Transcript From the Ground Up

Mother Nature’s EMP
From the Ground Up
Grounding
• What is a ground?
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A return path for current
Path can be intended or
unintended
Also a point to which circuits may
be referenced
No guarantee that any two points
will stay at the same voltage
Grounds are the sewer system of
electronics used current, like used
water, still wants to flow
Power & Safety
50/60 Hz -- 10-1000 amps -- seconds or
minutes
Lightning
Lightning -- 1 MHz -- up to 100,000
amps - tens of milliseconds
Radio Frequencies
10 kHz and up -- µ to amps – continuous
Electrostatic Discharge
300 MHz -- 10 to 50 amps typical -- tens
of nanoseconds
Electromagnetic Interference
DC to daylight -- µamps to amps – nano
seconds to years
GROUNDED means connected to the earth.
Safety Grounds vs. Radio Frequency (RF) Grounds
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There are two discreet types of ground systems and two reasons why a Ham
might desire to provide a station ground
These are for the most part mutually exclusive! In other words one does one job
for which it is designed, and one does a different job.
Design your grounding system with this in mind, or one function may inhibit or
nullify the other!
Safety Ground. Reduce risk of electrocution or equipment damage by short
circuited power mains, or lightening strikes to or near the antenna or feed line –
a DC ground
A good engineering practice – make a better path to ground than the person or
equipment being protected
RF Ground. A system that implies low impedance and inductance
You upgrade and fire up your new rig in great anticipation – first report – poor
audio, you have RF in your audio. This is RF feedback
A wire longer than 10%of 1 wavelength begins to lose effectiveness as a radio
ground. This is about 24’ on the 80m, 12’ on 40m, and drops to 6’ on 20m! This
is why so many hams who have no feedback on 80m or 40m, but have problems
on 20m or 10m.
An RF ground is something that presents a low impedance at all frequencies of
interest on the desired ground surface
Remember an RF ground wire is just a short antenna! We want to make it
as LOUSY an antenna as possible!
Grounds in Ham Radio
• All familiar with 3-prong plug
• Primarily focuses on equipment fault, but also considers lightning
• Lightning – a constant discussion topic among hams…
Why?
A "constant current" source. Develops
whatever voltage is necessary to overcome the
resistance of the struck object.
The strike is DC, but current wave shape has a
very fast rise time to peak current flow. It is
this characteristic that creates havoc with
electronics.
"It ain't what people don't know that bothers me. What gives me the most trouble is what they
know that just ain't so". Will Rogers…
Lightning
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Strike - series of fast rise time "pulses" that become a constant current
source once ground path is established.
Typical pulse has a rise time of 2us to 90% of peak and a 10 to 40us decay
time (to the 50% level). Three pulses per event is the median.
Average current is about 18,000 amps for the first pulse, then dropping to
9,000 for the second and third pulses. Ten percent of all strikes will exceed
60,000 amps on the first pulse and one percent will exceed 120,000 amps.
A Tower (or boat)
• When lightning strikes, a tremendous rush of electrons move down the
tower and out into the ground/radial system. Since we are dealing with a
fast rise time event, the inductance of the tower and paralleled coaxial
cables become the primary factor in determining the amount of
instantaneous peak voltage developed between the strike point and the
bottom of the tower.
• Typical (18KA strike), - 135kV voltage drop between the top of the tower
and ground
Nothing can stop a lightning strike! How you direct the energy is the difference
Most damage occurs from induced voltages (static) – not direct hits
Protect What?
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Coax cables and other feed lines
Towers
Rotor control lines
Electrical Power
Data lines
Telephone Lines
All paths where undesirable currents can flow….
RF Entry Points
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Bring cables to ground level before
entering equipment area
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Ground coaxial shields to earth ground
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80% of an induced or direct lightning
blast comes in on the shield
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Use lightning arrestors on lines that
feed sensitive electronics
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A blocking-type arrestor - no DC
continuity
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Use a bulkhead grounding system
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Use bulkhead for lightning protection
as well as RF neutral for interference
filters and similar items.
External Ground System
• All equipment must be connected to a
single ground system.
• The sooner the ground system can spread
the energy, the better the chances of keeping
it out of your equipment.
• Single point ground connects to series of
ground rods all interconnected to disburse
energy. All rods must be interconnected…
Single Point Ground
• A fundamental of protection is the creation of
a single point ground within the shack. This
single point ground is used to mount all of the
protectors and to provide a ground for all of
the equipment chassis. This interior single
point ground connects to external ground
system with a low impedance copper strap.
• Each piece of equipment is connected to the
internal single point ground – bus bar – not
daisy-chained.
• This is also your RF Ground
• Prevents ground loops and reduced RF in the
shack
Recommended Practices #1
• Always use the shortest and largest surface area conductor possible. Wide
metal strap, or large diameter wire or tubing.
• Use several ground rods, and cover a large surface area.
• Wet soil improves soil conductivity, and reduces RF noise
• Keep ground wires less than ¼ λ on 10 Meters (468 ÷ 30) ÷ 2 = 7.9 ft.
• Lowest AC impedance possible - large surface area (fat) conductors
• Anti-oxidizing compounds such as No-Ox, available at electrical suppliers a must for dissimilar metals!
• Minimum RF ground system - at least three 8 foot or longer ground rods
• Ground coax shield at antenna and shack entry – reduces noise
• Periodically inspect ground connections – clean and reinstall
• Good ground systems are not pretty
• Avoid bends in ground wires – run straight as possible
• Use parallel wires to reduce impedance
Recommended Practices #2
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Bond all ground rods in the system together
The most cost effective spacing between rods for normal
soil is two times the length of a rod into the ground. If 8 foot
rods are used, they place on 16 foot centers
Connect all ground rods together - the utility ground rod for
the house and telephone ground
Joints between ground wire and copper clad rods should be
made by exothermic welds or by using joint compounds in
high compression clamps. Solder connections, even torched
silver solder connections will not last as long as the above.
An exothermic weld is created when a graphite mold around
the connection is filled with copper oxide and aluminum
powders. An additional starter powder ignites the
exothermic process. The molten copper is deposited into the
lower mold cavity where it burns away any oxides and
creates a large fused connection.
Rumor and Conjecture
Just a word to those who tell us that they are safe from lightning because they
always disconnect the coax from their equipment. When asked what they do
with the disconnected lines, they usually respond that it on the floor.
Now stop and think about the last few thousand feet that the lightning has
jumped, you can see the fallacy of their thinking. In fact, they made it worse
since arcing involves ignition temperature plasmas inside your house. True, the
radio may still work, if it survives the house fire.
Throwing the coax out the window is not a solution, especially if the coax has
already entered the house from the antenna or the antenna is roof mounted
without a ground path.
Grounding switches will not last long with direct hits unless other good ground
paths are provided.
Grounding the antenna line and not disconnecting the coax shield can still allow
strike energy to be shared with the equipment The shield connects to the chassis
and if a single point ground is not present with power/telephone protectors, the
equipment will be damaged.
Conclusion
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Ground Systems – Safety = Shock and lightning protection
RF – Low impedance at RF frequencies
Proper grounding will improve transmit and receive signals
Three main components
– Single Point Ground – Inside – all equipment
– Distribution Point – Just outside at entry point –
copper plate as mounting base for protection devices
for coax, control cables with direct connection to
ground rod linked to the rest of the system
– Rods and wiring – Size is important! < 15’ between
rods and bond all ground rods together
Inspect all connections periodically – not maintenance free
Bonding wires to rods – exothermic is best, strong
mechanical connection with anti-oxidizing compound is
second
Remember to have fun!