I will be using 4 elevated radials. The ground screen is not attached
to the
bottom of the vertical (in this case the tower) and is not part of the
radials system. It is to
help reduce near field losses.
Bob
K6UJ
On 1/14/16 9:45 AM, David Robbins wrote:
what is the intended purpose of said ground screen not being connected to the
tower. if you area going to shuntfeed the tower then you want the ground screen
as big as you can make it and attached securely to the tower.
Jan 14, 2016 10:25:22 AM, k6uj@pacbell.net wrote:
Hi Roger,
I am concerned now with my planned ground screen around the base of my
tower.
I am planning a shunt feed to the tower for 160.
I will have separate elevated radials, 4 of them, at 14 feet above the
ground.
The ground screen will extend out 15 to 20 feet from the base of the
tower in all directions
and will not be attached to the tower. I am thinking of leaving a 1
foot clearance from the tower
to the ground screen. The rub is I am thinking of also connecting the
ground screen to a nearby 15' x 70'
parking slab welded wire mesh which is in the slab for reinforcement.
This would give me more
ground screen. Now I am thinking of the lighting possible arcing over
to the ground screen and then
possibly going out and cracking the concrete slab. Maybe I should leave
more than 1 foot space from
the ground screen and the base of the tower.......... Or forget about
tying in to the 15 x 70 slab too ?
BTW, I have 3, 10 foot ground rods attached to the tower.
Bob
K6UJ
On 1/14/16 3:36 AM, Roger (K8RI) on TT wrote:
Inductance and magnetism.
Lightening is not a single event, but at its simplest, it is a series
of pulses alternating between going up and going down with relatively
steep rise and fall time.
Remember, Lightening is not a single DC event, LIGHTENING IS AN "RF"
EVENT with the average stroke peaking "IIRC" around 1 MHz and tapering
off above and below that point.
The rapidly rising current in any conductor produces a rapidly
increasing magnetic field. This magnetic field builds a voltage of the
opposite polarity (reverse EMF), often reaching a point, where the
easiest path is no loner through that conductor. So, the lightening
leaves that conductor for an easier path. I've seen commercial towers
struck, where the lightening would jump to grounded guys and jump from
the guys across a rather large gap to ground rather than following the
already grounded guy. Hence the reason for multiple (earth) grounds.
The lightening stroke will divide itself between many possible paths.
Some will carry a lot of some of them, little, with the corresponding
voltages
Spend some time on Google, or other search engines reading about
lightening, what it is and how it works. Moving, opposing charges in
clouds and on the earth. Charges move around in the clouds and on the
earth. "Ground" is not just "Ground", or a single potential. It's an
interesting subject. Depending on your background in math and
electronics, you can spend many hours just studying, lightening!
Follow some of the links provided by Jim Brown K9YC
73
Roger (K8RI)
On 1/13/2016 Wednesday 7:42 PM, W3YY wrote:
The latest posts about grounding, and finally some free time here,
prompt me
to ask the following question.
Given lightning's desire to find the quickest way to ground, why
doesn't it
expend itself in a single 8ft ground rod at the base of a tower,
rather than
passing through another 250ft of transmission and control lines (also
buried
in the ground) leading to the shack? I would think that by then it
has had
plenty of opportunity to arc to ground itself.
I am not disagreeing with the experts on this subject, but I just don't
fully understand what is commonly recommended. With only a single 8ft
ground rod at the base of my 100ft and 120ft towers which are about
100ft
and 250ft from my house, I have only suffered two minor damages from a
lightning strike in over 40 years. And, I'm not sure that even had
anything
do with the towers, but was just an unrelated power line surge.
73, Bob - W3YY
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
Of EZ
Rhino
Sent: Wednesday, January 13, 2016 6:05 PM
To: Towertalk Reflector
Subject: Re: [TowerTalk] Grounds, 'remote' towers, 'house' power system
I'm not in disagreement with you Jim, but then why doesn't NEC
specify to do
things for lightning protection such as commonly followed by nearly all
commercial tower installations? Such as multiple ground rods, flat
strap,
star grounds, etc? (Think Polyphaser's docs). We know that one
ground rod
is woefully inadequate for a direct hit. If NEC is all about
lightning, why
doesn't is specify using more than one? It sure seems like NEC is
about the
bare minimum for AC protection and when it comes to RF and towers, it
doesn't really give much pertinent info at all.
Chris
KF7P
On Jan 13, 2016, at 15:49 , Jim Brown wrote:
On Wed,1/13/2016 2:35 PM, N3AE wrote:
The NEC is focused on electrical safety and not necessarily the most
effective system for lightning protection.
This is NOT true. The bonding required between your tower and power
system
sub-panel is for LIGHTNING protection.
In general, proper bonding is critical for lightning protection,
electrical
safety, fire safety, and to minimize hum, buzz, and RFI. Proper
bonding is
described in
http://k9yc.com/GroundingAndAudio.pdf
I'm not going to repeat it here for those too lazy to study it.
BTW -- I TAUGHT courses on Power and Grounding for about ten years.
73, Jim K9YC
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