Using the coax feed to my 10m/6m halo as a vertical I've worked out the ERP currently on 500kHz OPERA. The RF power from the transverter is around 5W from an IRF510.
The 10m and 6m halo lengths correspond to a horizontal capacitance of around 35pF. Some of the feed from the shack to the bottom of the vertical section is horizontal through the loft and this has not been included (I think it may add loss though). The vertical section outside is 6m long, equivalent to a vertical capacitance of 36pF. Using standard formula this means my effective height is around 4.48m.
Radiation resistance then works out as 0.088 ohms.
Measured antenna current is 0.48A (using a current transformer and detector)
ERP = 0.48 x0.48 x 0.088 = 20mW (or lower if the losses are higher).