G4EFE WSPR with 5 mW on 40 m

WSPR is a beacon system that is designed for low power. How low can you go?
Martin G4EFE ran WSPR with very low power.

Martin G4EFE wrote after experimenting with 1 mW, using his attenuator of and 20 dB:
So I've had a little time for some experimentation, and the results - I think - are spectacular. Attenuating the output to just 1mW netted me several spots from neighboring countries. 
Best DX was GM* at 711km, who reports me at -18b dB SNR, suggesting I can go even lower.
So today I'm running just 100 MICROWATTS. I can't believe anyone will spot me, but I'm the optimistic kind! Thanks again, Bert, for this easy and fun station accessory. 

The map, the tabel for 1mW and the picture of the beautiful attenuator can be found here: https://www.flickr.com/photos/71155570@N00/albums/72157679958156316
OK Martin, thank you for sharing this fine info.

5 mW down to 1 mW
Martin started with 5 milliwatt for three days, as you can see in both tables. On the 5th the power was reduced to 2 milliwatt. On the 6th Martin had great fun in 21 spots with just 1 milliwatt.
To make a power of 1 milliwatt Martin uses his IC703, at it's lowest setting with 100 mW and an attenuator of 20 dB. His antenna is a full-size 40 m square loop. As I saw on WSPRnet.

From hour to hour
In the table below you can see the number of spots in each hour.
The days go from the bottom to the top.
The number of spots vary from day to day. this is not only propagation.
Martin is constantly transmitting, but the listeners can be jumping from band to band.

G4EFE WSPR with 5 mW down to 1 mW - From hour to hour

The best DX from GM*with a SNR of -18 dB, that Martin refers to, gives a calculated lowest possible power of 0.1 milliwatt. This spot could be made with 0.1 mW and still be a solid copy with a SNR of -28 dB. See the spot in the red circle in the table below.

G4EFE WSPR with 5 mW to 1 mW, using an attenuator

The better the propagation, the better the SNR will be and the lower the Calculated lowest possible power. The Calculated lowest possible power is calculated from the power and the SNR.