Placing the first rod 16 ft from the tower adds significant impedance of the
wire radial to the first major earth current sink. Most towers that I have
seen place the first rod at less than 5 ft from the tower base, depending
upon whether the base has specifically designed as a Ufer ground.
For example, 16 ft of #6 or #4 wire would have an inductance of about 7 uH,
an impedance of about 3.7 ohms and assuming a strike of 65kA with a edge
rate of 2 microseconds, an IR drop of 227kV. Five feet of radial wire to
the first rod would be 1/3 of that or 65kV.
The optimal grounding system layout should be designed based upon tower
inductance, type and number of feed lines, ground characteristics, tower
foundation design, etc. I believe that is why the PolyPhaser document does
not provide "optimal layouts", but rather insight into how to design an
optimal system for the site.
Steve, W3AHL
-----Original Message-----
From: Jeff
Sent: Friday, November 3, 2017 9:51 PM
To: Steve, W3AHL ; towertalk@contesting.com
Subject: Re: [TowerTalk] Mot R36 related to NUMBER of installed ground
One of the docs that refers to the 50/75 is the Ham Radio Grounding doc. On
fresh look they say the radial should be 50-75' in length referring to the
50' as "minimum". So I guess that makes the basic radial construct assuming
8' rods to be a radial of 48' with 3 rods at 16, 32 and 48' spacing from the
base.
There is not much in the way of references in either document (Mot or the
Polyphasers) which provide optimal layouts for a given set of radials. One
could intuit 3 such radial sets for a total of 9 per tower.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Steve, W3AHL
Sent: Friday, November 03, 2017 3:49 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Mot R36 related to NUMBER of installed ground
The Polyphaser document is: “Lightning Protection & Grounding Solutions for
Communication Sites” by Ken Rand, copyright 2000. There is no mention of
the maximum distance of ground rods from a tower. However, for rocky sites
where ground rods are not possible and a radial grounding field is laid on
or just below the surface, the recommendation is that the radials be no
more than 75’ long and no less than 50’. More radials are added in parallel
from the base of the tower to achieve the desired impedance, while
maintaining the desired fast transient response time of the grounding
system.
The same principle is implied for ground rod installations in the diagrams
and calculations – more parallel runs from the tower base are better. And
more short ground rods are better than fewer long ones due to the impedance
of the rod.
I couldn’t find the complete PDF document online during a recent search,
although there are chapters from the document on PolyPhaser’s web site as
white papers.
It’s an excellent reference for lightning protection systems.
Steve, W3AHL
-----------------------------------
Date: Fri, 3 Nov 2017 11:58:40 -0500
From: "Jeff" <keepwalking188@ac0c.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Mot R36 related to NUMBER of installed ground
rods
In one of the Polyphaser documents they talk about the rise time and length
of a strike duration. Forget the details but the conclusion was that rods
further out than about 50' would not be effective because the strike would
be over before the outlying rods came into play.
If that's true then it seems there is an implied maximum density of the
field based on the 50' maximum radius for rods and the 2x rod length
spacing.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
...snip..
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