>Electricians and power companies are mainly interested
>in a measurement at 60 Hz.
WRONG! Earth grounding for power systems is solely for protection
against lightning and other potentially destructive on the power
line. Earth ground should NEVER carry current (except under a
lightning strike or power spike condition), and those are going to
be RF currents, not 60 Hz.
>Hams with vertical antennas are interested
>in the resistance at the frequency of operation.
Wrong again. Vertical antennas do not work against an earth
ground, they work against a conductive plane that may or may not
be earth. The earth itself is rarely sufficiently conductive to do
this in an efficient manner, which is why we install ground
radials. A ground rod (or a collection of ground rods) is quite
unlikely to improve the RF performance of a vertical antenna.
Those ground rods ARE important for lightning protection.
The earth is also a reflector of radio waves, causing addition and
subtraction at various vertical angles based on the phase
relationship between direct and reflected waves. This is the major
factor in establishing the angle of radiation of the antenna. But
no earth connection is required, and it is the earth at several
wavelengths from the antenna that does the reflecting! ON4UN's
book includes very good discussions of this mechanism for various
antenna types.
>lightning has a large RF component
Correct. IEEE data shows the energy spectrum of lightning to have
a VERY broad peak around 1 MHz, with lots of energy a decade or
two above and below that frequency. Thus the INDUCTANCE (and thus
the length) of the connection to earth is often of FAR greater
importance than the resistance.
73,
Jim Brown K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|