Coax Cable Grounding

Each coax traversing your tower needs to be properly grounded to the tower.  The first point is at the top of the tower where the coax connects to the antenna.  The second point is where the coax leaves the tower to go to the radio equipment.  This take-off point should be as close to the base of the tower as physically possible.  The third point, for towers taller than 150 feet, is every 75 feet down the tower as measured from the top of the tower.

Depending on the height of the tower (inductance) and the severity of the lightning strike (current), the tower could easily have an instantaneous voltage difference between the top and the ground that exceeds 100kV.  In very simplified terms, if we view the tower as if it were a very long resistor with one end connected to ground and the coax take-off point as a tap point on that resistor; you may begin to appreciate the problem associated with allowing the coax to leave the tower at any point above the bottom.  For a 100-foot tower with a coax take-of point at the 10-foot level, approximately 10% of the tower energy (10kV in this example) will follow the coax to your equipment.  If we lower the connection to the one or two foot level then the energy flow will be appropriately lower.

The grounding of the coax to the tower should be accomplished using a commercial-type grounding kit followed by the application of a commercial grade waterproofing material.  Be careful of dissimilar metals when connecting the grounding kit pigtail to the tower.

Special Considerations

Numerous radio stations will fall in an area that makes it impractical or impossible to follow some or all of the advice given so far in this article.  For those stations there are some things that you can do to achieve a reasonable level of protection for your equipment.

First, establish a SPGP for your equipment.  To do this, go through the exercise of creating a box-level schematic of your radio station, identify and procure the appropriate protectors for all of the I/O connections to the radio station, and then mount them on a common conductive surface.  Where your installation varies from the ideal may be that the SPGP cannot be connected to an external ground. 

As a substitute for an external ground, locate an alternative conductive path.  Here are some recommendations for potential sources.  These are in the order of most to least desirable.

·        Building steel

·        Stand pipe

·        Cold water pipe

·        Metal building skin

·        Electrical system safety ground

Even though many of these ground choices are highly inductive and will not function as a good RF ground, the goal is to survive a lightning strike event by ensuring no current flows on the radio equipment I/O connectors.  This is achieved through the grounding techniques of the SPGP, which maintains an equal potential between chassis during the strike and is independent of how the SPGP is grounded.  So, for the small upper-floor radio stations, this will work just fine.

Operating Safety

No matter how good your installed lightning protection plan is, you cannot be in electrical contact with the radio equipment during a lightning strike event.  Although there is no current flowing between the radios in your radio room, all of the equipment will be statically elevated above ground.  If you are holding onto the microphone or the key during the strike event, YOU are now the path of least impedance between the protected and now elevated equipment chassis and the nearest new path to ground.  This ground path can be to the rebar in the concrete floor below your feet or to a nearby electrical wire or water pipe. 

Consider getting a storm warning device capable of sounding a warning when lightning activity is within ten-miles of your radio station.  When the alarm sounds, leave the radio room.  If your protection plan is installed correctly, you may leave the equipment connected and powered-on; but you must leave the room.

The commercial radio operators get away with operating during the lightning strike event because most of their equipment is remotely operated – just like a repeater.