Last Updated on April 2, 2026 by John Berry
The Doppler effect describes changes in signal frequency caused by relative motion. It 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 amateur radio transmission systems.
My focus is on objects that appear mid-path and interact with 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 objects must be moving.
The Doppler effect must be accounted for in any path we choose to exploit. Any transmission system we use must be effective in overcoming it.
Basics of the Doppler effect
We often model radio propagation using acoustic analogies to aid initial understanding. Imagine a vehicle sounding its horn while approaching your position. The speed of the vehicle as it approaches causes the sound waves to compress, which raises the perceived pitch or frequency of the horn. As the vehicle passes and recedes, the waves stretch. This stretching reduces the pitch.
Radio waves behave in a similar manner. The relative velocity between stations determines the magnitude of the frequency shift. We calculate this shift using the formula:
Fd = 2v.f0/c
Where Fd is the Doppler shift in Hz, f0 is the signal frequency in Hz, v is the velocity in m/s of the object, and c is the speed of light in m/s and is a constant.
Doppler and reflecting objects
The Doppler effect applies when a transmitted radio signal is reflected from a moving object. As illustrated in the audio analogy above, 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. In the diagram, the greyed-out carrier and sidebands are at the original frequencies, so the shift is down. There may be a shift but demodulation will likely still be possible, if with difficulty.
Under some circumstances there will be little or no shift because the rise and fall balance out.
Doppler and multipath
However, when the reflecting object or objects are the spinning particles of the aurora or the rough surface of the Moon, for example, the situation becomes much more complicated. Both aurora and Moon 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 spread. This spread in turn causes modulation distortion.
The spreading of the frequency spectrum of the signal can cause “smearing” of the transmitted symbols. In digital modes, this results in inter-symbol interference (ISI). Your receiver may struggle to distinguish between individual bits or symbols. Consequently, you will notice an increase in the bit-error rate (BER) and a degradation in ability to communicate.
I’ve tried to describe this in the figure below. There is a shift in the signal frequency, and changes in amplitude. There will also be shifts in frequency and amplitude of the sidebands.
The result of this complex scenario is both Doppler shift and Doppler spread. The frequency shift may be obvious on the waterfall display of the radio, and the spread may be obvious in the recovered audio or data.
The Doppler shift and spread are obvious when sending CW during a radio 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. When working stations during 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 detect a required (but now shifted) frequency.
But accounting for Doppler distortion is more complicated. 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.
Summary
The Doppler effect in ham radio communications involves both frequency shifts and signal distortion. While shifts require simple re-tuning, distortion requires sophisticated signal processing to overcome. Understanding these effects helps you select the best modes for challenging propagation paths.