An Amateur Radio Station
In the block diagram shown, the "linear amplifier" is
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an amplifier to remove distortion in signals from the transceiver
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Correct Answeran optional amplifier to be switched in when higher power is required
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an amplifier with all components arranged in-line
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a push-pull amplifier to cancel second harmonic distortion
The Linear Amplifier - This is switched in to provide a stronger transmitted signal at times of difficult conditions. Not an essential item and not all radio amateurs use them or find them to be necessary. It provides an amplified version of the signal fed into its input. The term "linear" means that the output signal is a replica of the waveform of the signal fed into its input - except that the amplitude of it is greater.
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In the block diagram shown, the additional signal path above the "linear amplifier" block indicates that
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some power is passed around the linear amplifier for stability
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"feed-forward" correction is being used to increase linearity
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the linear amplifier input and output terminals may be short-circuited
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Correct Answerthe linear amplifier may be optionally switched out of circuit to reduce output power
Correct answer: the linear amplifier may be optionally switched out of circuit to reduce output power
The additional signal path above the linear amplifier provides a bypass route around the amplifier stage.
This allows the operator to:
- route the signal directly from the transceiver to the rest of the transmission chain
- operate without using the linear amplifier
This is commonly done to reduce output power when high power is not required.
- It is not used to pass power around the amplifier for stability.
- Feed-forward correction would require a more complex arrangement.
- The input and output terminals are not short-circuited.
Therefore, the additional signal path indicates the linear amplifier may be optionally switched out of circuit to reduce output power.
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In the block diagram shown, the "low pass filter" must be rated to
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Correct Answercarry the full power output from the station
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filter out higher-frequency modulation components for maximum intelligibility
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filter out high-amplitude sideband components
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emphasise low-speed Morse code output
Correct answer: carry the full power output from the station
In the transmit chain shown, the low pass filter is placed after the linear amplifier and before the SWR bridge and antenna system. At this point in the signal path, the RF signal is already at full transmitter power.
The purpose of this low pass filter is to suppress unwanted high-frequency harmonics generated by the power amplifier so they are not radiated by the antenna. Because the filter is carrying the final RF output, it must be rated to safely handle the maximum transmit power and current of the station without overheating or failure.
- filter out higher-frequency modulation components for maximum intelligibility applies to audio filtering, not RF output filtering.
- filter out high-amplitude sideband components is not the function of a low pass filter in the RF output stage.
- emphasise low-speed Morse code output is unrelated to RF filtering and is handled in signal generation or shaping stages.
Therefore, the low pass filter must be rated to carry the full power output from the station.
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In the block diagram shown, the "SWR bridge" is a
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switched wave rectifier for monitoring power output
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static wave reducer to minimize static electricity from the antenna
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Correct Answerdevice to monitor the standing-wave-ratio on the antenna feedline
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short wave rectifier to protect against lightning strikes
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In the block diagram shown, the "antenna switch"
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Correct Answerswitches the transmitter output to the dummy load for tune-up purposes
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switches the antenna from transmit to receive
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switches the frequency of the antenna for operation on different bands
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switches surplus output power from the antenna to the dummy load to avoid distortion.
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In the block diagram shown, the "antenna tuner"
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adjusts the resonant frequency of the antenna to minimize harmonic radiation
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adjusts the resonant frequency of the antenna to maximise power output
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changes the standing-wave-ratio on the transmission line to the antenna
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Correct Answeradjusts the impedance of the antenna system seen at the transceiver output
Correct answer: adjusts the impedance of the antenna system seen at the transceiver output
An antenna tuner (or antenna matching unit) does not physically change the antenna itself.
Instead, it provides an impedance match between:
- the transmission line / antenna system, and
- the transceiver output (typically 50 \(\Omega\))
This allows maximum power transfer by presenting a matched load to the transmitter.
- It does not change the antenna’s resonant frequency.
- It does not directly maximise output power.
- It does not change the SWR on the transmission line between the tuner and the antenna.
Therefore, the antenna tuner adjusts the impedance of the antenna system seen at the transceiver output.
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In the block diagram shown, the "dummy load" is
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Correct Answerused to allow adjustment of the transmitter without causing interference to others
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a load used to absorb surplus power which is rejected by the antenna system
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used to absorb high-voltage impulses caused by lightning strikes to the antenna
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an additional load used to compensate for a badly-tuned antenna system
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In the block diagram shown, the connection between the SWR bridge and the antenna switch is normally a
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twisted pair cable
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Correct Answercoaxial cable
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quarter-wave matching section
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short length of balanced ladder-line
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In this block diagram, the block designated "antenna tuner" is not normally necessary when
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Correct Answerthe antenna input impedance is 50 ohms
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a half wave antenna is used, fed at one end
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the antenna is very long compared to a wavelength
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the antenna is very short compared to a wavelength
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In the block diagram shown, the connection between the "antenna tuner" and the "antenna" could be made with
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three-wire mains power cable
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heavy hook-up wire
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Correct Answer50 ohm coaxial cable
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an iron-cored transformer
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