B-006
Subelement B-006
Section B-006-004
Section B-006-004
What is the major adverse consequence of using RG-58 coaxial cable for a transmission line operating on the 70 cm band?
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Excess reflections at the antenna feed point
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Excess RF radiation from the transmission line
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Excess transmission line heating
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Correct AnswerExcess RF loss in the transmission line
Losses in transmission lines increase with length and operating frequencies. RG-58 has a diameter of 5 mm. Its losses are more significant than a larger cable, such as RG-213 with a diameter of 10 mm.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What is the major advantage of open-wire transmission line?
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It does not radiate RF energy, which could cause interference to nearby equipment
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Correct AnswerIt can be operated at high SWR without excessive loss
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It can be located near metal objects without problems
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It has low impedance, which facilitates matching to a transceiver
The high Characteristic Impedances and greater separation of the conductors in parallel lines DO permit high power and high Standing Wave Ratio (SWR) BUT nearby metallic objects can affect them and impedance matching is most often necessary at the transmitter end. Their high Characteristic Impedance permits carrying power with less current (P = R * I squared), less current implies less loss due to resistance.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If your transmitter and antenna are 15 metres apart, but are connected by 60 metres of RG-58 coaxial cable, what should be done to reduce transmission line loss?
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Install a low-pass filter at the transceiver
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Install a balun at the antenna feed point
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Correct AnswerShorten the excess cable
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Roll the excess cable into a coil
Key words: 60 METRES of RG-58. Forty-five extra metres (150 ft.) of unnecessary RG-58 (diameter = 5 mm or 0.195 in.) introduce 4 dB of loss at 30 MHz, that's the problem here.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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As the length of a transmission line is changed, what happens to signal loss?
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Correct AnswerSignal loss increases as the length increases
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Signal loss is the least when the length is the same as the signal's wavelength
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Signal loss is the same for any length of transmission line
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Signal loss decreases as the length increases
Signal loss in a given transmission line goes up with increased length or increased operating frequency. For example, 30 m of RG-58 introduce a loss of -3 dB at 50 MHz. Doubling the length, double the loss: 60 m of RG-58 lose -6 dB at 50 MHz. The original 30 m of RG-58 wastes -10 dB at 450 MHz.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
As the frequency of a signal is changed, what happens to signal loss in a transmission line?
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Signal loss increases with decreasing frequency
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Signal loss increases as the square of frequency
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Signal loss is the same for any frequency
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Correct AnswerSignal loss increases with increasing frequency
Signal loss in a given transmission line goes up with increased length or increased operating frequency. For example, 30 m of RG-58 introduce a loss of -3 dB at 50 MHz. Doubling the length, double the loss: 60 m of RG-58 lose -6 dB at 50 MHz. The original 30 m of RG-58 wastes -10 dB at 450 MHz.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Assuming the same transmitter and RF output power are used, what is the effect of changing the transmission line from RG-213 coaxial cable to RG-58?
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More RF power is radiated from the antenna
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SWR at the transmitter increases
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Correct AnswerLess RF power is radiated from the antenna
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SWR at the transmitter decreases
Losses in transmission lines increase with length and operating frequencies. RG-58 has a diameter of 5 mm. Its losses are more significant than a larger cable, such as RG-213 with a diameter of 10 mm.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
The lowest loss transmission line on HF is:
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Correct Answeropen-wire line
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300-ohm twin-lead
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75-ohm twin-lead
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coaxial cable
Open-wire line has the highest characteristic impedance. The high Characteristic Impedances and greater separation of the conductors in parallel lines DO permit high power and high Standing Wave Ratio (SWR) BUT nearby metallic objects can affect them and impedance matching is most often necessary at the transmitter end. Their high Characteristic Impedance permits carrying power with less current (P = R * I squared), less current implies less loss due to resistance.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
In what values are RF transmission line losses expressed?
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Ohms per metre
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Correct AnswerDecibel per unit length
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Ohms per MHz
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Decibel per MHz
"Decibels per unit length". In North America, typically 'dB per 100 ft.' or 'dB per 30 m' at a given frequency. Loss rises proportionally with length. Loss goes up as frequency goes up.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If the length of a coaxial transmission line is increased from 20 metres to 40 metres, how would this affect the line loss?
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It would be increased by 10%
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It would be increased by 50%
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It would be increased by 20%
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Correct AnswerIt would be increased by 100%
If line length is doubled, the incurred signal loss is doubled. Loss for transmission lines is specified as "decibels per 100 feet (30 m)" at a certain frequency. Signal loss in a given transmission line goes up with increased length or increased operating frequency. For example, 30 m of RG-58 introduce a loss of -3 dB at 50 MHz. Doubling the length, double the loss: 60 m of RG-58 lose -6 dB at 50 MHz. The original 30 m of RG-58 wastes -10 dB at 450 MHz.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If the operating frequency is increased, how does the transmission line loss change?
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Loss increases, due to increased wave velocity
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Loss decreases, due to decreased SWR
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Correct AnswerLoss increases, due to internal line losses
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Loss decreases, due to skin effect
The higher the frequency, the higher the loss. Larger diameter coaxial cables are recommended at VHF (Very High Frequency) and UHF (Ultra High Frequency) to minimize losses. Signal loss in a given transmission line goes up with increased length or increased operating frequency. For example, 30 m of RG-58 introduce a loss of -3 dB at 50 MHz. Doubling the length, double the loss: 60 m of RG-58 lose -6 dB at 50 MHz. The original 30 m of RG-58 wastes -10 dB at 450 MHz.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
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