B-006
Subelement B-006
Section B-006-006
Section B-006-006
Which of the following antenna system conditions will cause a modern solid-state HF transceiver to automatically reduce power?
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Excessive antenna element movement in high winds
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Transmitting to a balanced antenna with an unbalanced transmission line
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Correct AnswerExcessive impedance mismatch between transceiver and transmission line
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Open circuit in lightning surge protector ground connection
A circuit, included at the output of the final amplifier, recognizes a serious impedance mismatch in the antenna system and protects the amplifier by reducing output power. This protection is generally present in all our transmitters.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What does an antenna tuner do?
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It switches an antenna system to a transmitter when sending, and to a receiver when listening
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It switches multiple transceivers to a common transmission line and antenna
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It helps a receiver automatically tune in stations that are far away
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Correct AnswerIt matches a transceiver to a mismatched antenna system
The 'antenna tuner' in an adjustable impedance matching network: it can adapt the impedance of the antenna system (which changes with antenna dimensions and operating frequency) to the design impedance of the transceiver. The 'antenna tuner' permits using an antenna on a frequency or band other than the one for which it was designed.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
An end-fed half-wave antenna (EFHW) has a very high feed point impedance. What device could be used to provide a good match to 50-ohm coaxial cable?
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A quarter-wave stub
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A counterpoise
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Correct AnswerA transformer
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A good ground on the coax shield
Impedance (Z) can be seen as the ratio of voltage to current in AC circuits: Z = E / I. While functioning with different ratios of voltage to current, the primary and secondary windings of a transformer can operate at different impedances. For example, a transformer with a turns ratio of 1:7 could accept 71 volts at 1.4 amperes on its primary (100 watts at Z = 50 ohms) and deliver 495 volts at 0.2 amperes on its secondary (100 watts at Z = 2450 ohms). Losses in the transformer are neglected and numbers are rounded in this example.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If both source and load impedances are purely resistive, what value of load impedance will result in maximum power delivery to the load?
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Load impedance equal to twice the source impedance
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Correct AnswerLoad impedance equal to the source impedance
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Load impedance equal to half of the source impedance
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Load impedance equal to the square root of source impedance
Impedance Match: maximum power transfer occurs when the impedance of the load matches the internal impedance of the source. For example, a transmitter designed to work with an impedance of 50 ohms will deliver maximum power if the antenna system offers an impedance of 50 ohms.
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 advantage of locating an antenna tuner near the antenna feed point, over locating it near the transceiver?
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Less atmospheric noise pickup
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More harmonic suppression
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Correct AnswerLess transmission line loss
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More usable bandwidth
An impedance mismatch at the far end of a line adds to the normal losses of the line. If these are already significant, it is advisable not to add to them by ensuring an impedance match at the far end.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
How does an antenna tuner compensate for an impedance mismatch in an antenna system?
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By adjusting the resonant frequency of the antenna
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Correct AnswerBy adding capacitive or inductive reactance
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By diverting reflections to a dummy load
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By increasing the conductance of the transmission line
An antenna tuner typically contains variable inductors and capacitors to build an impedance matching network. Different settings permit compensating or cancelling the effects of an impedance mismatch.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What advantage does a transformer present when used for impedance matching at radio frequencies?
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It provides constant voltage to the load
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It isolates the source from reflections
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Correct AnswerIt can be designed to do so over a wide bandwidth
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It supports larger power than reactive components
Unlike a matching network built with inductors and capacitors, components whose behaviour depends on the operating frequency, the transformer is less susceptible to such effects.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Where does impedance matching need to be done to minimize transmission line losses in an antenna system?
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Correct AnswerAt the junction between the transmission line and antenna
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At the junction between the transmitter and the transmission line
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At a quarter wavelength from the transmitter
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Anywhere along the transmission line
An impedance mismatch at the far end of a line adds to the normal losses of the line. If these are already significant, it is advisable not to add to them by ensuring an impedance match at the far end.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If an antenna is correctly matched to a transmission line, the length of the transmission line:
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must be a full wavelength long
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must be an even number of half-wavelengths
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must be an odd number of quarter-wavelengths
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Correct Answerwill have no effect on the matching
IF a mismatch is present at the end of the transmission lines, certain lengths may introduce an 'impedance transformation' effect. With a correctly matched antenna, line length is immaterial except for line losses if the line is unnecessarily long. [ References to multiples of the wavelength only tap into urban legends. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Why is an antenna tuner (external or internal) frequently used with modern solid-state transceivers?
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It absorbs the reflected energy from a mismatched antenna system
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It reduces common-mode noise pickup
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It reduces losses in the line to the antenna
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Correct AnswerIt enables the transceivers to deliver rated power to a mismatched antenna system
Modern power amplifiers are designed to deliver their rated power with a minimum of distortion over a limited range of load impedances at their outputs. An antenna tuner can transform an unsuitable antenna system impedance to an adequate value for the power amplifier.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If a transmission line with a characteristic impedance of 50 ohms feeds a folded dipole with a feed point impedance close to 300 ohms, what impedance transformation ratio is needed to match the two?
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4:1
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9:1
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Correct Answer6:1
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2:1
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