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KENNEDY 4 th^ EDITION
MULTIPLE CHOICES
INTRO TO COMMUNICATIONS SYSTEM
- In a communications system, noise is most likely to affect the signal
a. at the transmitter b. in the channel c. in the information source d. at the destination
- Indicate the false statement. Fourier analysis shows that a sawtooth wave consist of
a. fundamental and subharmonic sine waves b. a fundamental sine wave and an infinite number of harmonics c. fundamental and harmonic sine waves whose amplitude decreases with the harmonic number d. sinusoidal voltages, some of which are small enough to ignore in practice
- Indicate the false statement. Modulation is used to
a. reduce the bandwidth used b. separate differing transmissions c. ensure that intelligence may be transmitted over long distances d. allow the use of practicable antennas
- Indicate the false statement. From the transmitter the signal deterioration because of noise is usually
a. unwanted energy b. predictable in character c. present in the transmitter d. due to any cause
- Indicate the true statement. Most receivers conforms to the
a. amplitude-modulated group b. frequency-modulated gr c. superheterodyne group
oup
d. tuned radio frequency receiver group
- Indicate the false statement. The need for modulation can best be exemplified by the following.
a. Antenna lengths will be approximately λ/4 long b. An antenna in the standard broadcast AM band is 16,000 ft c. All sound is concentrated from 20 Hz to 20 kHz d. A message is composed of unpredictable variations in both amplitude and frequency
- Indicate the true statement. The process of sending and receiving started as early as
a. the middle 1930s b. 1850 c. the beginning of the twentieth century d. the 1840s
- Which of the steps is not included in the process of reception?
a. decoding b. encoding c. storage d. interpretation
- The acoustic channel is used for which of the following?
a. UHF communications b. single-sideband communication c. television communications d. person-to-person voice communications
- Amplitude modulation is the process of
a. superimposing a low frequency on a high frequency b. superimposing a high frequency on a low frequency c. carrier interruption d. frequency shift and phase shift
- Which of broad classifications of noise are most difficult to treat?
a. noise generated in the receiver b. noise generated in the tran c. externally generated noise
smitter
d. internally generated noise
- Space noise generally covers a wide frequency spectrum, but the strongest interference occurs
a. between 8 MHz and 1.43 GHz b. below 20 Mhz c. between 20 to 120 MHz d. above 1.5 GHz
- When dealing with random noise calculations it must be remembered that
a. all calculations are based on peak to peak values b. calculations are based on peak values c. calculations are based on average values d. calculations are based on RMS values
- Which of the following is the most reliable measurement for comparing amplifier noise characteristics?
a. signal-to-noi b. noise factor
se ratio
c. shot noise d. thermal noise agitation
- Which of the following statements is true?
a. Random noise power is inversely proportional to bandwidth b. Flicker is sometimes called demodulation noise c. Noise is mixers is caused by inadequate image frequency rejection d. A random voltage across a resistance cannot be calculated
AMPLITUDE MODULATION
- 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
a. 4V b. 3V c. 2V d. 1V
- In a low-level AM system, amplifiers following the modulated stage must be
a. linear devices b. harmonic devices c. class C amplifiers d. nonlinear devices
- If the carrier of a 100 percent modulated AM wave is suppressed, the percentage power saving will be
a. 50 b. 150 c. 100 d. 66.
- Leak-type bias is used in a plate-modulated class C amplifier to
a. prevent tuned circuit damping b. prevent excessive grid current c. prevent overmodulation d. increase the bandwidth
- The output stage of a television transmitter is most likely to be a
a. plate-modulated class C amplifier b. grid-modulated class C amplifier c. screen-modulated class C amplifier d. grid-modulated class A amplifier
- The modulation index of an AM wave is changed from 0 to 1. The transmitted power is
a. unchanged b. halved c. doubled d. increase by 50 percent
SINGLE-SIDEBAND TECHNIQUES
- Indicate the false statement regarding the advantages of SSB over double sideband, full-carrier AM
a. More channel space is available. b. Transmitter circuits must be more stable, giving better reception. c. The signal is more noise-resistant d. Much less power is required for the same signal strength
- When the modulation index of an AM wave is doubled, the antenna current is also doubled. The AM system being used is
a. Single-sideband, full carrier (H3E) b. Vestigial sideband (C3F) c. Single-sideband, suppressed carrier (J3E) d. Double-sideband, full carrier (A3E)
- Indicate which one of the following advantages of the phase cancellation method of obtaining SSB over the filter method is false :
a. Switching from one sideband to the other is simpler. b. It is possible to generate SSB at any frequency. c. SSB with lower audio frequencies present can be generated d. There are more balanced modulators; therefore the carrier is suppressed better.
