It would be interesting to see the same modelling over nonconductive
"rock" earth. I suspect once the soil gets bad enough that there is
essentially no electron mobility the "ground plane" (elevated radials)
antenna begins to act as if it was in free space - or at least a
substantial fraction of a wavelength above the ground - which can
reduce ground losses substantially due to the reduction of fields in
the soil.
While 1 km field readings are interesting, 1 km is only 6 wavelengths
at 160 meters (and less in the AM band) so those values still contain a
strong "near field" component and do not adequately reflect the true
ground losses. If one wants to really measure ground losses, it takes
at least two readings - one at 1 mile (~10 wavelengths) and another at
1.4 miles or 2 miles *along the same radial*. The departure from
square law losses can be attributed to additional ground losses.
Although 1 and 1.4/2 mi field strength specifically measures groundwave
losses, the soil conditions should be homogeneous enough in the skywave
launching region that the ground wave case will provide far greater
accuracy than any modelling. The problem is that accurate measurement
of field strength is difficult - particularly in urban/suburban areas -
and nobody wants to take the time to do it unless they are required to
do so. Even the AM broadcasters take pains to select monitoring points
well away from "clutter".
73,
... Joe, W4TV
On 1/23/2014 12:05 AM, Mike Waters wrote:
A very respected authority on radial systems, Rudy Severns N6LF, has this
to say about 4 elevated radials:
"Since my QST article I've done some modeling to explore the sensitivity of
a simple 4-radial system to asymmetries in the radial fan. The modeling
easily replicates Weber's results and the news is even worse than Dick
thought! The 4-radial system is indeed very sensitive to quite small
irregularities and/or nearby conductors. It's easy to demonstrate pattern
distortions of 2-3 dB and increased ground losses of 1-2 dB and these are
by no means worst cases. More importantly, the modeling shows that as the
number of elevated radials is increased the sensitivity goes down quickly.
Elevated systems with 10-12 radials are not very sensitive to reasonable
asymmetries. It turns out that a number of hams have observed significant
improvements in their elevated systems by going to 10 or more radials. Both
modeling and
experiment seem to agree.
"I doubt that the average 4-radial system is actually performing as
"advertized". No doubt there are exceptions but the advice I presently give
is to use 10 or more radials whenever possible in an elevated system."
(From
http://rudys.typepad.com/files/december-2010-letter-to-qst-technical-correspondence.pdf
)
See http://www.antennasbyn6lf.com/
73, Mike
www.w0btu.com
On Wed, Jan 22, 2014 at 7:18 AM, Richard Fry <rfry@adams.net> wrote:
C. Cunningham wrote:
If you get up to 4 symmetrical elevated radials there's not much to be
gained by adding more. There's been a lot of work done in the broadcast
industry using elevated radials to replace deteriorated buried radial
fields that shows that pretty clearly. It was published in some IEEE
transactions some years ago.
Probably this refers to the paper of Clarence Beverage titled "NEW AM
BROADCAST ANTENNA DESIGNS HAVING FIELD VALIDATED PERFORMANCE." It is
available as a PDF download from http://www.commtechrf.com/downloads.asp .
Below is a quote from that paper showing that the __measured__ groundwave
field at 1 km radiated by a base-insulated, 1/4-wave vertical using four
elevated radials was within 0.14 decibels of that from a perfect 1/4-wave
vertical monopole driven against 120 x 1/4-wave buried radials.
The r-f loss resistance of 120 x 1/4-wave buried radials used in a
monopole antenna system typically is less than 2 ohms in the MW and low-HF
bands, regardless of the conductivity of the earth in which they are
buried. The use of four elevated 1/4-wave radials in this system produced
almost identical performance to using a full set of 120 x 1/4-wave buried
radials.
"The first permanent use of an elevated radial ground system appears to be
at WPCI, 1490 kHz in Greenville, South Carolina. This installation,
designed by William A. Culpepper, involved replacing a standard buried
system with a four wire elevated system consisting of #10 solid copper
wire, one quarter wave in length, and supported on treated wooden posts
which keep the radials 4.9 meters above ground. The antenna radiation
efficiency, based on field strength readings on the eight cardinal radials,
was 302 mV/m at 1 kilometer versus the predicted FCC value of 307 mV/m. The
WPCI installation was unique in that the tower was base insulated but the
radials came right up to the tower, 4.9 meters above ground and terminated
in insulators. The tower was fed from the tuning unit, through a piece of
coax to the 5 meter point on the tower where the center conductor of the
coax was attached to the tower and the shield to the elevated radials. This
feed system resulted in a higher feed resistance than would normally be
expected. Data on this facility was taken from the FCC files."
Guy Olinger wrote:
Be careful not to extrapolate very specifically qualified broadcast
experience into ham radio. Originally FCC spec radials still make the close
foreground earth appear VERY conductive, which is NOT an advantage one will
have putting up two or four radials over plain old dirt, unless one is
talking about midwest USA 30 millisiemen super dirt.
A monopole system using ~ four evenly spaced, horizontal, elevated radials
or an "FCP" does not need (or use) a highly-conductive region ("FCC spec
radials") around the base of the vertical radiator, because in such antenna
systems the r-f currents flowing on its vertical and horizontal wires to
produce radiation do not travel through the earth.
Note that the system described in the quote from Clarence Beverage's paper
(above) was installed/tested near Greenville, South Carolina -- a region
having earth conductivity of not more than 4 mS/m per the FCC M3
conductivity map, and probably less than that. Yet it produced almost 100%
radiation efficiency as measured by a broadcast consulting engineer using a
calibrated field intensity meter.
Such characteristics would apply to the use of elevated radial systems by
ham radio operators as well as they do for AM broadcast stations.
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