>In reference to my posting wondering about the geometry and magnitude of
ground currents, induced by radials, whether buried, surface, or elevated:
two replies received were very informative. First, K1FK, who provided some
good info and sent me to the library for more; and second, W8JI who
described the geometry in some detail. I had not been able to find that
info elsewhere, so thanks, Tom.
K1KI said:
An excellent treatment of the currents and losses involved with short
ertical antennas is contained in Earling D. Sunde's (Bell Telephone Labs)
book EARTH CONDUCTION EFFECTS IN TRANSMISSION SYSTEMS, Dover Publications
Inc., NY (now out of print, but your local university or industry research
library can obtain a copy for you for a short time throught the
inter-library loan program.
There are actually three currents involved; displacement currents,
conduction currents, and earth induced current, all which interact in a
very complex manner and Sunde provides an excellent explanation and
mathamatical treatment (brush up on your calculus!).
Dave, K1FK
Fort Kent, ME
W8JI said:
In my back woods dirt road cow farmer mind....I look at it this way
> Current is flowing in an elevated radial. That current causes an induction
> field to exist around the radial.
Fields are things we use to explain actions at a distance. I guess
it's OK to picture them as being "real", as long as we don't get the
effects all tangled up.
> The induction field induces a voltage (note: NOT a current).
Or another way to say it is a force on other charges at a distance.
Charges affect other charges three ways.
1.) If they are just "there", by electrostatic fields.
2.) If they are moving at a steady rate in a fixed direction, by
magnetic fields.
3.) If they are accelerating (changing speed or direction) by
radiation.
We communicate over distance by radiation, not by magnetic or
electric fields. A magnetic radiator won't radiate at all over any
distance more than a few antenna sizes away! It would have all the
communications range of a magnet the same size.
The noise we hear from a thunderstorm (unless it is VERY close) is
due exclusively to radiation fields, and our "magnetic loops" hear
the noise from those storms exactly the same as they are heard on any
other antenna.
> about the third dimension? Well, there ain't no ground above ground. So
> not much current is gonna flow in the air.
That's why the term "displacement current" was invented. Displacement
current is "current" that really isn't moving charges. In the dirt or
in ANY conductor it's current, it isn't "displacement" current.
> Also, as we penetrate into the
> soil I find it very difficult to imagine much current flowing very far
> below the surface.
The 160 meter skin depth (distance where current drops to 37% of the
value traveling in the conductor) is about 30 meters in poor soil,
ten meters in good soil, and about a foot in salt water. Current
effectively goes to zero at about 10 skin depths.
73, Tom W8JI
w8ji.tom@MCIONE.com
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