Selective carrier shrinkage

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The selective carrier fading occurs preferentially with amplitude-modulated transmitters in the medium wave and short wave range and can considerably disturb the reception quality. The cause is destructive interference that weakens or completely eliminates the carrier frequency.

Spectral representation of the amplitude modulation

When modulating, radio transmitters generate so-called sidebands . As can be proven by measurement, the emitted carrier frequency itself remains unchanged. An example: If a medium wave transmitter with a frequency of 900 kHz is amplitude-modulated with music, the highest frequency of which is 8 kHz, the lower sideband occupies the range from 892 kHz to 900 kHz and the upper sideband the range from 900 kHz to 908 kHz. The actual (music) information is in the sidebands and not in the carrier frequency (900 kHz). The latter is only broadcast because the envelope demodulator in the receiver can then be particularly simple. With single sideband modulation (SSB), this carrier frequency is not transmitted and must be generated in the receiver. This makes the recipient more complicated, but in principle no selective carrier loss can occur.

Radio waves from distant transmitters can reach the receiver over different distances and therefore with different delays. For example, one wave can propagate along the ground, another is reflected off the ionosphere . With short waves, reflection on the metal fuselage of large aircraft is also possible. The individual wave trains then have the same frequency, but different phase shifts . If the sum of all incoming waves happens to be zero, one has the impression that the transmitter is working with reduced power or not at all. The individual waves can also constructively overlap, then the sum is particularly large.

The effect is strongly frequency dependent. It is quite possible that only 903 kHz is canceled, but not 902 kHz or 901 kHz. And that 905 kHz is particularly strong. In the example above, this would mean that some frequencies are missing and others are overemphasized. The music modulation then sounds distorted - something like that can often be heard on shortwave.

The ionosphere and planes move. Therefore, the path length of the reflected wave changes and the duration of the cancellation (destructive interference ) can last from fractions of a second ( flutter fading ) to several seconds. If the carrier frequency (900 kHz in the above example) is weakened too much for a few seconds, the demodulation in the receiver no longer works properly and only heavily distorted frequencies come out of the loudspeaker. This is then selective carrier shrinkage.

Web links

  • Prof. Dr.-Ing. Dietmar Rudolph, Technical University of Applied Sciences Berlin, scripts on "Modulations": Amplitude Modulation, page 35 online, PDF (1.8 MByte)