Hi Dick,
I get that at any point in the far field there is RF current in the
ground due to the space wave from the transmitter reflecting obliquely
off ground. I was, however, under the impression that at very low angles
the field from this reflection cancelled direct radiation for earth of
finite conductivity. I think you can see this in the Distance=20 mile
E-field plot you provided for the 1.83 MHz radiator. In that plot there
is an inflection in the E-field versus height very close to the horizon.
If you ignore that inflection and extrapolate a smooth curve from the
E-field at higher elevation angles down to zero elevation angle you can
see that the field would go to zero. I believe that this extrapolated
curve is the component of the E-field attributable to the space wave and
that E-field in the small segment of the original curve below the
inflection point is the E-field attributable to the surface wave which
arrived at that point as a "guided-wave" hugging the surface of the ground.
73, Mike W4EF
On 10/4/2012 2:54 AM, Richard Fry wrote:
Guy Olinger wrote:
But to prove it we can't use standing-man-with-meter. He may just be
sensing the current in the ground just below his feet that will never
be airborne. We need sitting-man-with-meter-in-helicopter to go up
there and prove that what you get from the ground up to twenty
thousand feet out 20 miles is a blend, and not a notch.
The r-f current flowing in real earth at a given distance beyond a few
wavelengths from a vertical monopole did not travel through the earth
from the monopole site to get there. It is the result of the EM wave
radiated by the monopole through space, at the earth boundary of that
wave.
Here is Terman's description: "The electric vector of the wave
produced at the surface of the earth by a vertical antenna possesses a
slight forward tilt. As a result the electric vector has, in addition
to the normal vertical component a slight horizontal component
parallel to the surface of the earth, and lying in the direction of
propagation. ... The component of the electric vector lying parallel
to the earth's surface that is present because of wave tilt represents
a voltage applied across earth's surface. This voltage causes a
current to penetrate down into the earth."
So those r-f currrents in the earth don't ORIGINATE radiation into
space, rather they are a consequence of it.
As for helicopter measurements, below is a clip from a website used
mostly by broadcast engineers, containing part of a post I made and a
response to it from an active, full-time broadcast engineering
consultant. The consultant's comment should be enlightening. Not
only does he recognize the validity of my NEC analysis, he reports his
experience with helicopter measurements confirmiing what is shown in
that NEC analysis.
CLIP:
The link below was prepared using values generated by 4nec2, but
plotted in
other software. It shows that even NEC2 can be used to accurately
calculate
low-angle fields radiated by a monopole over real earth -- and that
the NEC
fields are very close to the accurately measured fields for those
conditions
shown in the plot at the bottom of that page.
The NEC plot also shows that very significant space wave (skywave) is
generated by radiation from a monopole from elevation angles down to
about
one degree, where a NEC far-field plot shows very little relative field.
http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg
R. Fry
______________
Re: Monopole Radiation Patterns
Post by XXXXX » Thu May 17, 2012 7:16 pm
Interesting. As a side note I learned early on the hard way that when
taking FI readings in a helicopter to do the vertical calibration at
least a couple KM away from the antenna, otherwise all you're
measuring is the vertical plane radiation pattern and you end up
somewhere on the left end of the green line on the graph and screw
everything else up.
_______________________________________________
Remember the PreStew coming on October 20th. http://www.kkn.net/stew
for more info.
_______________________________________________
Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more
info.
|
