On a magic night, January 20th 2010 propagation conditions were just a little better than normal and the QRP stations on 500KHz WSPR were Spotted in the Czech Republic. First G3XBM with 1mW ERP then G3ZJO with 20uW ERP were logged by OK2BVG.
For me this is a distance of 1310Km or 814miles, just out of interest I worked out the Km and Miles per Watt of ERP - 6.55 Million Km or 4.07 Million Miles.
This prompted a comment from someone that his 1924 Admiralty Radio Handbook stated that, using 1KW on 500KHz the maximum range over an ideal salt water path is 1500Km. Well then, I told you WSPR is useful.
Over the next couple of days, something must have bugged us both, without consultation, G3XBM and me went into ERP checking mode, re-calibrating our Antenna Current meters, recalculating, and computer modelling our antennas. We both use small less 'gain' yet efficient antennas.
Exact figures are difficult but we both consider we are accurate to + or - 3dB.
Here is my WSPR Countries List on 500KHz 200 micro-watts or less. England, Scotland, France, Belgium, Netherlands, Germany, Czech Republic, Norway.