[TowerTalk] Ground resistance

Jim Brown jim at audiosystemsgroup.com
Thu Jun 8 13:37:25 EDT 2006

>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. 


Jim Brown K9YC

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