What causes local RF interference known as key-clicks?

Key-clicks are a form of local radio frequency interference that is commonly caused by parasitic oscillations originating from the power amplifier stage of a transmitter. When transmitting Morse code or handling rapidly changing signals, the abrupt transitions can lead to unwanted oscillations that occur at higher frequencies. These oscillations are not part of the intended transmission and can create sharp, spiky signals, resulting in what is perceived as clicks or key-clicks.

Parasitic oscillations can occur due to the design and construction of the power amplifier, particularly if there are issues like feedback paths or reactive components that interact in a way that creates instability. This instability generates spurious signals that can interfere with nearby receivers and is particularly noticeable during keying operations.

Understanding the source of key-clicks is essential for amateur radio operators so they can take steps to mitigate this issue, such as improving amplifier design, using proper filtering techniques, or employing linear amplifiers that reduce waveform distortion.