Gee, Richard, it's only been seven months this time since you last
posted essentially the same comments and questions. You were going a
year or so between.

Here are a couple of the previous ones, with my responses. Anyone
interested in more information can do a google groups search of my
postings on this newsgroup containing "surface wave".

4-20-2008:

Richard Fry wrote:
"Roy Lewallen"
However, you can't compensate for this factor when the ground is
poor by improving the ground system. The reason is that the reflection
takes place much farther from the antenna than nearly any ground system
extends. And low angle radiation, where the improvement is most needed,
reflects the greatest distance away.
___________

Roy, didn't the experiments of Brown, Lewis & Epstein of RCA in ~1937
show that the h-plane field measured 3/10 mile from a vertical monopole
of about 60 to 88 degrees in height, over a set of 113 buried radials
each 0.41 WL, was within several percent of the theoretical maximum for
the applied power as radiated by a perfect monopole over a perfect
ground plane? And conductivity at the NJ test site was poor -- 4 mS/m
or less.

That tends to show that the fields radiated at very low elevation
angles also will be close to their theoretical values when measured at
this radial distance, even though ground conductivity at the antenna
site is poor. The relative field (E/Emax) for radiators of these
heights and propagation paths approximately equals the cosine of the
elevation angle.

I believe we've discussed this before, so I'll be brief.

Their calculation of the field at the receiving site when the radial
system is perfect was adjusted for the effect of ground wave attenuation
caused by the imperfect ground conductivity. If the ground between the
antenna and receiving site were perfect, the field strength would have
been greater.

Also, I'm speaking of sky wave. Ground reflection isn't a factor in
determining surface wave, which is what they measured and which isn't of
interest to most amateurs.

The greatest radiated fields always will be directed in or near the
horizontal plane when measured/calculated for such conditions. This
also will be true for any monopole from infinitesimal to 5/8 wavelength
in height, although the elevation pattern of monopoles from /4- to
5/8-WL no longer are described by the cosine function (see
http://i62.photobucket.com/albums/h8...omparison.jpg).

Elevation patterns show maximum relative field centered at various
elevation angles above the horizon, when those fields are measured at
progressively longer radial distances from the monopole, due to the
propagation loss for the surface wave over other than a perfect, flat,
infinite ground for those ranges. Earth curvature and terrain
diffraction add to those losses for longer surface wave paths over real
earth, and for very great distances the h-plane relative fields falls to
~zero.

As I thought you were aware, the surface wave propagates considerably
differently than the sky wave.

But that pattern shape is not the pattern shape originally radiated
by the monopole, it also includes the effects of the propagation
environment at the range where it was measured (or calculated).

If this were not true then MW broadcast stations would have
essentially zero coverage area for their groundwave signals.

It would be a mistake to design HF antenna systems based on optimizing
surface wave propagation as AM broadcasters do, unless you desire
communication for distances not exceeding a few miles.

Roy Lewallen, W7EL

4-22-2006:

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