Last Updated on February 12, 2025 by John Berry
The signal received from a distant transmitter at a local antenna is the vector sum of all arrivals travelling over all paths. Path by path variation in loss, and interference between arrivals together result in fading. This fading is a departure from the nominal value for the path. When it results in reduction in signal received, fading is referred to by radio hams as QSB.
There’s the direct path assuming free space transmission modified by diffraction loss. The biggest diffraction loss contribution is from the Earth’s bulge protruding into the path. For skywave paths, there are various ionospheric losses. And then there are all the reflected paths from the ground, buildings, atmospheric layers, aircraft and frankly everything and anything in the vicinity of, and mid-way between, both stations.
The received signal at any point in time for two fixed stations varies and is described by a frequency distribution. This distribution has a combined slow fading component, and a rapid multi-path fading component. The slow fading is caused substantially by change in the refractivity of the atmosphere. That’s described by change in the k-factor. See the blog titled The Normal Troposphere for more on k, the Earth radius factor, and how k changes.
So both fading types modify the resultant and either fading may dominate the resultant at any time.
The resulting frequency distribution is described by combining Rayleigh (fast fading) and log-normal distributions (slow fading). Like many distributions, there’s a median signal level, and a variance that gives a signal exceeded for various percentages of time.
For radio amateurs, it’s the low percentage of time stuff that excites.
Fade depth
Radio amateur communications is of short duration – a few tens of seconds for data, and a few minutes for voice and CW. This is key when striving to understand fading.
When a communication is attempted, the slow fading can be considered fixed at that moment. Within the term of the communication, the slow fading component is mostly constant.
Its absolute value could be at the path median, above it, or below it – so things could get better – or worse over time. Over a longer communication, particularly in a net lasting an hour or so, the net can fall apart as the paths increase in loss.
When a communication is victim to fast fading caused by multi-path, this manifests as a flutter on this quasi-fixed state as the signal goes from usable to unusable with a period from milliseconds to seconds. The excursions go from a few dB above median to some 30dB (5 S-points) or more below.
A typical profile of a signal with time is shown below. This includes a path loss component for change in path loss with distance simulating the stations moving apart.
Impact of fading
Within the duration of the typical radio amateur communication, the slow fading is not apparent. So, if the CQ is answered, it’s likely that the communication can proceed. Tomorrow, maybe things will be different and there’ll be a ‘lift’. 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 has 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 there is the data bandwidth available, many others send redundant bits that are used to enable forward error correction. Then an authentic message is received, 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.
Each data mode is designed to cope in a particular way with in-communication momentary signal loss. Each must be understood in terms of the fading likely to be experienced on the path – normal ionospheric, disturbed ionospheric, anomalous ionospheric, tropospheric ‘normal’, tropospheric lift, tropospheric scatter, EME (with its plethora of fading mechanisms), meteor scatter, sporadic E and auroral communications etc.
This match between fading and data mode is considered further in other blogs.