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150 Cards in this Set
- Front
- Back
- 3rd side (hint)
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Explain the difference between dB, dBW, and dBm.
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dB are logarithmic units for expressing power ratios, dB will either show gain or attenuation. dBW and dBm express power levels. Zero dBW is referenced to one watt, where dBm is referenced to one milliwatt.
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What formula converts a power gain or attenuation ratio into decibels?
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dB=(10)log P1/P2
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What are the formulas for converting between dBm and milliwatts and vice versa?
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dBm=(10)logXmw or Xmw=Log-1(dBm/10)
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Convert 3.5 kw into dBW.
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kw = 3500 w, dBW =(10)log 3500 > dBW =(10)3.54407 > dBW =35.4407
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Convert 15 watts into dBm.
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w = 15,000 mw, dBm=(10)log15,000 > dBm = (10)4.17609 > dBm =41.7609
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Convert .0007 watt into dBm, then to dBW.
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.0007w = .7 mw, dBm =(10)log.7 > dBm= -1.5490 > dBm -30= -1.5490 - 30 > dBW = -31.5490
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An amplifier multiplies its input times 20. How much dB gain is that?
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dB=(10)log 20/1 >dB=13.0103 (power gain is 20 and 20=10x2. A 10X gain is 10dB/ a 2X gain is 3dB+3dB=13 dB )
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A 32 dBm signal is sent down a cable that attenuates it by 6 dB, how many watts would be output by the cable?
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32 dBm-6dB= xBm >26dBm-30 = xBm-30 > -4dBW = xBW >Xwatts = log-1(-4dBW/10) >Xwatts = .3981 watts
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What type of power measures the unmodulated signal?
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Carrier power.
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What type of power measures an average of power supplied to the transmission line over a long period of time compared to the lowest modulated frequency?
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Mean power.
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What is the basic formula for power?
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Power equals current times voltage, P = I • E
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What is the formula for EIRP?
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EIRP = Pt – LLt +Gt.
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What is the formula for ETP?
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ETP = Pt – LLt +Gt +Gr – LLr.
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Bandwidth is defined as the difference between what two components?
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Highest frequency signal component and the lowest frequency signal component.
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What unit is bandwidth measured in?
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Hertz (Hz).
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Which bandwidth type is used as part of the emission designator?
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Necessary
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The width of the frequency band that contains 99.5 percent of the mean power of the emission describes which type of bandwidth?
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Occupied
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The first four to six positions of the emission designator represent what technical value?
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Necessary bandwidth
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The first symbol after the necessary bandwidth indicates what value?
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The type of modulation of the main character.
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Which symbol indicates the type of information to be transmitted?
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Third symbol
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A code of “N” in the fifth symbol slot indicates what?
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That the signal is not multiplexed.
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What is the emission designator for a low-frequency CW transmission?
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100H00A1A
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The emission designator 3K00J3C indicates the transmission of what type of data?
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Facsimile
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What is the emission designator for UHF single-channel voice, uncovered?
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6K00A3E
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25K00G7W is the emission designator for what type of transmission?
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SATCOM voice, covered.
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What is the emission designator for Link 11?
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45K00F2D
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What term describes complete oscillations an EM wave makes per second?
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Frequency.
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Three to 30 KHz defines what international frequency division?
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Very low frequency (VLF)
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What term describes the distance the EM wave covers in one complete oscillation?
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Electrical wavelength
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Which formula do you use to calculate the free space EM wavelength of a known frequency?
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Electrical wavelength;or λ(feet) = 984/Freq (MHz)
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What is the electrical or free space wavelength of an 88 MHz signal?
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(984 feet / sec) / (88x106/ sec) =l > (984 feet / sec)*88 x 106 sec= l > 11.18 feet= l
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What is the physical or antenna wavelength of an 88 MHz signal?
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936/88 MHz = 10.64 feet (936 feet/sec 88x106/sec) = l > (936 feet/sec) * 88x106/sec = l (936 feet *88 x 106 sec=l > 10.64 feet= l
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What is the metric electrical wavelength for 88 MHz? 3.408 meters.
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(11.1818 x .3048 = 3.4082126) or (11.18 x .3048 = 3.407664)
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For amplitude modulation, the amplitude of what carrier is varied?
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RF carrier.
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What type modulation produces a constant amplitude transmission?
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Frequency modulation
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Which mode of operation is being used when both stations transmit on the same frequency?
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Simplex
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Which mode of operation commonly uses repeaters?
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Half duplex
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Which mode of operation supports communications between two or more stations in both directions simultaneously?
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Duplex
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What are the two basic stages in an AM transmitter?
