Fading (electrical engineering)

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Spectrogram with a horizontal time axis, the frequency is plotted vertically and the received field strength via the gray value: Horizontal fluctuations in the gray value correspond to fading

As fading (fading) is referred to by interference , shadowing, multipath and Doppler effect fluctuations caused the reception field strength upon radio transmissions .

causes

The interference is caused by the fact that the radio signal reaches the receiving antenna out of phase on several paths due to refraction or reflection from the ionosphere or terrestrial objects (buildings, mountains, etc.) . In this case, Selective carrier loss occur that distorts the modulation strong.

Fading, which occurs as a result of the superimposition of various propagation phenomena on terrestrial objects, is called slow, long-term or lognormal fading. The one that occurs due to the frequency used is called fast, short term or Rayleigh fading , this happens due to the multipath reception and the Doppler effect . Dips in reception with fast fading occur periodically at intervals of approximately λ / 2. This means that signal dips from fast fading are always dependent on the frequency used.

compensation

So-called Fading Mitigation Techniques (FMT) compensate for the fading in order to maintain the communication path even in the event of disturbances.

To compensate for fading when receiving amplitude-modulated radio stations ( long , medium and short wave ), the receivers are equipped with what is known as fading compensation. In practice, an averaged signal voltage is generated on the demodulator and used as a control variable . As the signal voltage rises, the so-called control voltage increases, through which the IF branch is throttled in its gain. Decreasing signal quality is noticeable in increased noise while the volume is adjusted.

To ensure the data transmission of a radio link, it is not sufficient to increase the sensitivity of the receiver. Instead, the following are used:

The interference from the atmosphere increases with frequency. While satellites transmit almost unhindered in the L, S and C bands, the transmission properties of the K bands depend heavily on the weather. With a suitable FMT, frequencies above 20 GHz can also be used for data communication with satellites without experiencing excessive restrictions from rain.

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