Doppler shift and distortion

Doppler shift is a phenomenon whereby the received signal frequency is perceived to be changed because of movement of either the sender or receiver, or the propagating medium. The latter case where the scattering medium is moving is of specific interest in amateur radio.

I’ve already illustrated on other pages that an aurora is a highly mobile medium.

Doppler shift is the rate of change of phase along the path from transmitting station to receiving station. This is complicated to determine when communicating via an aurora.

The following equation applies.

Fd = 2v.f0/c

Where Fd is the doppler shift in Hz, f0 is the carrier frequency in Hz of the transmitted signal, v is the phase velocity in m/S, and c is the speed of light in m/S and is a constant.

There are three points to note about the Doppler effect applied to auroral communications:

  • That received signals may be shifted up or down in frequency, depending on the relative angle between transmitting and receiving antennas and the scattering medium;
  • The relative phase velocity is not constant, and hence the Doppler shift in frequency varies throughout a QSO;
  • Propagation is by multiple paths between transmitter and receiver via a mobile medium, hence there are multiple Doppler shifts. This in turn causes Doppler broadening heard as signal distortion.

An aurora is comprised of spinning columns, spinning west to east. Intuitively, a signal sent from the UK scattering to the east of the aurora toward stations in central southern Europe will experience a positive Doppler shift. Conversely, a signal incident towards the western side of the aurora will experience a negative shift when received by signals in south-west Europe. And signals sent directly north will see no Doppler shift or changing positive and negative shifts depending on the exact path. The following diagram indicates why.

Doppler shift with beam angle

Doppler shifts of a few hundred Hz to 1kHz or so may be experienced on the 2m amateur band at 144MHz. This shift may rise to 3kHz at 432MHz. This is challenging for an operator who may need to listen on a slightly different frequency from that indicated on the transmit frequency read-out.

Of more significance for phone or SSB operators is the Doppler broadening. This results in a distortion of the voice as illustrate in the video below.

Example of Doppler distortion on received signals