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Audio frequency and radio frequency stages
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In an AM transmitter, what component electronically isolates the oscillator from the stages downstream from it, preventing load variations from affecting RF oscillator operation?
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Buffer amplifier
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What are the two basic characteristics of the AM transmitter’s modulation envelope?
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Frequency and power distribution
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In a DSB transmission, what percentage of power is in each sideband?
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25 percent
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What aspect of the transmitted intelligence carried by a conventional DSB AM signal makes SSB operation possible?
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Intelligence is contained in both sidebands
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What stage is considered to be the “heart” of the SSB transmitter?
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Balanced modulator
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What is the main purpose of the balanced modulator?
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To eliminate the RF carrier
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What SSB stage allows the operator to select the sideband that is transmitted?
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Sideband selector
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In the FM transmitter, intelligence to be transmitted is superimposed on what?
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The carrierBecause of the many sideband frequencies contained in FM waves, which frequencies are included only in the bandwidth?
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Significant frequencies
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A FM wave used to transmit multiple channels of intelligence has what type of bandwidth?
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Wide
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What must be limited in a FM wave to produce the same bandwidth as an AM wave carrying the same intelligence?
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Maximum deviation of the FM carrier
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What process mixes two frequencies in a nonlinear device, producing an output consisting of the two original frequencies, the sum of the two, and the difference of the two?
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Heterodyning
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If the RF amplifier in an AM receiver was tuned to receive 20,300 kHz, what frequency would the local oscillator pass to the mixer?
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20755 KHz
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What AM receiver stage demodulates and re-creates the intelligence from the IF?
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Detector
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In a SSB receiver, what component reproduces the original RF carrier frequency?
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Beat frequency oscillator (BFO)
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In the SSB receiver, what component receives the signal from the BFO and mixes it with the IF signal?
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Second mixer
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Where do the original carrier and the selected sideband combine to redevelop the original modulation envelope in a SSB receiver?
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Second mixer stage
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In a FM receiver, what component removes amplitude variations in the received signal that were picked up by outside interfering sources?
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Limiter
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In a FM radio receiver, what component removes the original signal from the sidebands and recreates the audio intelligence?
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Discriminator
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At what type of frequencies does pulse modulation usually work best?
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Extremely high frequencies (EHF)
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What are the different methods of separation or information extraction?
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Tuning, pulse width, pulse position, or pulse phase
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What type of frequency separation is most widely used?
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Tuning
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Pulse-amplitude modulation is highly affected by what factor?
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Noise
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What technique minimizes noise interference and improves accuracy of signal in a PDM system?
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Pulse-amplitude-to-width conversion
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In pulse-frequency modulation, as the pulse frequency increases, what happens to the modulating signal amplitude?
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It, too, increases
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What is another term used to identify PPM?
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Pulse-phase modulation
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What type of pulse modulation is also known as PNM?
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Pulse-code modulation (PCM)
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What type of pulse modulation is least affected by noise?
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PCM or PNM)
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What do we call the process of transmitting more than one channel of information over a single link?
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Multiplexing
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What can be done to transmission lines to minimize the effects of cross talk?
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Proper shielding, grounding, or separation
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What type of multiplexing transmits several signals simultaneously over a single channel?
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Frequency-division multiplexing (FDM)
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What type of multiplexing is used to transmit two or more channels of information over a single carrier?
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Time-division multiplexing (TDM).
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What type of multiplexing requires additional equipment to provide synchronization between the transmitter and receiver sections?
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TDM
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What receiver characteristic involves its ability to reproduce the signal of a very weak station?
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Sensitivity
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What receiver characteristic involves its ability to select and reproduce a desired signal from among several closely spaced stations or from interfering frequencies?
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Selectivity
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The amount of noise generated in a conductor is directly proportional to what physical characteristic of that conductor?
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Its temperature
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Two-wire transmission lines exhibit the properties of a circuit that contains what electronic components?
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Resistors, capacitors, and inductors
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When RF energy is applied to the transmission line, it causes the capacitors and inductors to exhibit what property?
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Reactance
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On what property is transmission line capacitive and inductive reactance dependent?
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The frequency the transmission line is carrying
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What term is described as the total resistive properties of a circuit when resistance, capacitive reactance, and inductive reactance are formulated together?
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Impedance
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Why is impedance matching between the transmitter and transmission line important?
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We want as much transmitter output power to the antenna as possible
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What measurements are used to determine if the transmission line and transmitter’s impedance are matched?
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VSWR, SWR, or the ratio of incident and reflected power.
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Given an incident power of 90 watts and a reflected power of 3 watts, what is the VSWR measurement?
