GROUNDING
In Florida, it is claimed that the average lightning
strike is 70,000 Amps. 40,000 amps is the US
average.
It usually hits a high point – antenna – and comes
down the transmission line to find ground.
It does so generally as a ‘Common Mode’ event, as
opposed to a “Normal Mode’ event. IE, whole cable
conducts strike, not just the center conductor.
A good ground is 5 ohms. This results in a voltage
spike of 350,000 volts.
If the strike finds ground by going thru the radio, it
becomes a fuse, blows up, and your warranty is void.
The key to minimizing lightning damage is to prevent
the surge current from entering the shack and flowing
through the equipment by giving it a shorter, lower
impedance, path to ground.
HOW:
A good, low resistance and low reactance
(inductance), single point ground system.
All shack inputs and outputs (AC power, all RF,
telephone, internet, and cable TV) equipped with
TVSS (surge protectors) located close to and bonded
to the single point ground buss.
Good coupling to ground system with multiple
ground wires or flat copper strap. Do not use
stranded or braided wire outside due to corrosion and
IM generation (salts get into the wire – mini diodes).
Route with minimum bends and current ‘flow’ in
mind.
Do not ever disconnect or cut an existing ground
wire. Bond using an approved clamp such as a ‘split
bolt’. Soldering is allowed, but only in addition to a
proper mechanical connection.
A good low impedance ground rod system for the
shack consists of multiple rods 8 – 16 feet apart,
connected together, bonded externally to electrical
entrance ground, any copper plumbing, and telephone
and cable grounds. Use 8’copper clad steel ground
rods and bronze ‘acorn’ type clamps (or Cad Weld).
If you have a tower, a ground system for it should be
installed around the base.
Verify or install surge protection at the building
electrical power entrance panel suitable for rated
service ( normaly 200 Amps). Install wiring on load
side of larger breakers (AC, water heater, range,
dryer). Best done by an electrician for code
compliance and safety. Many codes require TVSS
device be installed inside panel for explosion
protection. This will depend on the device type(s).
Have the electrician verify a good, tight ground
connection at this time.
Ground outer conductor (shield) of all coaxial cables
as they leave tower/mast, install with ‘minimum
radius bend’. Rotor and other control wires should
be coiled at this point.
Route all cables to their protectors at the single point
ground buss.
Now you can ground your equipment to the ground
buss, but do it directly – but don’t ‘Daisy Chain’.
Here you can use stranded or braid wire. #12 is fine.
Most of this is ‘shunt’ protection, giving the lightning
surge current a shorter path to ground. An additional
aid is to provide series protection, a longer path for
the surge through the equipment.
This is done by forming coils in the cables to act as
chokes and/or to add ferrite chokes.
The goal is to lower the impedance to ground and to
raise it to the equipment.
A buried ground ring around the building is excellent,
but can be very difficult on existing structures. 8’
copper clad steel ground rods every 8-15 feet and at
the corners, #4 or better solid copper wire buried 6”
or more (never use stranded or braided wire outside).
Use quality clamps – no Zinc, steel, or ‘pot metal’
items. Avoid dissimilar metals outside.
Avoid any vertical ground loops exterior to the single
point ground buss. These can act as a transformer,
often conducting the surge into the shack. This
usually occurs when tower and building grounds are
bonded together. Most industry sources have NOT
recommended that the tower and building ground
systems be bonded together, as the ground resistance
dissipates energy. This view may now be changing,
Motorola R56 does bond them together.
Each installation will require its own design.
Existing buildings may require compromise.
The station ground buss should be solid copper for
the best service. Buy a large enough buss so it’s not
outgrown.
The Harger brand works well and is affordable.
(HRO, Georgia Copper)
Two #4 solid copper wires or strap equivalent should
be run from the ground buss to the outside ground
ring. This tends to cancel inductance.
Electro-Static Discharge dissipaters are not
recommended.
Multiple plug-in MOV type AC surge protectors are
recommended, minimum of one per branch circuit,
all around the house. These are inexpensive and fail
in the ‘shorted’ state, indicating the failure (breaker
trips and won’t reset). An appropriate AC surge
protector for the station feed should be mounted in
the shack, close to the station ground buss along with
the station disconnect.
Do not buy expensive surge protectors – more of the
inexpensive units spread around differ circuits will
perform better. (Home Depot or Lowes)
RF surge protection will be based on the frequency
bands and power levels used. For the VHF/UHF
there are a number of manufacturers such as
Polyphaser and Cellwave but for HF, the Alpha Delta
series seem to work better. Also MFJ units work
well. Ground connections must be short for low
inductance.
(HRO, AES, MFJ)
Rotor and other control lines can be protected with
12V MOVs or commercial control protectors, again
close to ground buss.
(Digikey – maybe a club order?)
There is a 90% rule to lightning protection – each
point in the process will divert 90% of the strike
current. Three steps will get it down to 70 amps and
that is sustainable.
Spend time planning and not on fancy protection
equipment.
As each station will be different, feel free to consult
with your club technical committee, Ernie AJ4BP,
Jody W4SLC electrical, Greg KB4VVE lightning
prevention.
gtb/s 4/15
Desired grounding logic
Entrance and ground buss solution –
all in one box for Amateur station.
Guy wire grounding
No
braid
outside!
Outside
buss.
Typically optimum grounding connections
Small indoor
ground buss (Harger type). Place on backboard for
mounting RF, AC power, telco/internet and control
surge protectors.
Proper connection to ground rod with ‘Acorn’ clamp.
Solid wire only outdoor.