Lighting takes the form of a pulse which typically has a 2 u
second rise time and a 10- 40 u second decay to the 50% level.
The peak current will average 18,000 amps for the first impulse
and less than that for the second and third pulses. Three
strokes is the average per strike. The number of visible flashes
is not necessarily the number of electrical strokes or impulses.
A strike is a constant current source. Once ionization occurs,
the air becomes a conductive plasma reaching 60,000 degrees F.
and becomes visible. The resistance of a struck object is of
small consequence except for the power dissipation on that
object.
1
One lightning bolt and one 65-foot sycamore tree
make a convincing argument against taking refuge
under branches during a thunderstorm. A remarkable
detail in the photograph is a pair of upward
discharges; one from atop the sycamore to the left of
the main bolt and the other reaching from the
television antenna of the farmhouse at left. Such
discharges only occur in the area of a downward
stroke. Most trees survive direct hits with little
damage as the current passes over the surface to the
ground.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
2
This slide illustrates the numerous return strokes that to
the human eye appear as a single flash. A still camera
panned from left to right, with the shutter open reveals the
first stroke of a flash to the right, with its characteristic
branches, and some 20 subsequent strokes through the
same air channel. Such repeat surges make lightning seen
to flicker.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
3
In this slide, lightning is seen occurring along with rain.
It appears that lightning does more frequently occur in
the presence of rain. However, lightning has been
observed on Venus and Jupiter, both of which have very
little water.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
4
Valcanoes can create lightning with no storm in sight.
Friction from swirling ash particles generates electric
discharges.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
5
Reaching toward thunderclouds, skyscrapers can initiate
lightning. Branches streaking upward from an antenna
atop New York’s World Trade Center typify ground-tocloud lightning, common from tall buildings and
mountaintops.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
6
Golfers are prime targets for lightning --- they tend to
either stand in open grassy areas or huddle under trees.
A scored pattern on the fifth green at Phalen Park Golf
Couse in St. Paul Minnesota defined ground zero where
four golfers were injured --- one fatally -- by a June 1991
strike.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
7
An airliner’s lights trace a path around an intense summer
thunderstorm, a routine maneuver for pilots on approach
to Tucson International Airport. On the average,
commercial jets are hit by lightning once a year and suffer
only slight damage where the current enters and exits.
This photo scanned from NATIONAL GEOGRAPHIC
magazine July 1993.
8
Fifty percent of all strikes will have a first strike of 18,000
amps , ten percent will exceed 60,000 amps and only one
percent will have over 120,000 amps. Largest strike
measured was about 400,000 amps.
9
For a typical 100 foot grounded tower, there will be a 230,000
volt top to bottom potential difference.
About the only truly safe way to prevent damage to your
antenna and radio equipment is
1) DON'T put up your antenna
2) If your gear is out of it's box, put it back.
However, hams being the kind not to listen to good advice,
will try to put up the tallest, longest, ungrounded antenna
and will invariably forget to disconnect it and the AC mains.
Since there is no perfect ground, we will try to produce the
ground with the lowest resistance and inductance.
10
Unfortunately this protection is to help the antenna survive
and not your equipment. A direct or near by hit will cause
the antenna to ring whether it is grounded or not. Only a
grounded antenna can take a direct hit.
Both “on frequency” ringing and other frequencies will be
present and will propagate down the transmission line
towards your equipment.
11
Since although the best soil conductivity is not too difficult
to determine, it is rarely done properly. Therefore, the worst
case soil conductivity will be assumed and the best ground
possible will be attempted.
Ground rods or pipe that do not extend down below the soil
frost line may be a poor ground.
12
A good ground will produce a resistance to ground of 10-50
ohms. Usually, one ground rod is not enough for the
attempted 10- 50 Ohms resistance .
13
A one inch diameter rod driven 1 meter into the ground
(with 1000 Ohm/meter soil) would yield a ground
resistance of 765 Ohms.
A rod 2 meters long would yield a resistance of 437 Ohms.
A three meter rod would give a resistance of 309 Ohms.
14
Ground resistance is more effectively reduced by using
multiple ground rods than by increasing the length of a
single rod.
15
Three one meter rods, with the distance between them at
least equal to their length, reduce the ground resistance to
230 Ohms. If the wire connecting the three rods is also
buried just below the soil surface, then the ground resistance
may be further reduced to 200 Ohms.
16
The resistance of our 3-rod ground system can further
be reduced by adding 3 #2 gauge wire radials each
between 50 and 75 feet long buried just beneath the
surface. This will yield 30 Ohms ground resistance.
