On Sat, 13 Jun 2009 13:27:00 -0400, Joe Giacobello wrote:
>Jim, it is my understanding that unless the radials provide a perfect
>shield over the ground, some of the return current will flow through the
>radials and the rest through the ground. More radials provide more
>shielding and, consequently, carry a greater fraction of the current
>relative to the ground. Perfect shielding would mean the radials carry
>all the current. Presumably, Brown's 120 radials come to close to this
>ideal.
I've not heard that way of describing it. A better way of understanding it is
to
view the earth as a loosy path for return currents and fields, and the radial
system as a low resistance path. That path, whatever it is, shows up in series
with the radiation resistance in the equivalent circuit for the antenna.
It has long been well understood that return current is greatest near the base
of
the antenna, so copper near the tower is more important than copper at a
distance
in reducing I squared R losses. This is why AM broadcast stations often use a
heavy copper screen that looks like monster chicken wire (40 ft x 40 ft is a
number that comes to mind) in the immediate area of the tower, with radials
terminated to the perimeter of the mesh.
>If a perfect ground were available (zero resistance), one wouldn't need
>any radials at all and just a connection to ground. In the real world,
>one has neither perfect ground nor a perfect shield, so the return
>current is divided between the two parallel paths. If one then
>increases the conductivity of the ground through deliberate salting (or
>degradation of a chicken wire screen in this case) and the number of
>radials remains constant, then the overall return path resistance is
>lowered and the efficiency of the vertical is increased because of the
>reduced IR losses. Maybe the degraded chicken wire wouldn't make up for
>the original screen, but the net return resistance would be less than if
>the the radials were used by themselves over the same soil in an
>"unsalted" condition. (Note that that these radials are separate from
>the chicken wire screen.)
I don't think you could put enough salt in the earth over a wide area to make
anything approaching a metallic radial system.
Bottom line -- the EARTH is a lousy conductor, and thus degrades the efficiency
of any antenna system. A connection to earth will make an antenna work better
ONLY if it has NO good return for antenna current and fields. A metallic return
is always much better. The only condition under which a connection to the earth
matters for antenna performance is when it's salt water.
73,
Jim Brown K9YC
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