I then measured the signal level of my QRSS3 beacon on 137.675kHz to see how much lower the level was at the same point about 1.5km away from the home QTH. My QRSS3 beacon was measuring -42dB approximately, corresponding to a field strength level of 12uV/m at 1.5km.
To work back to my ERP from the earth electrode antenna I used the formula ERP = (E * d)^2 /49 and this gives a result of 6.3uW and an antenna "loss" factor of 56dB (power in to RF out).
There are several sources of error:
- loop orientation +/-3dB
- loop Q/tuning +/-3dB
- measurement accuracy +/-2dB