Conclusion: The less ground conductivity the higher is the antenna
elevation radiation angle. This is a negative impact for DX!
Cris, Tom, Paul et al
This belief is common when looking at the far-field elevation pattern of a
vertical monopole in MoM results, or in antenna textbooks. That pattern is
what remains of the radiated field at an infinite distance from the
monopole, over an infinite, flat ground plane.
--> But in reality all vertical monopoles of 5/8-wavelength and less radiate
their maximum relative field (E/Emax) in the horizontal plane.<--
A NEC near-field evaluation can show the field produced in and near the
horizontal plane closer to the radiator, for earth of defined
conductivity/permittivity, and it will not be zero as is shown in a NEC
far-field plot.
The NEC study at the link below illustrates this. The groundwave field
(0-deg elevation) is plotted out to 8 km. Note the good correlation between
the NEC GW field and the GW field measured by a broadcast consulting
engineer using an accurately-calibrated field intensity meter.
On that same chart is plotted the field existing from that radiator at an
elevation of 100 meters above the earth. Note that it is lower near the
radiator than the GW field, because the relative field radiated by a
1/4-wave monopole at higher elevation angles is less than in the horizontal
plane. In fact at the zenith it will be zero. At ~8km downrange it has
reached the value of the GW field, and further downrange it will exceed the
GW field.
From the NEC chart it can be seen that the field at low vertical angles
(less than 3 degrees) is at least as great as it is at zero degrees. There
is no physical reason for that low-angle radiation NOT to continue on to the
ionosphere to produce a skywave signal, given the right conditions.
http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg
Supporting this below is a clip from the Radio Engineers' Handbook (Terman,
1943) showing this same reality, in that the angle at which radiation leaves
that antenna for greatest 1-hop skywave range is less than 3 degrees.
http://i62.photobucket.com/albums/h85/rfry-100/TermanFig55.jpg
Probably the description "takeoff angle" commonly applied to vertical
monopoles is a rather misleading specification.
R. Fry
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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