To Rich's point, I created a buried field in 4NEC2 using the radial geometry
wizard and a vertical wire radiator 40m tall, all wires being of #12. The
radial field consists of 64, 0.5 wavelength radials, buried 0.1m deep in
moderate soil conductivity of 3 mS/m. As a bit of a sanity check, the base
impedance computes to 40.2-j18.9. Rich can keep me straight on my
assumptions, but I believe they're valid.
http://72.52.250.47/images/160m.jpg
Using NEC version 4.2, surface wave field strength is calculated at 1km as a
function of varying Z value from 0-500m with 1500W from the source at 1.8
MHz. Notice how the field strength at zero elevation does not produce a
null as we typically see in far-field calculations over average soil. So,
without knowing the surface wave component, it would appear the traditional
far-field plots only tell half the story about low-angle radiation from
base-fed monopoles.
My NEC input file is available to anyone who wants to see it. Before
asking, please ensure your modeling program runs under NEC 4 and not NEC 2.
Paul, W9AC
----- Original Message -----
From: "Richard Fry" <rfry@adams.net>
To: <topband@contesting.com>
Sent: Tuesday, October 02, 2012 2:58 PM
Subject: Re: Topband: Ground conductivity, permittivity measurement
>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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UR RST IS ... ... ..9 QSB QSB - hw? BK
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