6-5-2018: We see a lot of fading, and the waterfall plots contain the typical stripes:
Fading (or QSB) is a phenomenon where the strength (and phase) of the received signal changes over time.
The first plot shows a waterfall on the 80 meter band recorded on May the 6th 2018, this is a UK station some 370 km West of me. Fading often shows as a pattern of stripes, typically the repetition interval is 30 seconds or so.
It is not uncommon that multiple ranges of the spectrum of the radio transmission are affected at the same time. You miss a part of the high tones, the middle and the low tones at the same time, it is as if three notch filters slowly slide over the audio spectrum.
During fading several wavefronts arrive at the antenna, upon arrival they can add-up or they may cancel one another, in physics this is called interference.
So what type of interference mechanisms are known? This is the HF and we are talking about long waves, with the station in the UK we can expect a direct wave over the ground and a reflecting wave via the ionosphere. Both waves will interact.
Another possible mechanism on the HF is that only the properties of the reflecting layer are changing during the transmission. This is known as Faraday rotation in the ionosphere causing a change in the polarization of radiowaves. Any antenna has a preference to polarization, changes in the Faraday rotation will affect the received signal strength.
Interaction between the ground and the skywave could explain what is probably going on in the first plot. The critical frequency is around 2.5 MHz and we are near 3.8 MHz, furthermore at 370 km you can still expect a groundwave.
In my experience the pattern of stripes is typical for nearby stations where the ground and the skywave interfere. QSB manifests itself differently for more distant stations where the radio signal may arrive after multiple reflections involving the ionosphere. In the latter case you see that the signal strength varies, but you wouldn’t see the stripes.
9-5-2018: when you think about theory to explain what you saw then it is always good to look again at the observations and ask yourself whether the theory is still valid. Here is the waterfall spectrum of a QSO with EA3BOX Juan who is too far away for me for a groundwave, but still you see the fading stripes, which is the faraday rotation in the ionosphere. The critical frequency was 3,475 MHz at this time, here is the waterfall plot:
I also attached the ionograph of Dourbes (Belgium) at the time of this QSO, I don’t claim that I fully understand what I see in the ionographs, but the most important number is the foF2 which is the critical frequency, and other important ones are the MUF at the bottom legend.
If someone has a manual or a better explanation of what you see in the ionographs then it would be appreciated.
Last update: 9-May-2018