Ionization Potentials on Conductive Elements and their Effect on Lightning Attraction.

Preface:
Part 1:This article originated as a reply to a ham's query on eHam regarding lightning's attraction to grounded objects being greater than to ungrounded objects. He suggests that ungrounded objects are less likely to be struck as evidenced by his observation of other ungrounded towers and in particular a large tin roofed barn which had no lightning protection or grounding. Since there's much more than just grounded vs. ungrounded being critical factors here I replied as follows.

Part 2: This followed as a reply to a personal email from a ham who asked about the behavior of round static balls versus lightning rods after reading Part 1. He suggested the former would seemingly offer less protection since it retains a charge better than pointed lightning rods. He is in part right, but that ability to retain rather than dissipate the charge through ionizing surrounding air is what provides the protection. This is clarified in my response.

That being said the following describes in greater detail what was lacking in my article about grounding relative to this specific topic of lightning protection and lightning's behavior. I have learned the obvious isn't always obvious.If anyone reading this should note any errors or mis-statements I would appreciate being corrected. You may email me at msed01@comcast.net

Part 1

First, as stated by another respondent on eHam, law requires grounding of antenna structures as well as the cables upon entering building structures. The National Electric Code as well as the National Fire Prevention Association guidelines required that antenna structures and their entrance cables comply with rigid grounding policies. Your home owner's insurance will definitely not cover any damage if you are not in compliance. Many won't cover any such appurtenant (attached to the house) structure without prior approval. If not attached to the house you usually need a separate rider, also requiring some sort of approval. At least if you are (were) grounded properly the approval for damage coverage after the fact may be a plausible claim for relief by de facto inspection by a knowledgeable adjuster. I'm no insurance lawyer but I do know the NEC and their ain't no getting around grounding!

But let's assume for academic purposes grounding IS optional. Here's some interesting factoids on lightning's behavior. Lightning seems to observe two rules. One, it seeks to conduct to the greatest nearby* potential. 2) It never strikes within a 60 degree "cone of protection" created by a nearby* taller object even if greater potentials exist to attract it.

*Nearby generally means within a 60 degree angle from a point of origin. A fork in a lightning strike can be considered a point of origin. These rules do not hold after the strike whereby lightning can travel as current or high potential plasma ("ball lightning") elsewhere after the fact.

Now here are some things that have been observed that really lend themselves to this question. Dielectrics, such as fiberglass antennas and insulated structures, have a tendency in a storm to build up a significant charge on there surface. This charge makes them much more attractive to lightning than grounded objects. Even the 80 meter whip on my Jeep sizzles in a lightning storm unless I ground the vehicle. I've had arcs jump from my wheels rims to earth! Needless to say, an ungrounded tower will develop a similar charge. This same principle applies to trees with heavy foliage. The foliage, keeping the trunk and surrounding ground dry, will create a similar situation, particularly in the fall when the tree starts to shut down its circulatory system. You'll rarely see a very wet tree get struck if there's a dry one nearby.

This also holds true for plains of grasses. Within that sixty degree zone over a plain something's gonna get zapped. What gets zapped first? The tall dry stuff with wet tops at high potential. The green wet stuff, even if taller is safer cause they're nearer ground potential. Mariners, particularly those with tall masts, have long known of such phenomenon -- even giving it the name of "St. Elmo's Fire" where ungrounded metal railing and fittings on ships would develop an eerie blue corona around them. The subsequent correlation with lightning strikes now teaches us that all deck hardware should be grounded to the gear in the water whenever possible.

So all in all, the evidence of these observations would likely make your ungrounded tower far more likely to be struck than not.
But there are still other considerations to be digested. Sharp edges and points will readily produce coronal discharges at far lower potentials than smooth round surfaces. Which is why flagpoles have balls mounted atop and lightning rods come to a point. This is where the initiating tendrils for lightning strikes are born. What's the likelihood that you have some very small radii, if not sharp points, on an antenna installation? Pretty high I'd imagine. When I put up my aluminum and steel "Christmas Tree" I tried very hard to smooth all my edges and eliminate those sharp hose clamps and screw tops. I still saw some corona and heard sizzle in very damp weather on my vertical. On that same vertical I've even seen my disconnected PL-259's arc from center to shield in the gallon pickle jar I'd leave them all in during a lightning storm. It would start as a slow sizzle and grow until it culminated in a feisty SNAP! Like I said, that was on an UNGROUNDED vertical that thereafter was shorted during a storm.

