The "notch filter" found on many HF transceivers is to reduce interference from carriers in the receiver passband. The "notch" refers to a narrow area of frequency interference that you want to get rid of, while being able to maintain the quality of the nearby signal you want to keep.

General hint for most questions like this one: “WE HATE INTERFERENCE”

For more info see Wikipedia: notch filter

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When working with CW signals on a typical HF transceiver it may be beneficial to select the opposite or "reverse" sideband, as this may make it possible to reduce or eliminate interference from other signals. This method will work with CW, because it transmits just tones of dots and dashes. It won't work with a voice/phone signal because it would flip the speech pattern, making it impossible to understand.

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The noise blanker works on eliminating the annoying sound of repetitive pulses such that are heard with automotive ignition energy, some diesel engines, or the famous Russian 'Woodpecker' radar radio frequency emissions.

https://en.wikipedia.org/wiki/Noise_blanker

Noise Blanket "Blanker" works to reduce cold "Receiver gain" during a cold front "Noise pulse"

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A pronounced dip or drop in the reading on the plate current meter of a vacuum tube RF amplifier shows that you have correct adjustment of the plate tuning control. This dip happens when the impedance is matched at the right frequency.

Note: It is kind of like "zero beating" a CW signal, with the reading/mismatch being higher on either side of the correct level, and reaching zero when frequencies are matched to each other.

Remember to "peak the grid, dip the plate"

What is plate current?

Plate Current is the amperage that flows across a tube from the cathode to the plate. In the case of an audio tube, the control grid and screen grid mediate the amount of current that flows from cathode to plate.

QRZ Forum: Tuning for "a dip in plate current"

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The ALC avoids feeding too much power to the amplifier. Too much power can cause the amplifier to overdrive, which causes distortion in the signal and extraneous interferences.

Vehicle Automatic Traction Control helps to "Prevent excessive drive"

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Maximum power transfer occurs when the antenna system (antenna plus transmission line) has the same impedance as the transmitter.

If the antenna system has too much capacitance, you can offset that by adding inductance. If it has too much inductance, you can offset that by adding capacitance. The antenna tuner adds the necessary capacitance and/or inductance so the antenna system and tuner appear to the transmitter as a perfect match.

An antenna tuner is only necessary when there's a mismatch between the transmitter and antenna system. If the antenna and transmission line have the same impedance as the transmitter, there is no need for an antenna tuner.

Most amateur radio transmitters have an impedance of 50 ohms.

Hint: The answer is the only one that does not have the word antenna!

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As the noise reduction control is made to be more aggressive, additional parts of the signal become progressively targeted. Some of these parts are valuable to the quality of the audio.

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The load or coupling control of a vacuum tube RF power amplifier should be adjusted so that it is at the maximum power output usable without exceeding the maximum allowable plate current. The Load control for the RF power amplifier should be adjusted alternately with the Tune control adjusting the plate current until the highest matching amplifier power output level can be found without overloading the plate current.

HINT: both the question and answer use the word "power"

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The purpose of delaying RF output after activating a transmitter's keying line to an external amplifier is to allow time for the amplifier to switch the antenna between the transceiver and the amplifier output.

When the keying line of a transmitter is activated, it signals the external amplifier to start transmitting. However, there is a slight delay introduced to ensure that the amplifier has enough time to switch the antenna connection between the transceiver and the amplifier output.

While it is true that it can take the amplifier power supply some brief time to reach operating level the worst case there is for the signal to be a bit off at first; if the antenna is fully switched over or particularly if there is any way for energy to leak from the transmitter into the receiver then it could cause actual damage to the transceiver.

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An electronic keyer makes CW communications easier in that it automatically generates the strings of dots and dashes for CW operation. This type of keyer produces a wonderfully consistant signal, as the dots, dashes and spaces all are produced with the correct length and timing.

For more info see Wikipedia: Telegraph key

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Improper action of ALC distorts the signal and can cause spurious emissions

The Automatic Level Control (ALC) makes sure signals do not clip by compressing them. If you've dealt with audio production, this is a similar process to dynamic compression. The compressed signal sounds similar to the original signal, but it usually comes with distortion.

This distortion is acceptable in voice communication, but in digital modes, compression may generate emissions not intended by the pure signal from your computer.

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SILLY HINT: In a "Dual" relationship, one person needs to listen (receiver), while the other partner talks (transmitter) and for best communication either person should not try to talk and listen at the same time.

SILLY HINT2: It's "common use" - we don't commonly use full duplex, but we do "commonly" transmit and then receive.

VFO = variable frequency oscillator

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Attenuators are used in HF transceivers to reduce signal overload due to strong incoming signals. An attenuator acts as the opposite of an amplifier. Its purpose is to reduce the power or amplitude of a signal rather than increasing it, while still keeping the signal strong. This keeps the signal from having the distortion that would be experienced with too much incoming signal energy. Think of the attenuator as a downward volume control for the signal power.

SILLY HINT: The only answer with "receive" in it. You attenuate the incoming strong signal to something less dangerous.

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