Last Updated on April 30, 2026 by John Berry
The signal received at an antenna from a distant transmitter is the vector sum of all arrivals travelling over all paths. Changes in path loss, path by path variation in loss over multiple paths, and receipt of multiple reflections all result in fading. Fading is a departure from some nominal value of signal level. When it results in interruption of communication, fading is typically referred to by radio amateurs as QSB.
All propagation modes and all paths are victim to fading.
Interference and Phase
Multipath components combine at your antenna vectorially. If two signals arrive in phase, they strengthen the total signal. This is constructive interference. If they arrive 180 degrees out of phase, they cancel each other. This is destructive interference. Multipath arrivals can create an overall increase in signal level or a dramatic decrease.
Anatomy of a fade
The essence of fading is that there is some nominal value of signal existing at the receiver. The receiver threshold is exceeded for some significant period of time and that makes the communication workable. We see this idea when discussing chance. Simply, pre-fade, there is a good chance of communication. During a fade, communication may be interrupted with the received signal falling below receiver threshold.
The shape of the fade depends on the type of fading, the environment, and the signal frequency.
On tropospheric paths
In the troposphere via space wave propagation, propagation is typically by diffraction. The biggest diffraction loss contribution is from the Earth’s bulge protruding into the path. Fading over tropospheric paths is caused by changes in the Earth’s atmosphere, in the geometry of the path or paths, and by multi-path and multi-route arrivals at the receiver.
On ionospheric paths
For skywave paths, there are various ionospheric losses – each with their own variability. There are therefore multiple opportunities for fading. The ionosphere also gives scope for propagation by multiple routes with many discrete arrivals at the receiver. These arrivals may sum destructively giving fading.
Other fading
There are many types of fading, unique to path geometry.
In MF broadcasting around 1MHz the wanted signal propagates in the troposphere as ground wave. At certain times of day, it also propagates as an unwanted sky wave. The wanted and unwanted can interfere, giving rise to deep slow fades. That’s a mix of the two cases described above and is caused by the different path lengths and resulting different phases of received signals.
Frequently in mobile scenarios, shadow fading describes the situation when the path becomes obstructed by local geography. Literally, a station moves into the shadow of an obstruction. Then the diffraction loss rises, fading the received signal.
In amateur radio where bandwidths are narrow, all frequencies across the channel bandwidth fade at the same time. That’s termed flat fading. Other mechanisms exist in professional wideband systems.
Nature of fading
Radio amateur overs are typically of short duration – a few tens of seconds for MSK144 data, a few seconds for FT8, and a few minutes for voice and CW. This is key when striving to understand fading.
For the duration of the over, the slow fading component is often reasonably constant. Its absolute value could be at the path median, above it, or below it – so things could get better – or worse over time. But over a longer communication, a net can fall apart as the paths increase in loss and stations fade out. And during a data exchange of several minutes, slow fading can knock out some of the overs. A log-Normal probability distribution describes the variation in signal level. The cause is typically change in the refractivity of the atmosphere described by the k-factor.
Within each over however, the communication can be victim to fast fading caused by multi-path. This manifests as flutter on this quasi-fixed slow fading state as the signal goes from usable to unusable with a period from milliseconds to seconds. The fast fading excursions go from a few dB above median to some 30dB (5 S-points) or more below. A Rayleigh probability distribution describes the variation in signal level.
Above, I show a typical profile of a signal with time. This includes a path loss component for change in path loss with distance simulating mobile stations moving apart. This component would normally be constant for fixed stations. A Ricean probability distribution describes this complex resultant on the right above.
Data systems incorporating modulation scheme and coding are designed to optimise communication in this dynamic environment. The data mode Q65-60B is a good example of a design for fading over the complex Earth-Moon-Earth path.
Impact of fading
Within the duration of the typical radio amateur communication, slow fading is not apparent. So, if an operator answers a CQ , it’s likely that the communication can proceed. Tomorrow, maybe things will be different and there’ll be a ‘lift’ or a fade. But in that short time of a few seconds or minutes, all is well.
Fast fading, on the other hand can interrupt the communication in train. Voice and CW operators will manage their own error detection and error correction. ‘Again, again’, is a frequent refrain!
Data communication typically employs several similar techniques. Some simple data modes simply crash. They report errors at the receive end and present nothing to the operator. Others present the received recovered bitstream, errors and all. Some mimic the human operator and send an ARQ, an automatic repeat request.
Where a technology has spectral bandwidth available, it may send redundant bits to enable forward error correction. Then the receiver can decode a authentic message, even during some momentary signal loss. There is of course a limit to this, and even these FEC techniques run out of range and crash if the fading is too severe.
Designers like Joe Taylor, K1JT, have designed each data mode to cope in a particular way with in-communication momentary signal loss. They must understand the fading likely to be experienced on the path over which they propose mode application.
Designers systems engineers must match fading and data mode. I consider this challenge further in other pages.