Owen wrote:
"Is that true?"

Only for limited conditions. Owen is correct that enough samples must be
taken to catch all variations in signal strength, in elevation and
azimuth if total radiated power is to be determined.

A simple vertical antenna rests on the earth and has a null at its tip.
It is far from an isotropic radiator.

The sampling I described is done on AM broadcast antennas. Only the wave
which travels along the earth is usually of any interest. The minimum
distance from the broadcast antenna for any field strength measurements
is usually one mile to be sure the far field is being measured.

The FCC`s ground wave intensity charts assume that if you have ground
conductivity such as sea water, a certain power and antenna efficiency
deliver 100 millivolts per meter at a distance of one mile from the
antenna. Field intensity in millivolts declines linearly with distance
so that at 10 miles you might have 10 mv/m if you have 100 mv/m at one
mile.Power is proportional to the square of the voltage.

Best regards, Richard Harrison, KB5WZI