WSPR Propagation Analysis - G3XBM 28 MHz Dec 2014

Roger G3XBM ran his WSPR signal for several days on 28 MHz.
This provides interesting information on propagation on 28 MHz.
I collected data from the WSPR database, to make the diagrams.

The first two diagrams, show spots that are collected over several days.
The strongest spots are given in red. 

The first two diagrams show the calculated electrical field strength in micro volt per meter.
If spots show the same field strength, the voltage on the antenna will be the same, for a simple wire antenna.
The difference between the successive values of the field strength is 5 dB.
This is about one S-point, since 6 dB is one S-point. 
The lower the value, the better the propagation.
So 0.03 micro volt/meter is about 1 S-point stronger than 0.06 micro volt/meter.

The strongest spot over 900 km was 2 S-points down, compared to the spot over 5300 km

The strongest (in red) spots show the same field strength.

Notice that the spots over 5300, 6500 and 14700 km were made with the same field strength at the receiving antenna, so the strength of the signal that arrived on the antenna  was the same in these three spots.

Further analysis shows which receiving stations have Excellent Ears. See the spots in red.
The spots in red in the table below correspond to the spots in red in the tables above.

The stations with excellent ears.

Power in CW
I was wondering, how much power would be needed to make  CW QSO's.
For a CW QSO you need  twenty times more power,  (Click) than for a WSPR spot. (13 dB)
The diagram below shows the  Calculated lowest possible power  (Click)in milliwatt for each spot.

For the strongest WSPR signal, received by K9AN the calculated lowest possible power is 2 mW.
Thsi means that the that 2 mW would be received with a SNR of -29 dB. (Solid copy in WSPR)
To be received in CW, a power of 20 * 2 = 40 mW would be needed.
This does not surprise me a bit, since I made a CW contest QSO with K3WW on 21 MHz in 2012 with 36 mW.

Rogers signal was also heard in VK. The calculated lowest possible power was 5 mW.
So in CW his signal would be heard with 20 * 5 = 100 mW.
This seem too low, but please notice that to double the distance it takes an increase of 1 S-point (6 dB) to get the same field strength.
One  S-point means four times more power.
Further, please notice that WSPR is patient and tireless.
It is even possible that one of the operators was a sleep during the time that the spot was made. hi.