Topband: Modeling "Ground" and losses

[Richard Fry]

Regarding the quotes below:

>2.) We see any radial or counterpoise system, close to the radial or 
>counterpoise, has to have external fields. Those fields must extend out of 
>the counterpoise, and always cause loss when a counterpoise is near a lossy 
>media.

>3.) We see we only mitigate the loss by making current density in the lossy 
>media as low as possible, which generally means spreading the current out 
>as evenly as possible in the lossy media or moving the antenna and a 
>counterpoise away from the lossy media to reduce current.

Please note that vertical monopoles driven against a set of four- or 
six-wire counterpoises are in daily use by several AM broadcast stations in 
the U.S.

These stations provide * measured * fields very close to the maximum 
theoretical field possible from that monopole and applied power when driven 
against a PERFECT ground plane -- even though these counterpoises are 
elevated only some 10 or 15 feet above earth of rather poor earth 
conductivity.

Here is a quote about one of the early cases, from the NAB paper titled

NEW AM BROADCAST ANTENNA DESIGNS HAVING FIELD VALIDATED PERFORMANCE
   by
Clarence M. Beverage
Communications Technologies, Inc.
Marlton, NJ

\\  In November of 1988, our firm supervised the construction of a temporary
antenna system in Newburgh, New York under FCC Special Field Test Authority
using call sign KPI-204. The antenna system consisted of a lightweight, 15 
inch
face tower, 120 feet in height, with a base insulator at the 15 foot 
elevation and
six elevated radials, a quarter wave in length, spaced evenly around the 
tower and
elevated 15 feet above the ground. The radials were fully insulated from 
ground
and supported at the ends by wooden tripods. Approximately ten feet above
ground, a T network for matching the antenna was mounted on a piece of 
marine
plywood to isolate the components from contact with the lower section of the 
\
tower which was grounded. Power was fed to the system through a 200 foot 
length
of coaxial cable with the cable shield connected to the shunt element of the
T network and to the elevated radials. A balun or RF choke on the feedline 
was
not employed and the feedline was isolated from the lower section of the 
tower.
The system operated on 1580 kHz at a power of 750 watts.

The efficiency of the antenna was determined by radial field intensity 
measure-
ments along 12 radials extending out to a distance of up to 85 kilometers. 
The
measured RMS efficiency was 287 mV/m for 1 kW, at one kilometer, which is
the same measured value as would be expected for a 0.17 wave tower
above 120 buried radials. //

Clearly there is little effect on the radiation efficiency of these systems 
due to the proximity* of their counterpoise to the lossy media of nearby 
earth.

  *about 8.7 degrees, in this case

R. Fry