Topband: Monopole Elev Pattern w.r.t. Earth Conductivity

[Richard Fry]

Guy Olinger wrote:
>So far NEC4 has predicted any real measurements, regardless
>of operator skill.

Presumably the above text was meant to read "So far NEC4 has _NOT_ predicted 
any real measurements, regardless of operator skill."

Some may not recall the comparison of groundwave fields calculated by NEC to 
those measured with an accurate, calibrated field intensity meter by a 
broadcast consulting engineer, posted here some months ago (link below). 
Best-fit earth conductivity for the measured path was about 6 mS/m.

The fields calculated by NEC are shown on the top half of the page.  Note 
the close agreement between the data shown by the blue line there, and the 
solid line below the inverse distance field in the consultant's data plotted 
at the bottom of that page.  This is at at least one case where NEC and 
real-world data agree.

Another important observation to be made from that NEC data is that 
space-wave radiation from elevation angles below ~2 degrees equals the 
radiation in the groundwave at 1,300 meters downrange.  For further 
distances downrange the space wave exceeds the groundwave, as the space wave 
is decaying at a 1/r rate, and the groundwave decays at greater than 1/r, 
due to earth losses and eventually, earth curvature.   This is shown by the 
green line on the NEC chart starting to exceed the blue line at an 
h-distance of 8,000 meters.

For long enough point-point paths along the surface the earth, the 
groundwave essentially is zero, and the much greater space wave radiation 
from such low elevation angles can produce nighttime skywave coverage over 
the longest, single hop paths.

This type of performance also applies to the monopoles of 5/8WL and less 
used by ham operators on Topband and the HF bands (regardless of earth 
conductivity at/near the monopole site).

>The first thing would be to prove that NEC4's null at 50 km is a fantasy.

The fields in the groundwave itself do not provide coverage by ionospheric 
skip, and there is little point in examining the groundwave 50 km downrange 
(or anywhere else) to prove or disprove this.

NEC properly calculates zero field for its far-field plot of the elevation 
pattern of a monopole over a non-perfect ground plane, and not much more at 
vertical angles below 5 degrees or so.  And a NEC near-field plot properly 
does _not_ show this "notch" for distances sufficiently close to the 
radiator.  These calculations are not in conflict when properly understood.

The fields providing the greatest single-hop skip range are produced by the 
elevation plane fields within a few wavelengths of the radiator that are 
directed toward the lowest elevation angles.  Those space-wave fields are 
much greater than the groundwave for sufficiently long paths along the 
surface of the earth.  And those fields exist at much lower elevation angles 
than shown by a NEC far-field plot, alone.

NEC calculates all of these fields accurately.  It is only their 
(mis)understanding and incomplete use that leads to the concept of "takeoff 
angle" for the elevation patterns of monopoles over a non-perfect ground 
plane.

http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg

R. Fry