Below is the text I posted on another board in response to the quotes shown. Thought
it might be of interest here (comments welcome).
QUOTE I have reviewed at least a couple of dozen Michigan area applications
where the measured conductivities were found to be 0.1 to 2 mS/m on many,
many radials where the M-3 value was 8 mS/m. I am curious as to what your
explanation of this is, Rich. /QUOTE
I haven't seen or analyzed such data, and I don't doubt such conclusions for
the conditions and assumptions used in those tests.
But the fact that groundwave fields measured along a radial might indicate
much lower than M3 conductivity at some measuring locations doesn't
necessarily mean that the fields further downrange on that path will
continue to be much lower than expected, based on the M3 map for the
conductivity of that total path length.
MW field intensity in the space immediately above the surface of the earth
is affected by the conductivity of the earth at every location. But that GW
field is produced as a result of the interaction of the earth with the total
radiated envelope at that range, which for the GW is a localized effect.
That field hasn't traveled from the transmit antenna through the surface of
the earth (or very near it) to reach that point.
At elevation angles where the radiation envelope is located far enough above
the earth it is not affected by the losses at the earth/space boundary. Such
higher-angle fields decay at a uniform 1/r rate.
The field at the earth interface is dependent on, and an extension of the
1/r field existing above the earth interface. So GW fields well downrange
are not dependent on GW fields closer to the source. Rather they are
dependent on the 1/r fields radiated by the source, and earth conductivity
only at each location along the radial, independently. Therefore with
increasing distance it is possible for GW fields to increase rather than
decrease, depending on effective earth conductivity at each range along a
given radial.
This is illustrated by the clip linked below, showing the radiation envelope of WJR
at/near the earth calculated by NEC software for a path length of 90
miles -- the distance from the WJR tx site to the location near Saginaw
where I measured their GW field. I measured 1.9 mV/m, and NEC shows about 2
mV/m. However NEC does not account for earth curvature, which would affect
groundwave fields at this distance. The NEC model was set for 8 mS/m d.c. 13
earth. This NEC field value for the GW is in rather good agreement with the
FCC GW curves for WJR, using the M3 value for this physical path.
Note that the GW field at zero elevation in this NEC study is an extension
of the field plotted here up to an elevation of 1500 meters, and exists
despite having crossed parts of metro Detroit and Oakland county having very
poor effective conductivity. Yet it still meets its expected value for WJR
based on M3, at the end of this 90-mile path.
http://s20.postimg.org/j46qvh4vh/WJRFar_Field_NECSetup.gif
Rich
it might be of interest here (comments welcome).
QUOTE I have reviewed at least a couple of dozen Michigan area applications
where the measured conductivities were found to be 0.1 to 2 mS/m on many,
many radials where the M-3 value was 8 mS/m. I am curious as to what your
explanation of this is, Rich. /QUOTE
I haven't seen or analyzed such data, and I don't doubt such conclusions for
the conditions and assumptions used in those tests.
But the fact that groundwave fields measured along a radial might indicate
much lower than M3 conductivity at some measuring locations doesn't
necessarily mean that the fields further downrange on that path will
continue to be much lower than expected, based on the M3 map for the
conductivity of that total path length.
MW field intensity in the space immediately above the surface of the earth
is affected by the conductivity of the earth at every location. But that GW
field is produced as a result of the interaction of the earth with the total
radiated envelope at that range, which for the GW is a localized effect.
That field hasn't traveled from the transmit antenna through the surface of
the earth (or very near it) to reach that point.
At elevation angles where the radiation envelope is located far enough above
the earth it is not affected by the losses at the earth/space boundary. Such
higher-angle fields decay at a uniform 1/r rate.
The field at the earth interface is dependent on, and an extension of the
1/r field existing above the earth interface. So GW fields well downrange
are not dependent on GW fields closer to the source. Rather they are
dependent on the 1/r fields radiated by the source, and earth conductivity
only at each location along the radial, independently. Therefore with
increasing distance it is possible for GW fields to increase rather than
decrease, depending on effective earth conductivity at each range along a
given radial.
This is illustrated by the clip linked below, showing the radiation envelope of WJR
at/near the earth calculated by NEC software for a path length of 90
miles -- the distance from the WJR tx site to the location near Saginaw
where I measured their GW field. I measured 1.9 mV/m, and NEC shows about 2
mV/m. However NEC does not account for earth curvature, which would affect
groundwave fields at this distance. The NEC model was set for 8 mS/m d.c. 13
earth. This NEC field value for the GW is in rather good agreement with the
FCC GW curves for WJR, using the M3 value for this physical path.
Note that the GW field at zero elevation in this NEC study is an extension
of the field plotted here up to an elevation of 1500 meters, and exists
despite having crossed parts of metro Detroit and Oakland county having very
poor effective conductivity. Yet it still meets its expected value for WJR
based on M3, at the end of this 90-mile path.
http://s20.postimg.org/j46qvh4vh/WJRFar_Field_NECSetup.gif
Rich
