HF: Doppler, Signal Level and Time
What you see in the picture at the top, is a mostly hidden gem of HF propagation. I took the carrier of Sofia-Kostinbrod transmitter form Bulgaria (250kW) on 9400kHz and observed it for three hours. In the upper window you see the frequency wihtin a window of 2Hz height only. You see two strong carriers: one nearly in parallel to the x-axis, the other snaking some fraction of one Hertz below it.
With one transmitter only on this frequency: How does this happen?
It’s multipath propagation. The signal takes one way via a groundwave-like way, the upper trace. It reveals a very slight drift downwards. As I use a GNSS-controlled receiver, the FDM-S3 from Elad, this miniscule drift should be happen within the transmitter, not the receiver.
The snaking trace stems from a second way, most likely via the F2 layer of the ionosphere. As the ionosphere is prone to winds and an ever dynamic change of its ionization, it is moving. And with all moving objects, also this causes a Doppler effect to waves. This is exactly what we see – the angular speed of the ionosphere, relative to the “groundwave-like” signal.
You may also see at least two weaker traces, caused by two further ways, hence showing other Doppler shift.
In the diagram at the bottom, you see the combined level of all traces. Because they reach the reeiver at different time and, hence, different phases, their addition leads to an ever changing signal level, called: fading.
I hope to continue this work with some other examples in the future, also taking fade-in and fade-out into account.