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For a lossless electrically short vertical monopole over a perfect ground plane:
The field strength at 1 km for 1kW radiated power is 300 mV/m.
Because field strength varies as the inverse of distance, the 65 mV/m at 100 m measured by the FCC is equivalent to 6.5 mV/m at 1 km.
Field strength varies as the square root of radiated power.
Therefore, the field strength of 65 mV/m at 100 m corresponds to a radiated power of [(6.5/300)^2]X(10^6) = 469 mW of radiated power from a lossless short vertical monopole over a perfect ground plane.
For a lossless quarter wave vertical monopole over a perfect ground plane:
The field strength at 1 km for 1 kW radiated power is 313.2 mV/m. [edit…unit correction]
Therefore, the field strength of 65 mV/m at 100 m corresponds to a radiated power of [(6.5/313.2)^2]X(10^6) = 431 mW from a lossless quarter wave vertical monopole over a perfect ground plane.
The FCC agrees with Rich’s statement that an elevated antenna does not necessarily have more efficiency than one that is at ground level. Section 73.189 of the FCC Rules and Regulations considers an AM BCB antenna to have the same effective height whether it is mounted on the ground or on top of a building. However, there is likely to be some conductive radiating path between an elevated antenna and earth ground unless there is a deliberate effort to eliminate one. For example, a battery-powered transmitter feeding a vertical dipole on top of an insulated pole would not have a conductive path to ground, provided that there are no audio or control wires going to the transmitter. The audio source to the transmitter should also be battery powered, and on on the insulated pole. If unlikely extreme measures like this are not taken, there will probably be a spurious conductive path to ground by some route.