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incident P/ reflected P = 90W/3W =30/1=30:1 > VSWR = (1+1/30 ) ÷(1-1/30) >VSWR=1.446
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What formula is used to calculate SWR when the impedance of the transmission line and antenna are known?
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SWR= Z(larger)/z(Smaller)
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What transmission line can have less than 0.1 dB loss per 100 feet when transmitting the HF band, but is also characterized by high radiation losses and electrical noise pickup due to the lack of shielding?
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Two-wire open line
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What balanced transmission line has braided copper shielding that isolates its conductors from stray EM fields?
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The shielded pair
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What are the three types of coaxial cable?
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Rigid (air-dielectric), flexible (solid dielectric), and semi-rigid (foam dielectric)
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Which type of coaxial cable is most likely seen in a tactical environment?
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Flexible coaxial lines
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What type of coaxial cable is best suited for underground cable runs?
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Semi-rigid coaxial line
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What is the disadvantage of waveguides when used with frequencies below 1,000 MHz?
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The large physical size of waveguides, at frequencies lower than 1,000 MHz, makes them impractical.
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As a waveguide gets bigger, what happens to its optimum frequency range?
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Its optimum operating frequency gets lower
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What transmission line is immune to EMI?
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Fiber optic
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Name the two parts of the electromagnetic wave.
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Electric fields (E-lines) and magnetic fields (H-lines)
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As RF is applied to an antenna, the E- and H-lines are components that combine to form two different EM fields. What are the two fields and which field travels and makes radio communications possible?
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Induction field and radiation field, the radiation field becomes detached from the conductor and travels through space making radio communications possible.
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What type of polarization is best for HF ground wave communications? Why?
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Vertical polarization is best for HF groundwave transmissions because radiation fields are attenuated less when E-lines are perpendicular to the earth’s surface
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What type of polarization would normally be better for a radio system covering the downtown Chicago area?
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Horizontal, because dense forests or buildings attenuate the horizontally polarized radiation fields less than the vertical polarized ones
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Does the antenna polarization of members participating in an HF sky wave network need to be the same? Explain.
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In HF skywave propagation, the polarization of the transmitting and receiving antennas does not have to match because of the random changing of the signal as it is bent by the ionosphere.
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When does radio wave refraction occur?
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Refraction of the radio wave only occurs when the wave enters the new medium at an oblique (nonperpendicular) angle.
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What propagation characteristic describes the bending of the wave as it travels near the edge of a solid object that it cannot pass through?
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Diffraction
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What causes attenuation of RF energy in free space?
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Spreading out of the wave
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Compute the predicted path loss using the FSL formula given 88 MHz transmitted over a path distance of seven statute miles.
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FSL = 37 + 20 Log f (MHz) + 20 Log D (miles). FSL = 37 + 20 Log 88 + 20 Log 7 or FSL = 37 + 38.8897 + 16.9019. FSL = 92.7916 dB.
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When you shine a flashlight on a pool of water, some of the light is reflected off of the water, and some penetrates the water and is bent in a different direction. This bending is demonstrative of what type of propagation effect?
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Refraction
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Refraction only occurs when radio waves enter new mediums at what type of angle?
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Oblique (nonperpendicular)
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What type of propagation effect is described by the radio wave bending around the edge of a solid object that it cannot pass through?
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Diffraction
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What type of propagation effect is determined by the sharpness of an object, the height and distance of the object, and the frequency of the transmitted signal?
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“Knife edge” diffraction
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What type of precipitation attenuation has a pronounced degrading effect on microwave energy but little effect on HF radio waves?
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Rain
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Which type of precipitation causes frequencies above 2 GHz to be seriously attenuated?
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Fog
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What aspects of hail determine how much it will attenuate a signal?
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Size and density of the stones
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Any antenna transfers RF energy from space to its terminals with the same efficiency that it transfers RF energy from its terminals into space. What antenna property does the preceding statement define?
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Reciprocity
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What are the two types of radiators?
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Isotropic and anisotropic
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The sun is a good example of what type of radiator?
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Isotropic
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What term describes all practical RF radiators that emit stronger radiation in one direction than another?
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Anisotropic radiators
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A flashlight beam is a good example of what type of radiator?
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Anisotropic radiator.
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The difference in strength between the signal received from an isotropic radiator and the signal received from the directional antenna is the gain of the directional antenna. In what unit of measurement is this gain expressed?
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Decibels (dB) or decibels isotropic (dBi).
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Where on its RF emission is an antenna’s beamwidth measured?
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At the half-power point
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What are the two methods for measuring the antenna beam half-power point?
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The half-power point can be measured where the strength of the E-field falls to 0.707 of its maximum. Or it can be measured where the signal power dissipates to 0.5 (half) of its maximum.
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If the resonant antenna is fed with other than its resonant frequency, what will happen?
