Well, repeating the original calculation, taking your "best" signal level as -12dB on a noise level estimated at 3uV/m in 2.5kHz, your received field strength at my QTH is 0.75uV/m. At a distance of 69km, and assuming only "geometric spreading" propagation losses (i.e. an inverse relation between FS and distance), your ERP is
ERP = (Ed)^2 / 49 , with E = 0.75u, d = 69000, ERP = 55uW
Then we have to consider what additional propagation losses exist; originally I suggested 3dB, which would increase the ERP to 110uW. However, looking at G4FGQ's grndwav4 program, and the ground resistivity around this area, the "ground type" in the program might be 6 - 8, making the ground wave propagation losses between 4.2 and 8.5dB. This would make the ERP between about 140uW and 390uW.
Obviously there is a lot of uncertainty in this kind of estimate - the two main things are the possible variability in noise levels between when I estimated the noise level and now, and the propagation losses. Both these could contribute several dB variation, so the ERP estimate should only be regarded as "order of magnitude" accuracy.
Still, your signal is certainly much stronger now - one of my nearer-term projects is to make some more accurate FS measurements by injecting a calibration WSPR signal into the loop antenna, which should give a more direct and accurate measurement of the signal levels.
Cheers, Jim Moritz
73 de M0BMU
1 Nov 2009
Some weeks ago Jim Moritz M0BMU made an estimate of my 500kHz ERP based on received signal strength levels at his QTH 69kms away. As I've recently increased my ERP, I asked Jim if he would kindly do a recalculation. These are his latest estimates.