On 12/20/2022 4:25 AM, n4is@comcast.net wrote:
The assumption that "next to the water" is the same as "in the water" , is a
not right. It is not the same !
In this discussion, it's important to realize that the earth, salt
water, and radials, serve two functions. Soil in all its forms is a big
resistor. Radials serve to carry return current for the antenna IN PLACE
OF LOSSY EARTH (caps added for emphasis). Think of that loss as a
resistor in series with the antenna, forming a voltage divider with the
radiation resistance, where loss is the voltage divider ratil. They also
serve to SHIELD (screen) the field produced by the antenna from the
lossy earth. Salt water, being an excellent conductor, serves as a very
low loss return for antenna current, and needs no such screen. All of
this matters ONLY very close to the antenna, and it only applies to
vertically polarized antennas.
The second contribution of the earth (including salt water) is in the
far field, where it produces the reflection from the antenna that forms
the vertical pattern. The strength of that contribution is directly
related to the earth's conductivity, which is why salt water is so much
better than soil. A vertical with its base very close to the water
produces a very strong and very narrow low angle lobe; if the antenna is
raised, very strong and very narrow lobes are produced.
The contributions of radials and a counterpoise to ground loss is
addressed in these slides for a talk I did at Visalia and Pacificon
about ten years ago. http://k9yc.com/160MPacificon.pdf
I published a modeling study of this about ten years ago. It was written
for NCJ, but ARRL's graphics department complained about the number of
graphs (which are crucial to understand it). It's here.
http://k9yc.com/AntennaPlanning.pdf
73, Jim K9YC
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|