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Grounding & RFI
My system was
designed to provide a high level of lightning protection - there are three
levels of defense to guard against let-through. It isn't
perfect, but does, however, follow good guidelines and shows measured
improvements in RFI reduction.
See Schematic.
See Materials
used and Sources.
Tower Ground Field
The system begins with a well grounded tower. Multiple 2/0
(double-ought) ground wires run from the tower to earth ground. The path
is short and ends at three, 5/8th inch diameter ground rods. These three
ground rods are wired together with 2/0 wire. And each of the three ground
rods has a pair of underground #4 bare copper radials. These six radials
are evenly spaced and emanate out and away from the tower and nearby
structure. Each of the six radials has four ground rods along the length
of the radial. The three ground rods at tower base, the radial
ground wires and their twenty four ground rods provide the dispersion field for
radiating electrical strike energy down the tower and away from the structure.
The rotator cable used is heavy gauge, and 100% shielded (metalized Mylar
wrap and drain wire). Shielded rotator cable helps keep RFI out of the
cable, and connecting the drain wire to ground everyplace the cable passes
through a box /arrestor helps reduce RF currents.
The coax selected is Bury Flex. This coax has low loss and a double
shield. It can also be directly buried. Burying coax further reduces
RFI.
Tower Box
(1st line of defense)
Each coax and rotor cable passes through the first set of protective devices
inside a waterproof box mounted on tower base. The box has multiple 2/0
ground wires that also connect to the three tower ground rods.
Entry Box
(2nd line of defense)
There is a second set of arrestors mounted inside a steel box attached to
the structure. The inside of the steel box has a heavy copper bus to which
all grounds connect. The box goes to earth ground using 2/0 wire to a
nearby ground rod. This ground rod connects back through 2/0 wire to the
tower ground rods. As you will see, the system has a single point ground
which starts in the radio room and daisy-chains out to the ground field.
Also inside the entry box are Line Isolators, whose job is to
"strip" electrical RFI currents flowing along the outer surface of the
coax shield and divert to ground. Measurements were made with and without
the line isolators, and insertion showed great reductions in currents. The
line isolators connect to the copper bus bar inside the steel box via the line
isolator's copper ground strap as well as connect to the steel box by way of the
coax connectors.
The rotator cable passes though an arrestor and the cable's shield connects
to ground bus inside the box.
Entry Panel
Attached to the inside of the structure is a thick aluminum plate. A
length of 2/0 wire connects this plate to the ground bus inside the entry
box. Each coax bulkhead connector is attached to the aluminum plate, and
the rotator cable shield is also connected.
The coax cables, rotator cable, and a length of 2/0 ground wire run from the
Entry panel to the Radio Room Panel.
Radio Room Panel
(3rd line of defense)
Mounted on a radio room wall is a plastic laminated panel, to which a third
set of arrestors are attached. A piece of aluminum angle stock provides a
ground bus as well as convenient mount for the arrestors, etc. The
aluminum stock serves as a mount as well as ground bus. It connects to the
outdoor ground field via 2/0 wire.
A second set of Line Isolators is included.
Single Point Ground
A No. 4 insulated ground wire connects the
radio room panel ground bus to the Nye Viking MB5A Tuner chassis. This
ground point is the single point for equipment grounds. The single point
ground connects via 2/0 wire all the way back outdoors to the ground field.
Additional Line Isolators
A line isolator is inserted between FT-1000MP
rig and Commander HF-2500 amplifier.
Another line isolator is in series with an I.C.E. 422
Low Pass Filter, inserted between the Commander HF-2500 amplifier and Nye
Viking MB5A Tuner.
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