Radio Grounding for Lightning Protection?
(The “electrical safety ground” and an “RF ground” are not effective as a
lightning ground.)
What are we dealing with?
• 50% -18,000+ amps
• 10% - 65,000+ amps
• 1% - 140,000+ amps
• Voltages easily exceeding 100 KV+
• Avg near 50 thunder-storms/yr MN-WI
• Often 2 or 3 strokes per strike
How does “lightning” strike?
• Charge in cloud ionizes area beneath
• “Leader” extends as air breaks down path
• Multiple leaders inter-connect approaching opposite polarity of earth
• As charge increases, “upward seeking leader” extends toward the approaching step
• When leaders connect, huge currents flow
• Polarity usually positive
“A lightning strike can really mess up your day”
………. K0KFC
Think AC• A lighting strike has predominant AC
components, hence inductance (L) and impedance (Z) are primary considerations.
• Main components are 1- 2 Mhz and higher.
Start with Single Point Ground Bar
• All metallic chassis direct connected to SPGB
• Includes transceivers, rotor controls, TV, telephone, racks, computer, everything
• Must have low inductance (L) paths
• Accomplishes common bonding
• Power cord 3rd wire “ground” of little value
Coax Protectors
• Each coax must have coaxial feed thru arrestor. • DC continuity vs DC blocked• Gas tubes? (single vs multi-electrode)• Protector placed as close to transceivers as
possible• Well grounded at SPGB• ¼ Wave Stub must be low Q to minimize
“ringing” (Usually very little effectiveness)
Rotor Cables, Tel lines, Relay control boxes
• MOV (Metal Oxide Varistors)
• Well grounded
• Clamp rotor lead during voltage rise
• Metal enclosure
• MOV must have short lead length
Establish Perimeter Ground Field
• Minimum 8 foot rod length (1/2” or 5/8”)
• Interconnect w/ #4 AWG minimum
• 2” copper strap desirable (min L)
• Rod spacing X2 its length (16’ min)
• Exothermic bonds most desirable
• Plan perimeter field w/duck foot radials
Ground Field (con’t)
• Interconnected ground rods become energy dissipation system
• How many ground rods?• Common bonds to electrical safety ground
and metallic water• Provide low L path to direct energy away
from the shack• How many ohms? XL? L? R?
TowersGuyed vs Self-Supporting?
• Properly grounded guy wires place added low L paths in parallel with tower L resulting in overall lower impedance
• Locate tower 20’ minimum from shack to minimize EM flashover
• Tower leg joints not a concern• Ufer ground is useful as a part of the overall
system (less effective when used alone)
Towers (con’t)
• Be aware of corrosion issues• All dis-similar connections isolated with
stainless steel hardware• Use joint compounds (protective coatings)• Ground ALL coax sheath at both top and
bottom of tower• Bring coax off at very bottom of tower to
minimize voltage divider effect
Towers, (con’t)
• Orient copper strap broadside to tower to minimize mutual coupling
• No sharp bends anywhere (>2”)
• Avoid scratching off galvanized coatings
AC Power Considerations
• Surge Protection (TVSS) at main entrance service panel
• TVSS should be physically closest to main breakers in panel
• Interconnect electrical ground with tower and perimeter ground fields
• Run power in grounded metallic conduit to shack to minimize inductive flashover
AC Power, (con’t)
• Surge protectors on all devices at the operating desk
• Minimize AC cord lengths (Cut to length)
• NO cheapie plastic (K-mart specials) power strips (fire hazzard)
• Power strip must have robust metal enclosure to minimize fire hazzards
TVSS
Two more observations…..
• The arc/flashover resulting from an un-controlled medium sized lightning strike is hotter than the surface of the sun.
• Never operate when you can hear thunder or see lightning flashes. (Grounding is intended to protect equipment, not operator.)
References/Questions?
• www.polyphaser.com
• Lightning Protection code NFPA 780
• www.harger.com
• www. arrl.org/tis/info/lightning.html
