137.766kHz ERP - now measured at 6.6uW

So, today I repeated my field measurements to establish the ERP of my 8W 137.766kHz beacon feeding my earth-electrode antenna. This time I did the measurement correctly. Here's how.

1. Travel 2km from the home QTH with E-field probe, FT817 and PC running Spectran.
2. Choose a location in the best direction for the "loop in the ground" so the measured signal strength is close to the maximum possible i.e. not off the sides of the loop.
3. Set up FT817 so the RF gain is adjustable and turned well back and the AGC is inactive.
4. Tune in my LF beacon and adjust the RF gain to the lowest possible detection level.
5. Measure the S/N of my signal with Spectran and note reading.
6. Retune to DCF39 (138.830kHz) and, without adjusting the RF gain at all, measure the S/N with Spectran.
7. Repeat these measurements 5 times.
8. Calculate the difference in dB between my signal and DCF39 (in my case 41.3dB)
9. Using the assumption that DCF39 has a field strength of 1mV/m, work out my own field strength (in my case 9uV/m)
10. Using the formula ERP = (E^2 * d^2)/49 work out the ERP.

Answer? My measured ERP on 137.766kHz is 6.6uW

Caveats:

1. If DCF39 is not approx 1mV/m then the ERP needs to be adjusted up or down proportionally.
2. Measurement error is +/- 2dB.
3. The ERP is the figure measured in close to the best direction. Off the sides of the loop the ERP will be much lower.

Conclusions:

1. Stations able to detect the QRSS3 signal at any distance are doing well as the signal is very weak indeed.
2. Using my proposed WSPR transverter at 32W, for example as a reasonable target output, would give me 6dB more ERP at around 25uW.
3. Based on results with QRSS3, I should be able to be copied using WSPR-2 at up to 100km on ground wave. At night with sky wave, considerably further is possible. 
4. Using WSPR-15 and 32W my ground wave range should be up to around 200km, possibly a little more. 

The QRSS3 beacon is still running over the weekend and more reports would be welcomed