Last Updated on November 26, 2023 by John Berry
Doppler shift is a phenomenon where the signal frequency is perceived to be changed through movement of stations, or propagating medium. The latter case where the scattering medium is moving is of specific interest in amateur radio. The resulting Doppler shift and distortion makes operating during an aurora difficult.
I’ve already illustrated on other pages that an aurora is a highly mobile medium.
Definition
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. The shift depends on the relative angles between transmitter and receiver and the scattering medium;
- The relative phase velocity is not constant, and hence the Doppler shift 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 modulation distortion.
Shift up or down
An aurora is comprised of spinning columns, spinning west to east. A signal sent from the UK to the east of the aurora will experience a positive Doppler shift. Conversely, a signal sent towards the western side of the aurora will experience a negative shift. And signals sent directly north will see little or no Doppler shift. The following diagram indicates why.
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. An operator may need use their RIT control to listen on a slightly different frequency from that indicated.
Operation on 144MHz SSB during a strong aurora with many stations either side of north is a constant challenge. It involves fiddling the RIT control to make voices intelligible. Operation is much saner on 50MHz with Doppler shift of just a few tens of Hz.
Doppler distortion
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.