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Summer thunderstorms and grounding

Most (if not all) radio engineers cringe when they here a clap of thunder.  Then the waiting begins.  What are we waiting for?  The cellphone to start ringing, of course.  Over the twenty or so years I have been doing this, I have learned a few things.  One of them is you cannot over ground something.

That being said, you can, of course, ground something improperly.

The worst areas we have for lightening damage is the Gainesville/Pensacola markets.  Those places are in the lightning capital of the US.  Time was our class C FM station was getting knocked off a couple of times a month.

US thunderstorm Days map

US thunderstorm Days map

There is hope.  When we upgraded the stations and installed new transmitters in 2004 I insisted that the tower and building be properly grounded.  I even got into an argument with the CFO about the “mission creep” as he put it.  Never mind that I put $20K in the initial work specification for grounding.

There are a couple of strategies to use when dealing with lightning at transmitter sites:

  1. Grounding:  First, foremost and always.  Grounding should consist of multiple ground rods driven as deeply into the earth as possible.  At the Trenton Florida transmitter site we used 20 foot long ground rods driven in 20 feet apart all the way around the building and in five 60 foot spokes around the tower.  All of these ground rods and tower base were bonded with #2 solid copper wire CAD (exothermically) welded to the ground rods.  All turns were kept to a large diameter radius to keep inductance down.   When lightning strikes the tower, this creates a large electron sink to dissipate the strike energy into.
  2. Bonding:  All equipment cabinets, racks, and everything metal is bonded together and to the same ground point presented by the grounding system.  When lightning strikes, often the ground cannot dissipate the energy fast enough.  When this happens, the entire ground area around the tower gets charged up.  Current will only flow down a less resistive path.  If everything is bonded together, the potential between any piece of equipment or component is the same, even if that potential is +10,000 volts.  No flow of current means no damage.
  3. The transmitter building is located away from the tower.  Almost every FM and TV transmitter site I have visited, the building is right smack at the tower base.  By moving the building away about 100 feet or so, the EMP from the tower strike has dissipated (log function) significantly before it passes through the transmitter building.  It is a little more expensive to install due to the added transmission line lengths and losses, however it works.

I have been at the Trenton Florida transmitter site when lightning struck the tower.  The result, not even a transmitter overload.  Nothing noticed on the air, no damage sustained by any equipment.  For the last five years, there has been no off air time due to lightning damage at this site.

Studio building with lightning rod, Gainesville, Florida

Studio building with lightning rod, Gainesville, Florida

The studio site has a similar story.  We built a new studio building in 2005, there is a 100 foot monopole that holds the STL antennas.  You know that it gets hit during a storm.  I remember the manager and IT guy from Pensacola commenting about how nice the new SAS Rubicon consoles were.  Both of them also said that they wouldn’t last through the first summer because of lightning damage.  Four years later, not a single incident of damage to the consoles, computers or anything else in the building because we grounded everything as I described above.

Proper planning and installation pays off.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

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~Alan Weiner

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