40m, 500mW WSPR TX mode, last 12 hours relative to Mar 9, 2018 10:26
From this WSPR graph you can see that 40m radio waves propagate well over the ocean, that the F layers are likely reflectors at 250 km altitude and that the skip distance is around 500km. Here is the explanation in more detail.
The first WSPR fact is that you are only heard either within approximately 30 km via the ground-wave (over the surface direct line of sight) or at distances greater than 500km. Between 30 and 500 km you usually get no reports whereas there should be some listeners. I rarely get reports from for instance the university of Twente where there is a
permanently running WSPR receiver. Instead, the first signal reports seem more to arrive from southern Germany and the center of France, roughly at 500 to 800 km from Rotterdam. The mechanism that likely explains this is a reflection of the radio wave against an ionospheric layer at a height of probably around 250km, but this depends of course on the critical angle. An ionosonde can tell you what the height of the reflecting layer is, my guess is that it is around 250km, so this is a F2 layer. All of what you see in the WSPR plot is observed during the night time the D and E region descrease in intensity (number of free electrons). Within the EU we see single, double or maybe triple hop reflections, but nothing seems to go any further than that.
But now the long paths over the north Atlantic, these paths can only be explained by more than 3 hops, there should be tens of hops to explain a path across the Atlantic ocean. The first surface reflection towards the west seen from Rotterdam will be over a water surface, the north Atlantic in particular. There are no reports from the UK, my explanation is that it is too close to the transmitter. Over the ocean the propagation is much better, water (and ice) are very good reflectors, and the signal goes on until you hit the east coast of the united states and also Costa Rica in central America. The WSPR signal survives maybe one or two hops over the continental US, and then it dissipates because land is a worse reflecting surface than water or ice. This is the general tendency of HF radio propagation, on 20 and 40m it is very difficult to get any reports from the central US, Texas, Oklahoma and Kansas and northward. Most of my QSOs are with the
Eastern states because of the mechanism described before, land is not a good reflector, water is better.
Some logbooks like hrdlog.net allow you to plot the states that you have worked in the USA, this summarizes the contact mostly on 20 and 40m over the last several months (1000 QSOs back actually, on average I do some 8 per day). It also shows that the eastern states are confirmed, and this is in agreement with the WSPR propagation plot, propagation over land is less effective than over sea water or ice.
Last update: 10-3-2018 9:15