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Much of the input signal’s power will be turned back down the transmission line and cause destructive standing waves and inefficient operation.
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How much bandwidth do most resonant antennas have?
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Within 2 percent of its design frequency
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To what lengths are resonant antennas cut?
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The ½ wavelength, any multiple of the ½ wavelength, and a ¼-wavelength antenna that is grounded at one end and will act as a resonant antenna.
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Using the physical ½ wavelength formula, how long is a half-wave antenna for 20 MHz?
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L /2 (ft)= 492/20MHz > L/2= 26.4ft
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For impedance matching on the resonant antenna, by what is antenna impedance or radiation resistance greatly varied?
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Where the transmission line is connected to the antenna
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When one end of a nonresonant antenna is terminated in a resistance that is equal to its characteristic impedance, what happens to the antenna’s standing waves?
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A minimum amount of standing waves set up because its waves can travel in only one direction.
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When a nonresonant antenna is impedance matched, what would be an acceptable VSWR reading?
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The VSWR should be below 3:1; the closer to 1.5, the better
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From which two antenna types have nearly all antennas been developed?
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Hertz and Marconi
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Which antenna is commonly referred to as the “Hertz” or half-wave antenna, and is made of two lengths of wire rod or tubing, each ¼ wavelength long?
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Dipole
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What type of radiation pattern does the vertically polarized dipole antenna radiate and what purpose is it best suited for?
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Shaped like a doughnut, and best for omnidirectional and groundwave propagation
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What is the horizontally polarized dipole’s radiation pattern and what purpose is it best suited for?
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Resembles a short, over-stuffed link of summer sausage, best for HF skywave networks.
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How is the horizontally polarized dipole’s radiation pattern adjusted for HF operations?
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Turning it on its side changes it to a bidirectional antenna
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What advantage does the folded dipole have over a simple dipole?
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The folded dipole can be used over a much wider frequency range than is possible with a simple dipole.
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What is the whip antenna also known as?
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Monopole antenna
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Normally, what is the gain of a whip antenna?
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Generally, less than 2 dBi
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When VHF and UHF vertical ¼-wave antennas are positioned several wavelengths above the ground, undesirable high angles of radiation can result. What can be used to remedy these high radiation angles?
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Use a simulated ground or ground plane mounted below the antenna
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Why are whip antennas not very good for HF skywave transmissions?
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The omnidirectional characteristics of the whip wastes RF power in long-range point-to-point communications; more directional antennas are better suited for the mission.
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Which antenna is highly directional and used for HF skywave nets that span from 500 to 1,200 miles?
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Long-wire antenna
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At what length does a long-wire antenna exhibit a pronounced power gain on its end lobes?
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At least 2 wavelengths long
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When siting a long-wire antenna, why must you offset the physical axis of the antenna at an angle from the distant station?
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To ensure the high-gain radiation lobe is aimed at the station you want communication with. Depending on the length of the long-wire antenna, the offset angle will be different.
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When one end of a long-wire antenna is terminated in the antenna’s characteristic impedance,how is the radiation pattern affected?
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The terminated long-wire radiates only in the direction of the terminated end and eliminates the back radiation lobe.
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How are helical antennas polarized and what band do they normally work in?
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Circularly polarized, used mostly for UHF operations
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Normally what is the basic element of an antenna array?
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It is generally the ½-wave dipole
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What are the two types of elements in an array antenna?
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Driven elements and parasitic elements
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Which type element is connected directly to the transmission line?
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The driven element
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In what direction does the maximum radiation occur from a broadside array to the driven elements?
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Perpendicular to the driven elements
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What is the dBi gain of a five-element broadside array?
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7–10 dBi
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How are the LPA elements cut? And what is the normal gain for an LPA?
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LPA elements are cut to dipole length for frequencies in the band of operation. Normal gain for an LPA is 5– 7 dBi. |
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Name the two types of parasitic elements.
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Directors and reflectors
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For an LPA, where is the director placed in reference to the driven element?
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Between the element and the path of maximum transmission and reception
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What type of antenna is used to impedance-match waveguides to free space?
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Horn antennas
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How much gain can be realized by a parabolic reflector antenna?
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40 dBi plus is common
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What are the six general criteria that should be used when selecting an antenna?
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Frequency, gain, power, impedance-matching, terrain, and utility
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What type polarization should be used in a wooded or jungle area?
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Horizontal
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What is the maximum gain antenna you should use for any communications?
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The minimum amount of gain necessary to meet the operational needs of the user
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Lowering the antenna from ½ to ¼ wavelengths above the ground has what effect on the radiation pattern?
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Reduces the pattern from two lobes to one large lobe
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