Near field soil conditions become less of a factor with the use of
elevated radials. Consider a "ground plane antenna" with the base
mounted at the rooftop of a 1 or 2 story building - it is reasonably
independent of near field ground conditions because it is an antenna
within itself - much the same as a vertical dipole. A vertical with
buried radials is quite dependent on ground conductivity unless the
number of radials is large.
The far field ground conditions still do play a part and what Jim has
pointed out below is valid.
IMHO, the main advantage of a vertical is that it has a null at the top
of the elevation plots which will reduce the response to high angle
reception (nearby stations). If you look at the elevation plot for a
horizontal dipole along with a vertical, you will see that the dipole
has almost as much gain at low angles as the vertical - so from a pure
gain standpoint, the vertical offers no advantage.
BUT that is comparing a vertical with a high dipole (at least 1/2
wavelength high). For the lower frequency bands, the height of a dipole
with those characteristics is not practical for most hams and the
vertical wins "hands down" for DX - provided a good radial field or
elevated radials are used.
73,
Don W3FPR
On 7/23/2015 11:10 AM, Jim Brown wrote:
On Wed,7/22/2015 6:15 PM, Al Lorona wrote:
This suggests that ground loss is sort of an impedance-matching
problem. Clearly, as your ground gets really bad your antenna can
still work just fine, even better in some cases!
Hi Al,
Several observations. First, soil influences vertically polarized
antennas in two ways.
First, loss in the soil near the antenna as the field from the antenna
causes current flow in the ground. The result is simple I squared R
loss, and that power does not get radiated.
Second is the reflection from the earth in the far field where the
radiated field hits it. That reflection adds (algebraically) to the
direct signal to form the vertical pattern. The strength of the
combined direct signal plus reflected signal depends on the relative
phase angle between them, which in turn is a factor of distance, the
elevation angle, and the soil.
The second point is that conductivity is not the only soil parameter.
There is also the dielectric constant. In EZNEC, if you open the
Ground Description tab and right click on either Cond or Diel Const,
you'll get a window showing a continuum of soil types from very good
to very poor. BOTH Cond and Diel Const vary with those soil types, and
both quantities affect how the antenna behaves.
A year or two ago, I did an extensive study of the interaction of
vertically polarized antennas with soil of different types and at
different mounting heights. I presented it to the Pacificon Antenna
Forum using these slides. http://k9yc.com/VerticalHeight.pdf
73, Jim K9YC
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