In the Saginaw Valley, which is an extremely large (perhaps close to, or
more than 6,000 sq miles and ringed by roughly 600 ft hills-shown on
Google Earth) is shallow valley off the SW end of Saginaw Bay in
Michigan. The top soil varies widely, but generally is no more than a
foot or two thick. The sub soil is "usually" clay, or sand (possibly
layers of both) usually for 6 to 10 feet. Below that is usually about
400 feet of gravel, sand, and large rocks over shale. On the surface
those rocks would be called erratics. In our area there is a wide area
of salt water at roughly 100 feet down, but that varies widely from 50
feet to over several hundred feet, over short distances.
*IOW, the soil in this area generally makes for a poor ground, so an
effective, safety ground system needs to cover a relatively large area.
The one, two, or three ground rods required at the electrical entrance
are often relatively useless as safety grounds. In late summer they
don't come near the water table in some locations (mine)**
**
**Even though the water table may be within a few feet of the surface
part of the year, three, 8' or 10' ground rods around the base of a
tower, often make for a poor safety ground and are not suitable for
lightning protection, **
*
Many hams in this area use only one or two Ground rods at the base of
the tower, sometimes, none. There are several 40 to 60 ft towers
within 3 miles of me with no ground rods. I took down a simple 30
footer on a sand hill that had no ground rods. The rational was that it
was dry sand as far as any ground rod would reach. It was a fine sand,
what we usually refer to as "blow sand". A temporary tower for a few
years and he didn't have the money for a ground system. With
thunderstorms in the area he said he'd just throw the cables out the
window.
I believe there many in this area that just can't afford a good ground
system and much of their stations consist of old, scrounged, or basic
home built equipment. I know, that 55 years ago, as a 20 year old about
to turn 21, mine was that way. Much because I couldn't afford it and
much because I didn't know any better.
73
Roger (K8RI)
On 2/7/2016 Sunday 12:47 AM, Jim Brown wrote:
On Sat,2/6/2016 4:22 PM, jimlux wrote:
Using N6LF's formulas I get skin depths around 5-10 meters at 2MHz
(0.005 S/m, epsilonr = 3-10)
You might get 60 feet (20 meters) if the soil is particularly
non-conductive (rock). Dropping the conductivity to 0.5 mS/m gives
you skin depths of 22-34 m at 2MHz
Rock and sand are quite prevelant throughout the west, and pretty much
what we have in the mountains. :)
73, Jim K9YC
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|