Eight ground rods placed at 16 foot intervals along the
radials, will produce a ground resistance of about 13
Ohms.
17
The ultimate ground would consist of 3 eight foot, 5/8"
diameter rods (the tops of which are 18" below the
surface) connected to 50 meters of #10 wire below the
surface with eight foot rods every 5 meters and at the end.
This would produce a ground resistance of about 4 Ohms
in 1000 Ohm/Meter soil.
18
Solid copper wire , copper strap or steel tubing may be
used as ground connectors. The primary purpose of
copper clad (copper on steel rod) , known as copperweld
is to reduce the chance of rust or corrosion which
drastically increases inductance.
19
In this slide, you can see that each tower leg has a
large solid ground wire and a 2” wide copper strap
connected to it by a transition clamp. Each wire and
strap runs to the edge of the tower base and into the
ground where it is connected to an 8 foot ground rod
at 6, 12 and 18 feet from the tower leg. Also notice
that the exposed wire and strap have a gentle curve
(8" radius).
20
This is a front view of the same area. Notice that
there is a solid copper ground wire and 2” wide
copper strap leading from the tower base ground
point into the wall. This ground wire and strap goes
to the common point ground system inside.
21
Dissimilar metals should not be placed in contact with each
other as part of the grounding path, especially copper and
galvanized steel. The copper-galvanized steel connection
will soon form a copper-zinc battery and the connection will
last only a day of so. A transition clamp of brass, bronze or
stainless steel can be used between copper and galvanized
steel.
22
Chemicals should not be used to increase the ground
resistance because they can cause corrosion. The process
of cadwelding or silver soldiering will produce the best
above or below ground connection.
23
Now that we have our 10-50 Ohm ground, it is extremely
important to make connections to the tower, equipment
and guy wires with the lowest inductance connection
possible.
Tower ground connections should be made as close to
the bottom of the tower as possible using a low
inductance connection. Coax runs coming down the
tower should have their shields grounded as close to the
"ground-tower" connection as possible.
24
This slide shows more of the same tower base. Note
the coax grounds which go to the tower ground.
25
Any coax or ground wire bends should have no sharper
than an eight inch radius. The inductance of the selfsupporting or guyed tower and any parallel coax runs can
be calculated.
26
This slide shows the gentle curve of each of the
coax cables and ground wire connections.
27
In order for the guy wire to effectively reduce the total
tower inductance, the individual guy line inductance
must be well grounded and in turn connected back to the
tower base ground.
28
This slide shows the connections between the guy
wires and the ground wire coming up from its ground
rod at each of the guy anchor locations. In this slide,
you will see that the galvanized ground wire is used
to connect each guy wire to the ground rod by a
transition clamp...in this case aluminium.
29
This slide shows the proper technique (according to
the Rohn people) for securing the guy wire to the
turnbuckles.
30
This slide shows the proper technique for locking the
turnbuckles in place so that vibrations don’t loosen the
guy wires. Even if the turn buckles do come disconnected,
the interconnecting loop of guy wire will at least keep the
tower from falling.
31
This drawing illustrates the method used to properly ground
the guy wire at the lower end. Note that several ground rods
are used and that each is connected together and back to the
guy wire.
32
With a set of guy wires, the total inductance is
decreased from 28 uH to 12 uH. With a multiple
leg inductor, each part of the vertical inductor
will share the current and therefore the voltage
differential. Coax should be connected to the
top of the tower and at or near the bottom of the
tower.
33
The best equipment ground is achieved through the use of
a single point bulkhead ground system.
34
In this slide, you can see the 2 inch copper strap
coming into the equipment cabinet. Short straps are
used to connect each piece of equipment with the
ground strap which runs from the top of the
equipment cabinet to the bottom. It is connected to
the cabinet at both the top and bottom.
Also, in this slide, you can see the coax cable
connector grounded to the equipment ground.
35
36
View of common point ground system. In this
slide you can see the power line and telephone
line inductor. Each is 100 feet of wire which
serves as a surge inductor. Be sure to use large
enough wire for the power line inductor so that
there is no voltage drop when you transmit.
37
View of common point ground system. Notice
that the phone line and power line are
connected to junction boxes. Each end of the
phone line and power line inductors have MOVs
connected from each side of the line to ground.
38
39
40
41
42
43
44
Disconnect and ground all antennas and remove equipment
from power mains.
Since the tower and all antennas are at or near ground
potential, the cloud charge is gradually reduced. This
prevents the cloud-to-ground buildup of large potential
differences.
45
46