Working on a project which will focus on Doppler spread of HF channels (see at the bottom) and other impairements, I also bumped into some more prominent Doppler catches, namely on the VHF aero band. I took the AM carrier of nearby Hannover VOLMET on 127.4 MHz and observed doppler traces about plus/minus 200Hz the carrier frequency. Following the acitvity in the airspace via Flightradar24 in parallel, it is easy to match traces and aircrafts. In this case, I nailed cargo flight NH8406 from Frankfurt to Narita/Tokyo. It is important to remember what is shown in left part of the screenshot: it is the signal of Hannover VOLMET, reflected by this moving Boeing 777-F. Thus, the reflected frequency shows a Doppler frequency shift – depending on the relative speed in respect to transmitter and receiver. A positive Doppler frequency signals that the aircraft is approaching my location. When it turns to the lower frequencies, I see the aircraft passing.
Things get more complex wen it comes to the Doppler shift at HF propagation. You will also see planes, but effects from high winds in the upper atmosphere, coming and fading of ionospheric layers and the influences of the geomagnetic field are prevailing. Due to the much lower frequencies, the effects are just about a tenth compared to thie above example on VHF.
See below a result from my observations on HF as a preview.
We all know that propagation largely follows solar activity – on a diurnal scale, as well as along the seasons and, above all, the solar cycle – see diagram at the top of the page. We just had entered solar cylce #25, and again some unchartered waters, filled with high hopes as well as some shallows. This is a good opportunity to have a look into the rear view mirror, as in general terms we can see part of the future in the past.
To do so, I did some explorative data analysis, available for free from the following sources:
The following diagram shows the correlation of the daily sunspot number with the maximum, the mean, and the minimum MUF as measured that specific day. Therefrom, you see a decent correlation between solar activity and the MUFmean, whereas the impact of solar activity onto the MUFmax splits up at sunspot numbers above ca. 125. Values above in some cases do enhance the MUFmax, and in some cases just to the opposite. The good news, however, is that even at low solar activity, there may be experienced a MUFmax well above 30MHz, the threshold from HF to VHF.
One of the reasons that “more isn’t more in each case” lays geomagnetic activity, also triggered by the sun. From the data trove, I took seven days of November 2012: A stream of days with good propagation is interrupted by one day where the MUFmax in knocked down from more than 30MHz to just under 15MHz. The reason is a coronal mass ejection (CME), resulting in a magnetic storm, disturbing the earth’s magnetic field and, hence, ionospheric propagation. Already on the next day, propagation is recovering. Please have also a look at the “silence before the storm”, namely November 13, 2012. There you see a slight enhancement of the MUFmax of about ten percent. The geomagnetic storm is lagging behind one to three days the solar activity, as calculated from the sunspots. So, you should use this information surfin’ HF just after a massive enhancement of solar activity – before propagation recedes for a day or so. By the way, a geomagnetic storm doesn’t hit the earth in each case. This has to to with the distorted magnetic field which is no plane but resembling the tutu of a ballerina (not my association …).
Tacitely, I had used the term “MUF” in the sense what is exactly dubbed “MUF(3000)F2”, or: “MUF of the F2 layer when communicating between two stations at a distance of 3000 kilometers”. Ionosondes mostly measure the vertical MUF with transmitter and receiver at the same location, which has to be multiplied by a specifc factor. This factor of about 1,5 to 4 largely depends on the distance of the two stations, and the height of the layer. The following illustration shows propagation between my location and Funchal/Madeira, just 3000 kilometers away in the Atlantic Ocean. At a given height of the refracting layer, one hop gets longer with lowering the elevation of the antenna beam. The lower this angle, the larger the hop. It is a difficult and often expensive task to get this angle under 10° or so, especially at lower bands.
The illustration below shows the MUFs of ten consecutive days for bridging different distances from 0 (critical frequency, measured directly at the ionosonde) to 4000km. The last distance reckons with a layer height of 330km, being quite optimistic in the years of low solar activity but will be reached easily in other times. Nowadays, the standard is to use MUF(3000), wheras you often find MUF(4000) in legacy literature.
In the last paragraph, I mentioned the height of the refracting layer, and this changes also with solar activity. How, is illustrated by the figure below. There you can see the heights of the four ionospheric layers F2 (highest), F1, E and Es, or Sporadic-E. At the bottom, you find the smoothed sunspot number. You can see that this mostly influences the F2 layer – raising it at high solar activity, and, hence, allowing for larger 1-hop-distances. The height of all other layers follow a much more regular yearly pattern, and are not that much dependant on solar activity.
This, in turn, leads to the fact that the critical MUFs of each layer, by and large, is matching solar activity. F2 and F layer are mostly influenced by solar activity, as all layers are by the season. See illustration below.
Just a look on how the MUF changes from day to day under more or less stable solar conditions. With the overall pattern remaining nearly the same, the MUFmax may change considerably, ranging from 20 MHz to well above 30MHz. Nevertheless, at each day you will see a sharp rise before local sunrise (fast iononization) and a flat slope (slower re-combination) directly from around sunset.
The following figure shows that there is a high probability to expect today’s propagation also tomorrow – to about 75 percent. There, I used each day of the full twelve years’ period to cover the whole solar cycle.
Did you ever asked yourself, why important contests and DXpeditions are taking place in spring and autumn, preferably in years of high solar activity? The two following figures will give an answer: the MUF peaks just in these seasons. The peaks are prominent in a year of high solar activity, and not that pronounced in a year of less solar activity.
Last, but not least, we now will leave the MUF, heading for power, or, in this case, field strength. The figure below shows hourly measured fieldstrengths on six HF channels from 22.5MHz (top) to 4.3MHz (bottom), normalized to 1kW EIRP from June 1, 1980 to December, 1993, after Deutsche Bundespost had cancelled this project. I added daily smoothed sunspot numbers to the top and the bottom figure. 22.5 MHz has even not been used at all during a time of low solar activity, whereas they stopped using 4.3 MHz from the beginning of a new solar cycle. You can easily spot that some solar activitiy greatly enhances propagation on the channels up from 6.4MHz. In this case, the daily time where you can use the higher channels up from 13.0MHz , also increases. In years of lower solar activity, the time of propagation on the lower channels, down from 8.6MHz, is increased. But this effect is by far not outweighed by the effect on higher frequencies. For these data, we are indebted mainly to Dr. Thomas Damboldt, DJ5DT (1941-2015).
To make use of the future you have to know the past. This is easy with cyclic, physical processes. So this look into history is also a look forward into same aspects of solar cycle #25. It will bring better propagation on higher bands, where we may use smaller antennas, face less atmospheric noise and have larger frequency allocations. I am sure that F2 propagation will even take miniscule signals (QRPP) around the world, allowing for daily contacts from Europe to Australia even on 6m – as I had experienced at the peak of cylce #23. Be prepared!
P.S. All calculations/diagrams had been made with Matlab. You may also try it with free Python, matplotbib and Seabornor other software, you have at hand. With millions of data fields needed, spreadsheet software must be avoided.