Last Updated on December 8, 2025 by John Berry
The Doppler effect is ever present in ham radio communications. Sometimes it’s significant and matters. Sometimes it’s insignificant and can be ignored. It’s named after the physicist Christian Doppler, who described the phenomenon in 1842.
This page aims to clarify what it is, and how it affects the various transmission systems.
My focus is on particles and bodies that appear mid-path and reflect radio signals transmitted between two radio amateurs. Those radio amateurs may be located anywhere on the surface of the Earth. And for the Doppler effect to be observed, they or the reflecting particles or body must be moving.
The Doppler effect must be accounted for in any path we choose to exploit. Any transmission system we use must also work to overcome it.
Basics of the Doppler effect
As an emergency services vehicle (transmitter) drives towards a pedestrian while sounding its siren (transmitted signal), the pedestrian will hear the siren pitch (frequency) rise. As it passes them and drives away, the siren pitch will fall. The speed of the transmitter modifies the signal frequency, first increasing it, then decreasing it as it moves past. That’s the Doppler effect.
There’s a good audible example of the Doppler effect in the video clip at the start of my presentation on aircraft scatter where a reflected signal rises in pitch and then falls as the aircraft crosses a communications path.
The formula for Doppler shift in a radio path is:
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.
Doppler and reflecting media
The Doppler effect also applies when a transmitted radio signal is reflected from a moving object. If the reflecting object is moving towards the receiver, the frequency of the transmitted signal increases by Fd. If it is moving away, the frequency falls by Fd. The figure shows the basic geometry. I’ve ignored the effect of the various angles.
The figure shows the basic case – a single ray from transmitter to object and a single ray from object to receiver, at a single frequency. We need to add modulation of some sort. A single modulating tone results in sidebands. The exact nature of the sidebands is unimportant to the argument – their form will depend on the form of modulation.
The diagram below illustrates now what happens when reflecting from a moving object.
There will be a shift in the signal frequency and a shift in the sideband frequencies. The greyed-out carrier and sidebands are at the original frequencies, so the shift is down. There may be a shift in sideband frequency, up or down, but decode will likely be possible.
Doppler and multipath
However, when the reflecting object is the spinning particles of the aurora or rough surface of the Moon, the situation becomes much more complicated. Both scatter radio waves and produce multi-path reflections with different path lengths. The received signal for each sideband from each of many reflecting points is the aggregate of all arrivals. The result is random shifts in recovered modulation frequencies – a huge number of shifts causing Doppler distortion.
I’ve tried to describe this in the figure below. There is a shift in the signal frequency, and changes in its amplitude. There will also be shifts in frequency and changes in amplitude of the sidebands.
The result of this complex scenario is both Doppler shift and Doppler distortion such as experienced during a radio aurora. The frequency shift is obvious on the waterfall display of the radio, and the distortion is obvious in the recovered audio.
The Doppler and multi-path are also obvious when sending CW during an aurora where the CW is detected as a burst of noise.
Accounting for the Doppler effect
Doppler shift is often accounted for by adjusting the receiver frequency. In EME this can be done automatically from astronomical data given the Moon’s speed and direction. In the case of an aurora, it’s done manually by ear.
Digital modes must employ techniques for annulling the shifts. Q65 makes use of a synchronisation tone, for example. In this case, the frequency of the modulating tones is relative. So long as the decoding software can lock on to the sync tone, it can sense the tone difference, rather than trying to demodulate a required (but now shifted) frequency.
But accounting for Doppler distortion is more complicated – and yet there is one characteristic that helps in digital systems: movement of the particles or body.
Movement of the particles or body means that the multipath scenario changes millisecond by millisecond. Distortion is dynamic. Adding forward error correction (FEC) to the signal coding (with its associated redundancy) means that errors can be detected and corrected. Likewise, multiple transmission, and error detection and automatic repeat request (ARQ) allows re-transmission until the data is recovered correctly. These techniques are used in transmission systems like MSK144 and Q65.
So, there are two parts to the Doppler effect of interest to radio amateurs – shift in the signal frequency, and distortion of the modulation because of multipath reflections and their associated differing (shifted) path lengths and amplitudes. Both must be accounted for.