Lightning

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Lightning Strikes

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The following are popular false myths....

• By grounding  feedlines and bleeding off the static buildup we are helping reduce chances of a lightning strike
• We can get rid of charges with static dissipators, and reduce the chance of lightning hits

Where is the charge piling up and how can we equalize or reduce the pile?

The charge difference is between different areas of clouds, and between those clouds and the entire earth and anything near or on the earth. The real problem is the piling up of electrical charges in the droplets (or even dust particles in bad dust storms) collected in one area or another of "clouds". If we wanted to reduce the charge gradient, we would have to create a conductive path capable of allowing steady charge movement between the areas with different charge, so charge difference could equalize. It is quite possible to reduce charge gradient between an airplane and the surrounding atmosphere by attaching wire brushes or whiskers to protruding parts of the airplane. As the perfectly-insulated airplane travels directly through differently charged areas, charges can easily migrate into or out of static dissipators, bringing the airplane to the same potential as the area it is flying through. This is much the same as a conductive strap on a well-insulated motor vehicle contacting the surface of a road, preventing tire friction from charging the insulated vehicle to a different charge potential in relationship to the road (and earth).

It's my firm belief, based on both reasonable logic as well as several reports and studies, that grounding an antenna (or adding metallic whiskers or metallic "porcupine balls") does nothing to reduce charge gradient that causes lightning. Moving charges between the huge volume of the earth through a tower into the air immediately surrounding the tower and earth is meaningless, because the real charge gradient is between clouds thousands of feet away and the entire earth. Unless the discharge contacts or forms a path connecting differently charged areas, the ionized area does nothing at all. The most it offers is a wider blunter area at the location of the dissipater. While this might slightly increase the the voltage gradient of the area immediately around the dissipater to the sky, it does not alter the charge gradient or voltage between the cloud and tower.

Everything I have read detailing successful deployment of lightning charge dissipators has been anecdotal at best. In every case there are several alternative explanations that have been ignored. For example, several people have told me beacon or lightning system damage has been reduced by addition of a mast and dissipators around a beacon light. On the other hand investigative reports that collect data, including data from NASA, find lightning strikes occur at similar rates with or without dissipators.

In my own systems, I have a commercial metal antennas or metal masts mounted above my beacon lights. Looking at my Rohn 65G to the left, you can see almost 15-feet of antenna above the beacon light. The "thin" black line going horizontally to the right is a 160-meter dipole mounted at about 310 feet. The more vertical lines are ropes, and the fiberglass upper guylines are clearly visible.

Prior to the installation of the upper VHF antenna, with only a commonly-used short spike sticking a few feet above the beacon light, I replaced several MOV's and tower flashing modules in the tower lights.

After installing the antenna protruding 15 feet above the beacon light, and bonded to the tower below the beacon light, this tower has taken hundreds of strikes without damage to any electronics.

The taller low-impedance conductor bonded to the tower below and away from beacon wiring reduces damage to things lower on the tower. Lightning current is harmlessly routed around the beacon, rather than flowing through a thin lightning rod mounted immediately next to the beacon and grounded to the beacon mounting plate. This antenna produces the very same improvement credited to fancy expensive whiskers, without the need for false tales about "charge dissipation" or "lightning mitigation" or "charge equalization".

A second effect of adding the mast above the beacon was reduction in damage to my 160-meter  dipole. Without the tall antenna mast at the top, the coax in the 160 dipole's balun would occasionally melt during a strike. After installing the tall mast, there have been no balun failures.

Had I installed lightning dissipators, I would have probably credited these improvements to mitigation or reduction of strikes. Unless I was watching the tower throughout a storm, the only way I would know if the tower was hit would be by observing damage to equipment on the tower after the storm passed.

Reducing how frequently Lightning Strikes

The problem we face is the small cloud mass far away from our massive earth is charged more and more as a storm progresses. The cloud either has to stop charging before it reaches a voltage breakdown point, or there must be a direct path that allows it to equalize charges without doing damage. Nature eventually takes care of this. When the charge gradient between the cloud (the source of the potential) and the earth (just a big charge sink or reservoir) becomes large enough, a streamer forms and paves the path for full blown lightning bolt.

Connecting an antenna to earth does nothing at all to reduce the likelihood of a strike. The antenna is already at earth potential, the real problem is the huge potential difference between the cloud and earth. The tower is simply a protrusion that lowers the breakdown voltage between the cloud and earth. As a matter of fact, grounding if anything only makes the problem ever so insignificantly worse. A grounded antenna is solidly clamped at earth potential, instead of being ever so slightly closer to cloud potential like an insulated or electrically isolated antenna could be. In the large scheme of things, none of this affects the likelihood of a strike. What big improvement would come from several thousand volts of change when compared to millions of volts of potential difference? The only significant change, if we want to reduce direct hits, is by reducing structure height.

A second (but less effective) way to reduce how frequently a target is stuck is to create a very wide blunt target. Having a blunt target will reduce the electric field density appearing at one concentrated point. This is the same effect that causes a wider gap in a spark plug, or a blunt smooth tip in a spark plug, to greatly increase gap voltage breakdown. Grounding the shell of the spark plug better does not help increase gap voltage breakdown, and neither does putting sharp whiskers on the electrode tip!

Other than reducing tower height until it is well below the height of surrounding objects, there really is only one reliable course we can take to reduce damage risks. We can provide a low impedance path to a wide area of earth, routing lightning current around things that can be easily damaged. Installing towers a reasonable distance from buildings is a good idea, as are perimeter grounds and proper cable entrances. I've seen some terrible structural damage, and even one fire, caused by terminating guylines or tower support brackets into building walls. Steel guylines should be terminated at earth anchors that are reasonably well away from buildings or building walls. If that isn't possible, use fiberglass guylines or install low-impedance earthing systems that are tied into a building perimeter ground. Bracketed towers need good grounds at the tower base, and house brackets should not be near large metallic objects in the house. The last thing we want is a tower bracket arcing through a dry wooden attic surface to a metal duct or attic electrical wiring.

Anyone who thinks a few six-foot or 60-foot deep ground rods can dissipate hundreds or thousands of amperes at frequencies from near dc up to radio frequencies with negligible impedance probably should spend time rethinking the frequency spectrum of lightning. Unless there is a very large highly conductive surface area, like a radial field or ground mat, it is almost impossible to even spread or dissipate strike current. With small area grounds, no matter how deep they are or how good they are at dc or 60 Hz, there will be a huge voltage rise between different points around the strike. We have to do our best to have everything in the protected area rise in voltage at the same same rate. Keeping charge levels of different things rising and falling at the same rate is the primary reason we must bond the utility entrance ground to the radio shack entrance ground. It is the reason a protected area requires a low impedance perimeter ground buss encircling the entire protected area.

We can't make problems go away or reduce the odds of a strike noticeable amounts by grounding or snake oil cures like static dissipators. We can't discharge the clouds intentionally. We just have to deal with what happens during a strike.

See Lightning Strikes

grounding