Earth electrode loop effective area at 500kHz

From some measurements of my earth electrode "antenna" this evening and some estimates of the ERP calculated from received field strengths 69km away, I've received an estimate of my effective loop area including that part that is within the ground. This was calculated by Rik Strobbe OR7T as 70m^2 in total, suggesting some 20m^2 of the loop was within the ground. These were Rik's calculations:

"Assuming

- 50uW ERP

- antenna is traded as a loop (gain = -0.4dBd)

Due to the negative gain (-0.4dB versus dipole) a loop antenna must "radiate" 55uW to get 50uW ERP.

Since the antenna feeding current is 0.15A the radiation resistance is 2.44 milliOhm (R= P/I^2)

The radiation resistance of a small loop is : Ra = 320xPi^4*A^2/L^4

where A = loop area (m^2) and L = wavelength (m)

For 500kHz Ra = 5*10^(-7)*A^2 or A = 1416*sqrt(Ra)

If Ra = 0.00244 Ohm then effective the loop area is 70m^2"