- The most commonly used filters in SSB generation are
a. mechanical b. RC c. LC d. low-pass
- In an SSB transmitter, one is most likely to find a
a. class C audio amplifier b. tuned modulator c. class B RF amplifier d. class A RF output amplifier
- Indicate in which one of the following only one sideband is transmitted.
a. H3E b. A3E c. B8E d. C3F
- One of the following cannot be used to remove the unwanted sideband in SSB. This is the a. filter system b. phase-shift method c. third method d. balanced modulator
- R3E modulation is sometimes used to
a. allow the receiver to have a frequency synthesizer b. simplify the frequency stability problem in reception c. reduce the power that must be transmitted d. reduce the bandwidth required for transmission
- To provide two or more voice circuits with the same carrier, it is necessary to use
a. ISB b. carrier insertion c. SSB with pilot carrier d. Lincompex
- Vestigal sideband modulation (C3F) is normally used for
a. HF point-to-point communications b. monoaural broadc c. TV broadcasting
asting
d. stereo broadcasting
- Since noise phase-modulates the FM wave, as the noise sideband frequency approaches the carrier frequency, the noise amplitude a. remains constant b. is decreased c. is increased d. is equalized
- When the modulating frequency is doubled, the modulation index is halved, and the modulating voltage remains constant. The modulation system is a. amplitude modulation b. phase modulation c. frequency modulation d. any of the three
- Indicate which one of the following is not an advantage of FM over AM: a. Better noise immunity is provided b. Lower bandwidth is required c. The transmitted power is more useful d. Less modulating power is required
- One of the following is an indirect way of generating FM. This is the a. reactance FET modulator b. varactor diode modulator c. Armstrong modulator d. reactance bipolar transistor modulator
- In an FM stereo multiplex transmission, the a. sum signal modulates 19 kHz subcarrier b. difference signal modulates the 19 kHz subcarrier c. difference signal modulates the 38 kHz subcarrier d. difference signal modulates the 67 kHz subcarrier
RADIO RECEIVERS
- Indicate which of the following statements about the advantages of the phase discriminator over the slope detector is false :
a. Much easier alignment b. Better linearity c. Greater limiting d. Fewer tuned circuits
- Show which of the following statements about the amplitude limiter is untrue :
a. The circuit is always biased in class C, by virtue of the leak-type bias. b. When the input increases past the threshold of the limiting, the gain decreases to keep the output constant. c. The output must be tuned d. Leak-type bias must be used
- In a radio receiver with simple AGC
a. an increase in signal strength produces more AGC b. the audio stage gain is normally controlled by the AGC c. the faster the AGC time constant the more accurate the output d. the highest AGC voltage is produced
- In a broadcast superheterodyne receiver, the
a. local oscillator operates below the signal frequency b. mixer input must be tuned to the signal frequency c. local oscillator frequency is normally double the IF d. RF amplifier normally works at 455 kHz above the carrier frequency
- To prevent overloading of the IF amplifier in a receiver, one should use
a. squelch b. variable sensitivity c. variable selectivity d. double conversion
- A superheterodyne receiver with an IF of 450 kHz is tuned to a signal at 1200 kHz. The image frequency is
a. 750 kHz b. 900 kHz c. 1650 kHz d. 2100 kHz
- If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of
a. improving the effectiveness of the AGC b. reducing the effect of negative-peak clipping c. reducing the effect of noise at low modulation depths d. improving the selectivity of the receiver
- Indicate the false statement. The superheterodyne receiver replaced the TRF receiver because the latter suffered from
a. gain variation over the frequency coverage range b. insufficient gain and sensitivity c. inadequate selectivity at high frequencies d. instability
- The image frequency of a superheterodyne receiver
a. is created within the receiver itself b. is due to insufficient adjacent channel rejection c. is not rejected be the IF tuned circuits d. is independent of the frequency to which the receiver is tuned
- One of the main functions of the RF amplifier in a superheterodyne receiver is to
a. provide improved tracking b. permit better adjacent-channel rejection c. increase the tuning range of the receiver d. improve the rejection of the image frequency
- A receiver has poor IF selectivity. It will therefore also have poor
a. blocking b. double-spotting c. diversion reception d. sensitivity
- Three-point tracking is achieved with
a. variable selectivity b. the padder capacitor c. double spotting d. double conversion
- The local oscillator of a broadcast receiver is tuned to a frequency higher than the incoming frequency
a. to help the image frequency rejection b. to permit easier tracking c. because otherwise an intermediate frequency could not be produced d. to allow adequate frequency coverage without switching
- If the intermediate frequency is very high (indicate false statement)
a. image frequency rejection is very good b. the local oscillator need not be extremely stable c. the selectivity will be po d. tracking will be improved
or
- A low ratio of the ac to the dc load impedance of a diode detector results in
a. diagonal clipping b. poor AGC operation c. negative-peak clipping d. poor AF response
- One of the following cannot be used to demodulate SSB:
a. Product detector b. Diode Balance modulator c. Bipolar transistor balanced modulator d. Complete phase-shift generator
- Indicate the false statement. Noting that no carrier is transmitter with J3E, we see that
a. the receiver cannot use a phase comparator for AFC b. adjacent-channel rejection is more difficult c. production of AGC is a rather complicated process d. the transmission is not compatible with A3E
- When a receiver has good blocking performance, this means that
a. it does not suffer from double-spotting b. its image frequency rejection is poor c. it is unaffected by AGC derived from nearby transmissions d. its detector suffers from burnout
- An AM receiver uses a diode detector for demodulation. This enables it satisfactorily to receive
a. single-sideband, suppressed-carrier b. single-sideband, reduced-carrier c. ISB d. Single-sideband, full-carrier
- The main disadvantage of the two-hole directional coupler is
a. low directional coupling b. poor directivity c. high SWR d. narrow bandwidth
- To couple a coaxial line to a parallel-wire line, it is best to use a
a. slotted line b. balun c. directional coupler d. quarter-wave transformer
- Indicate the three types of transmission line energy losses.
