wrote
I can understand why we want a short connection between the station
ground (the bonded chassis) and earth ground in the case of an RF
ground, especially for unbalanced antennas. But for a lightning
ground, I am not sure I understand why we want a short connection
between station ground and earth.
Because a large portion of the frequency spectrum of lightning is at RF.
Therefore the inductance of a conductor, not just it's dc-resistance, is
critical to maintain as low as possible. This is done by the shortest,
lowest impedance connection possible to the earth. The potential from all
bonded equipment in a shack and the potential at the grounding electrode of
the station ground should bever be more than 4' apart in elevation, and even
that is a stretch. 1' would be ideal!
Let's take the example of an operator with whose only choice is a
second story ham shack and an antenna mounted on a pole some distance
from the house. If a balanced antenna system is used (so we don't need
a good RF ground) and the coax has an arrestor placed at the service
entrance ground where the shield is connected (cold water pipe in
basement), why do we need a ground wire at all? (The coax would run
from the antenna to the basement level [where grounded] and then up to
the second floor.) The chassis' can be bonded together and the bond
point can be connected to earth ground through the coax shield. The
lightning is not going to travel up to the second floor to get the
radio when it has a low impedance path right at ground, is it?
-JJ
Good that you brought that up. A second story ham shack is the most
difficult, and almost an impossible location to safely provide a low
impedance ground for. Feedlines supplying a second-floor station should
never be allowed to enter from an above ground elevation. Think of inches of
elevation being equal to hundreds of volts each. A few feet above ground can
easily be 10,000v potential above ground. As long as the conditions you
described are followed, the feedlines have a safe path up to a second story,
wrt ground potential rise or attachment of lightning to a nearby antenna.
There are other issues that make the second floor difficult to protect, but
education can solve most of those problems. Lightning striking the nearby
antenna tower, telephone pole, tree in the yard, rooftoop, etc, can also
induce massive energy by electro-magnetic induction on house AC wiring,
phonelines and feedlines inside a structure. It won't matter if they were
grounded on the first floor if the energy in also induced at the second
floor level. Bonding, and a ground system, and adequate surge protection
control EMI from lightning. The problem that relying on a copper strap
connection to ground from 13' above ground is, it still allows a LOT of
potential when confronted with thousands of volts of potential from EMI.
"Wire" to ground from the second floor is useless at RF frequencies, and
even wide copper strap allows a dangerous level of potential to develop. But
it is better than nothing, because the electrical wiring on a second floor
is at a frightening potential wrt ground. If bonding and a lower impedance
path to ground is not provided, the prospect of at least minor damage is
possible from any nearby strike. Disconnecting the feedlines outside (only)
do not help prevent EMI damage at all. The wiring still carries it, and
feedlines still connected to the radios carry it as well. A Drake R8B I own
was originally crisped in a friend's second story shack, and it's feedlines
were disconnected outside. The entrance point of the damage? The RG-8X
feedline! Where did the lightning strike? 200' away on the tower. The
connection to AC ground (a horrible potential between earth and AC ground
exist on a second floor, remember?) was established via an AC powered sound
board that the Drake was connected to via speaker wire connection. My
friend thought disconnecting from main AC power and breaking the feedline
connection outside was enough to protect equipment. That's not always the
case.
73,
Jack Painter
Virginia Beach, Virginia
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