Richard Fry wrote:
"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.

NEC analysis has been supported many times by measurement and observation.

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.

Yes. The question is what is the calculated value. B, L, and E
normalized their measurements to the unattenuated field strength at one
mile for 1000 watts radiated power. I couldn't find anywhere in their
paper where they explained how they determined the ground attenuation
between the antenna and their observation point.

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.

No, the field strength is strongest at low elevation angles only close
to the antenna, as you further explain below.

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.

Of course the standard far field analysis doesn't accurately depict the
field close to the antenna -- it's a plot of the field at points very
distant from the antenna, as clearly explained in the manual. NEC allows
you to include the surface wave if you want, and it accurately shows the
total field including the surface wave at a distance of your choice.
(Accurate, that is, up to a hundred km or so, beyond which the deviation
of the flat ground model from the curved Earth begins affecting results.)

Don't feel bad -- Reg has a lot of trouble understanding this, too.

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).

No, the measured fields from quarter wave broadcast antennas are
considerably less than 195 mV/m for 1 kW at one mile, unless perhaps
there's only salt water between the antenna and measurement point. As
you explained above, the surface wave is attenuated with distance. What
you seem to be missing is that the attenuation is strongly dependent on
ground conductivity (between antenna and measurement point, not just at
the antenna site) and frequency, so the actual field strength at one
mile for 1 kW radiated will always be considerably less than the perfect
ground case. The 195 mV/m and associated values for various antenna
heights is the "unattenuated" or "inverse" field, which doesn't include
the surface wave attenuation beyond simple inverse distance field
strength reduction. It's the field strength you'd get if the ground
between antenna and measurement point were perfect, not what you get
over real ground. I'm not very conversant with FCC antenna measurement
methodology, but somewhere the measured field strength is normalized to
the unattenuated field strength by fitting to a ground attenuation
curve, which in turn depends on frequency and ground conductivity. (I've
been told that this is the way broadcasters determine ground
conductivity -- by seeing how far the measured field strength deviates
from the unattenuated value.) I believe that the surface wave
attenuation curves used by the FCC are from the 1937 I.R.E. paper by
K.A. Norton. That paper is also the basis for NEC's surface wave
calculations.

Roy Lewallen, W7EL

Dear Crew:
What may have been lost in this discussion - but often provided by Roy -
is the difference between a true surface wave and a sky wave (from the same
vertical antenna).

An excellent ground screen at the base of a vertical antenna reduces the
losses and thus increases the amplitude of both the surface and sky wave.

A few to several hundred wavelengths from the vertical antenna, the
amplitude of the received surface wave is highly dependent on the
conductivity of the intervening earth. One could hedge this a bit for
frequencies above something like 5 MHz.

At distances from the antenna where the surface wave has become weak,
the sky wave dominates. As Roy has pointed out several times: the amplitude
of the sky wave is mostly determined by ground properties distant from the
antenna - much farther away than any reasonable ground screen will extend.

The sky wave effect may be demonstrated in NEC by effecting a 0.25 WL
vertical with a good set of radials over mediocre ground and then changing
the ground, at some distance from the antenna, to salt water. (Visualize
the antenna being in the center of a circular island and change the diameter
of the island.)

Broadcasters at MF, with some exceptions, are interested in maximum
surface wave and minimum sky wave. Radio amateurs, for the most part, are
interested in maximizing the sky wave. A good radial system assists the
radio amateur's goal by improving the overall efficiency. It is the ground
distant from the vertical antenna that determines the sky wave.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
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