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.
R. Fry
Broadcast Systems Engr (retired)
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|