[TowerTalk] Grounding on a rock hilltop

[John Langdon]

Scott,

My station is on a limestone ridge with about 3 inches of soil on top.  I
run remote, so everything is connected all the time.

In this type of environment, grounding really consists of having enough
ground system area to spread the charge out quickly, because there is no
"real ground" to be had.  I used 9 each 2" wide copper strips, 30' long,
with a 10' ground rod at the base of the tower and additional rods 10', 20'
and 30' out from the tower.  The strap is CADwelded to the ground rods. The
ground rod holes had to be drilled, not driven, and backfilled with
"Bentonite" clay. Despite being witness to many spectacular central Texas
thunderstorms, no damage from lightning in about 13 years despite some
direct hits (other than replacing some Polyphaser units!). The cables go
through Polyphaser units at the tower base, and then are buried in conduit
(about 225' run about 3' deep) to the shack, which has a perimeter ground
system and a single point ground with a Polyphaser PEP entry panel for all
cables. 

All the low band antennas have to be horizontal polarization because the
ground conductivity is so poor.  K5IU did a presentation several years ago
at the W5KFT "DX Bash" that showed that with soil like mine, a 80M four
square installed on 1/2 wave radius copper discs would be 1 dB below a half
wave dipole up 120' and broadside to the desired DX station.  The good news
is that short beverages work very well, but you have to do a "spread it out"
kind of ground for them too!

The other thing to remember is that winds on the ridge are 20+ mph or more,
and I have observed many times the beam element deflections indicating three
clearly different wind directions along the 195' tower, so everything has to
be designed for maximum wind loads.

73 John N5CQ


-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of scottw3tx at verizon.net
Sent: Monday, July 19, 2010 10:12 PM
To: TOWERTALK at contesting.com
Subject: [TowerTalk] Grounding on a rock hilltop

I'm working on a friends' station this week, which is on the top of a very
high hill (the hill-top is about 100ft above the terrain for 270 degrees of
azimuth).

Naturally, the hill is solid rock (the tower foundation was blasted).  There
is from no to perhaps 2 inches of soil covering the rock. 

I'm interested in hearing what has worked to improve station grounding in
similar situations.

73, 
Scott W3TX

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