Skywave fading is a fun “feature” of shortwave propagation. Take some time to examine the many fading effects you experience every day.
Shortwave signals propagate through the Ionosphere, the highest region of our atmosphere. Solar radiation and other factors cause ionization at this height. We end up with layers of charged particles that are capable of bouncing shortwave radio signals around the world.
Over the long term, the ionosphere’s performance varies over the eleven year solar cycle. It also varies by seasons and, importantly by time of day. This latter diurnal cycle is why skywave performance is different by day and night.
But over the shorter term (hours, minutes, seconds) we experience skywave fading. This is where received signal strengths fluctuate up and down. Let’s look at some of the fading we experience on shortwave, where there is really no such thing as a constant signal strength. Random and other processes serve to cause fading.
In part, fading results from transmitted radio rays taking different paths through different regions of the ionosphere. So several radio waves from the same transmitter will arrive at different times after following different routes. Sometimes these multi-path signals reinforce each other. Sometimes they cancel each other when out of phase.
You can see skywave fading effects in the picture above. Both transmitters are running 500 kilowatts and are about 3,000 kilometers apart. CRI signals travel from Kashi-Saibagh (western China) to Calgary along a 10,000 km. path. AIR travels 13,000 km at a lower frequency from Bangalore, a few hours later as the path is starting to close.
Fading on the All India Radio signal is faster, but still in the range of 100-200 milliseconds. SDR Console provides a useful graph of signal strength history.
Skywave Fading – Effects and Causes
Normally, listeners don’t really hear the effects of skywave fading because receivers use Automatic Gain Control. AGC varies the signal amplification to maintain a constant audio level. Typically, you will find your AGC has an attack time of 100 milliseconds, with a decay time to 500 to 2000 ms. These characteristics serve to provide constant output.
Most fading you notice arises either from multi-path propagation, or changes in ionospheric conditions along the signal path. These fades are in the range of seconds. Sometimes, the ionosphere can act as a lens to magnify signals. Other times, lower layers cause path loss by absorption.
Really fast fading or fluttering is caused by shifts in signal polarization, especially over polar paths. Typically, fluttering happens too fast for the AGC to compensate.
So, remember that shortwave signals are comprised of multiple rays traveling down many paths to your receiver. And, remember that the ionosphere is a very complex region of space with lots of variations. Skywave fading is normal.