"Roy Lewallen" wrote:
The maximum far field (sky wave) gain of a ground mounted
quarter wave vertical over average ground, with a completely lossless
ground system, is on the order of 0 dBi, and this occurs at roughly 25
degrees above the horizon (both depending on
frequency as well as ground characteristics).
_____________

The above is an understandable conclusion using NEC analysis, however it is
not supported empirically. If it was, AM broadcast stations would perform
very much differently than they do.

The measured data in Brown, Lewis & Epstein's 1937 benchmark paper "Ground
Systems as a Factor in Antenna Efficiency" proved that the *radiated*
groundwave field from a vertical monopole working against 113 buried radials
each 0.41 lambda in length was within a few percent of its calculated peak
value for a radiation pattern with maximum gain in the horizontal plane.
The path length for the measurement was 0.3 miles, which was in the far
field of the vertical monopole configurations measured.

BL&E's measurements, and the results of thousands of measurements made of
the groundwave fields of MW broadcast stations using such radial ground
systems ever since demonstrate that their peak gain always lies in the
horizontal plane.

It is true that, as a groundwave propagation path becomes longer, the field
measured at increasing elevations above the earth at distant ranges might be
higher than measured at ground level at those ranges. But that is not
because more field was launched by the original radiator toward those higher
elevations -- it is because the the groundwave path has higher losses, which
accumulate as that path lengthens. Therefore a NEC plot showing the
conditions reported in the quote above do not accurately depict the
elevation pattern as it is launched from the radiator, and the groundwave
field it will generate.

There are software programs designed for calculating MW groundwave field
strength given the FCC "efficiency" of the radiator and the conductivity of
the path. The radiator efficiency is the groundwave field developed by the
radiator with a given applied power at a given distance (1 kW @ 1 km).
These values must meet a certain minimum level for the class of station. I
think in all cases, they must be within ~0.5 dB of the theoretical value for
a radiation pattern with its peak gain in the horizontal plane. In the case
of directional MW antennas, this performance must be proven by field
measurements.

Finally, standard equations show a peak field of ~137.6 mV/m at 1 mile from
a 1/2-wave dipole radiating 1 kW in free space. The calculated groundwave
field at 1 mile radiated by 1 kW from a 1/4-wave vertical MW monopole over
a perfect ground plane is ~195 mV/m. This is the same field as generated by
the free space 1/2-wave dipole, when all radiation is confined to one
hemisphere (137.6 x 1.414).

The groundwave fields measured from thousands of installed MW broadcast
antenna systems confirm that their intrinsic radiation patterns are within a
fraction of a decibel of that perfect radiator over a perfect ground plane,
no matter what is the conductivity at the antenna site (N.B. Reg).

RF