target range, but not position - a transponder is required at the target - it does not give the target position

251. A scope displays - the target position and range - the target range and but not position - the target position but not the range - neither range nor position but only velocity 252. The Doppler effect is used in (false)

- moving target plotting on the PPI - the MTI system - FM radar - CW radar

253. The coho in MTI radar operates at the

- intermediate frequency - transmitted frequency - received frequency - pulse repetition frequency

254. The function of the quartz delay line in an MTI radar is to

- help subtracting a complete scan from the previous scan - match the phase of the coho and the stalo - match the phase of the coho and the output oscillator - delay a sweep so that the next sweep can be subtracted from it

255. A solution to the “blind speed” problem is to - change the Doppler frequency - Vary the PRF - Use monopulse - Use MTI

256. Indicate which one of the following applications are advantages of radar beacons is false:

- target identification - navigation - very significant extension of the maximum range - more accurate tracking of enemy targets

257. Compared with other types of radar, phased array radar has the following advantages (false)

- very fast scanning - ability to tract and scan simultaneously - circuit simplicity - ability to track may targets simultaneously

CHAPTER 17: TELEVISION FUNDAMENTALS 258. The number of lines per field in the United State TV system is

- 262 ½ - 525 - 30 - 60

259. The number of frames per second in the US TV system is

- 60 - 262 ½ - 4.5 - 30

260. The number of lines per second in the US TV system is

- 31500 - 15750 - 262 ½ - 525

261. The channel width in the US TV system, in MHz is

- 41.25 - 6 - 4.5 - 3.58

262. Interlacing is used in television to

- produce the illusion of motion - ensure that all the lines on the screen are scanned, not merely the

alternate ones - simplify the vertical sync pulse train - avoid flicker

263. The signals sent by the TV transmitter to ensure correct scanning in the receiver are called

- sync - chroma - luminance - video

264. In the US color television system, the intercarrier frequency, in MHz

- 3.58 - 3.579545 - 4.5 - 45.75

265. Indicate which voltages are not found in the output of a normal monochrome receiver video detector

- sync - video - sweep - sound

266. The carrier transmitted 1.25MHz above the bottom frequency in a US TV system channel is the

- sound carrier - chroma carrier - intercarrier - picture carrier

267. In television 4:3 represents the

- interlace ratio - maximum horizontal deflection - aspect ratio - ratio of the two diagonals

268. Equalizing pulses in TV are sent during

- horizontal blanking - vertical blanking - the serrations - the horizontal retrace

269. An odd number of lines per frame forms part of every one of the world’s TV system. This is

- done to assist interlaced - purely an accident - to ensure that line and frame frequencies can be obtained from the

same original source - done to minimize interference with the chroma subcarriers

270. The function of the serration in the composite video waveform is to

- equalized the charge in the integrator before the start of vertical retrace

- help vertical synchronization - help horizontal synchronization

- simplify the generation of vertical sync pulse 271. The width of the vertical pulse in US TV system is - 21H - 3H - H - 0.5H 272. Indicate which of the following frequencies will not be found in the output of a normal TV receiver tuner:

- 4.5 MHz - 41.25 MHz - 45.75 MHz - 42.17 MHz

273. The video voltage applied to the picture tube of a television receiver is fed in

- between grid and ground - to the yoke - to the anode - between the grid and cathode

274. The circuit that separates the sync pulse from the composite video waveform is

- the keyed AGC amplifier - a clipper - an integrator - a differentiator

275. The output of the vertical amplifier, applied to the yoke in the TV receiver, consist of

- direct current - amplified vertical sync pulses - saw tooth voltage - a saw tooth current

276. The HV anode supply for the picture tube for a TV receiver is generated in the

- main transformer - vertical output stage - horizontal output stage - horizontal deflection stage

277. Another name for the horizontal retrace in a TV receiver is the

- ringin - burst - damper - flyback

278. Indicate which of the following signal is not transmitted in color TV:

- Y - Q - R - I

279. The Shadow mask in a color picture tube is used to

- reduce x-ray emission - ensure that each beam hits only its own dots - increase screen brightness - provide degaussing for the screen

280. In a TV receiver the color killer

- cuts off the chroma stages during monochrome reception - ensure that no color is transmitted to monochrome receiver - prevents color over loading - makes sure that the color burst is not mistaken for sync pulse, by

cutting off reception during the back porch