It seems highly improbable that a Rangemaster could actually be radiating 25 mW, even if it were operating over a perfect ground system. The output stage and matching network are just not that efficient. I suspect that either the field strength reading or the calculations are in error.
As was stated, the value calculated by NEC is the
transmitter output power for the NOUO conditions, with the typical coil/ground losses shown -- not the power radiated by the antenna system.
If the earth ground connection and the coil both had zero r-f loss, then, other things equal, the groundwave field intensity at 450 ft would be about 7 mV/m. This shows that about 2/3 of that 25 mW of transmitter output power is dissipated in the real losses of the ground and coil. With the stated losses in the r-f ground and coil included, the
radiated power is about 0.025/3 = 0.0083 watts (8.3 mW).
Most of that 8.3 mW is radiated by the long conducting path from the transmitter chassis to earth ground, rather than from the 3-m whip generally considered to be "the antenna." An r-f ground does not exist at the top of a grounded tower or "massive wire" -- it is supplied by something buried in the earth, such as radial wires, ground rods etc.
The bottom line is that the 25 mW transmitter output power and the 4 mV/m field at 450 ft are consistent with each other, for the given conditions.
RF