a. I^2 R , R (^) L , and temperature b. Radiation, I^2 R , and dielectric heating c. Dielectric separation, insulation breakdown, and radiation d. Conductor heating, dielectric heating, and radiation resistance.
- Indicate the true statement below. The directional coupler is
a. a device used to connect a transmitter to a directional antenna b. a coupling device for matching impedance c. a device used to measure transmission line power d. an SWR measuring instrument
RADIATION AND PROPAGATION OF WAVES
- Indicate which one of the following terms applies to troposcatter propagation:
a. SIDs b. Fading c. Atmospheric storms d. Faraday rotation
- VLF waves are used for some type of services because
a. of the low powers required b. the transmitting antennas are of convenient size c. they are very reliable d. the penetrate the ionosphere easily
- Indicate which of the following frequencies cannot be used for reliable beyond-the-horizon terrestrial communications without repeaters:
a. 20 kHz b. 15 MHz c. 900 MHz d. 12 GHz
- High-frequency waves are
a. absorbed by the F 2 layer b. reflected by the D layer c. capable of use for long-distance communications on the moon d. affected by the solar cycle
- Distances near the skip distance should be used for the sky-wave propagation
a. to avoid tilting b. to prevent sky-wave and upper ray interference c. to avoid the Faraday effect d. so as nor to exceed the critical frequency
- A ship-to-ship communications system is plagued by fading. The best solution seems to be use of
a. a more directional antenna b. a broadband antenna c. frequency diversity d. space diversity
- Electromagnetic waves are refracted when they
a. pass into a medium of different dielectric constant b. are polarized at right angles to the direction of propagation c. encounter a perfectly conducting plane d. pass through a small slot in a conducting plane
- Diffraction of electromagnetic waves
a. is caused by reflections from the ground b. arises only with spherical wavefronts c. will occur when the waves pass through a large slot d. may occur around the edge of a sharp obstacle
- When microwave signals follow the curvature of the earth, this is known as
a. the Fara b. ducting
day effect
c. tropospheric scatter d. ionospheric reflection
- Helical antennas are often used for satellite tracking at VHF because of
a. troposcatter b. superrefraction c. ionospheric refract d. the Faraday effect
ion
ANTENNAS
- An ungrounded antenna near the ground
a. acts as a single antenna of twice the height b. is unlikely to need a ground screen c. acts as an antenna array d. must be horizontally polarized
- One of the following consists of nonresonant antennas:
a. The rhombic antenna b. The folded dipole c. The end-fire array d. The broadside array
- One of the following is very useful as a multiband HF receiving antenna. This is the:
a. conical horn b. folded dipole c. log periodic d. square loop
- Which of the following antennas is best excited from a waveguide?
a. Bicon b. Horn
ical
c. Helical d. Discone
- Indicate which of the following reasons for using a counterpoise with antenna is false :
a. Impossibility of a good ground connection b. Protection of personnel working underground c. Provision of an earth for the antenna d. Rockiness of the ground
- One of the following is not a reason for the use of an antenna coupler:
a. To make the antenna look resistive b. To provide the output amplifier with the correct load impedance c. To discriminate against harmonics d. To prevent reradiation of the local oscillator
- Indicate the antenna that is not widebandL:
a. Discone b. Folded Dipole c. Helical d. Marconi
