[TowerTalk] 16om vertical and the number of radials

[Brian Beezley]

Rob, K5UJ:

"For US hams wanting to get a rough idea of where they land in the
ground conductivity lottery, you can look at the high def. FCC map and
either cheer or weep.  But, keep in mind, the boundaries were drawn a
long time ago so use as an approximation."


Rob, that map is fascinating to ponder. It's fun to try to correlate 
contour lines with known soil types or terrain features. But the map has 
limitations besides age. First, it is for the AM broadcast band. Ground 
conductivity typically differs greatly at other frequencies. Second, 
conductivity varies a lot faster than the contour lines suggest. The map 
resolution isn't nearly fine enough to tell you the conductivity in your 
neighborhood.

A ground probe can yield more accurate results. It has its own 
limitations, mainly limited depth sampling. Antenna-induced ground 
current typically extends deeper than the ground probe rod length. Your 
antenna sees ground the probe doesn't. If the ground is inhomogeneous, 
results may be inaccurate. But I think it's still a lot better than 
using a map. Plus a probe gives you permittivity.

Brian