Even if you don't have a 75-foot tower and any ungrounded whips you all can try this neat experiment. During the next rainstorm, or certainly a lightning storm, take a neon pilot lamp and place it across your disconnected coax. You can sit there and watch it blink as the charge reaches the lamp's ionization potential and discharges over and over again. Sometimes it charges so rapidly it just looks steady unless you move your eyes around to see the strobe rate. It's pretty cool! A SK showed me that one, a real old timer who knew more practical stuff than most engineers will ever see, but that's another issue.

And finally, do you know what happens when lightning finds earth through a "non-conducting" medium, like concrete? It explodes, like a sizable bundle of dynamite. I had a co-worker killed by a brick that blew off a building near a bus stop from this. Why? Because the buildings roof flashing WASN"T GROUNDED! The lightning chose the damp brickwork to find earth. Bricks were found over 500 feet away, about a good wheelbarrows worth. (For those interested you can probably look this up. It occurred in Staten Island, NY on Clove Road and Howard Avenues in the late 80s or early 90s. I'm terrible with dates.) It has been stated that reinforced concrete provides and excellent low resistance path for grounding. Indeed it may be the case that the moist concrete contains ample conductive ions to exhibit a low resistance. However, this relies upon the water in the conrete which will rapidly be vaporized and explode under the excessive currents present in a direct lightning strike. Perhaps a very large slab of concrete in an industrial site (Oil rigs, substations, etc) can tolerate this surge but certainly not a tower base of a few cubic yards at best. Tower manufacturers generally tell you to permit the tower legs to extend through the footing and NEVER use the re-bar for grounding for this reason.

As for that tin roof being safe I can only speculate that the roof didn't have sufficient potential over such a large surface to initiate a tendril or if it was raining (he didn't say) it was "grounded" by the water flow. But I wouldn't use that anecdotal of an observation to preclude avoiding the good (and legal) practice of grounding my tower.

So, do you still think that leaving a tower ungrounded is a good idea? I certainly hope not. My advice obviously is to ground the tower. Even if you do have a strike at least you won't likely lose your house or tower to fire and explosion. And if you do the right thing you may even be able to recoup some loss from insurance that would otherwise be out of the question.

I've taken this time and lengthy text not to be a smart-ass, but to substantiate my opinion with as much information as possible. For those who have read my posts before you know I am generally wordy. But in as serious (possibly life and death) a matter as this I feel no economy of words is warranted. I hope you see the reasoning I've tried to convey and follow accepted practice and enjoy a long and successful endeavor in Amateur Radio.

Part 2

Following my preceding response a ham asked me, "You discussed that a flag staff has a round end (I presume your argument is that it somehow makes it less likely to attract lightening), and that a lightening rod has a pointed end. Seems that potential builds easier on a round surface. Confused." Part 2 addresses this misperception.

I opted to avoid the physics involved in my preceding reply. It's been my experience that most hams know of this phenomenon even if they don't know why. But here's why.

To explain this I'll used some idealized (easily conceptualized) physical shapes. When a surface charge is present on any shape it seeks to distribute it's charges (actual free electrons, - charges, or lack of electrons, "+ charges") evenly about the surface. In the case of a flat plane, or a section of a large sphere this would have charges evenly distributed and a net potential per unit area equally uniform. Potential is equivalent to # of charges per unit area of surface.

Let's now make this surface a cube. At the edges of the cube where two planes meet there will exist a thin edge where charges from both planes meet. This creates a concentration of charges at the edge and net higher potential. The edge, depending on it's sharpness can be considered almost as a dimensionless line where the charges from both planes meet. If we give the edge dimension, by smoothing or rounding the edge, the charges from each plane have a radius to bend around, actually an area to spread over, and will not aggregate as tightly, resulting in a lower potential. But depending on the radius it will still be higher than on the plane's surface, until we round it to a sphere.

We use a sphere on flagpoles since the sphere is the best shape we have available to distribute charge as we cannot place an infinite plane on top of a flagpole. But the larger the sphere the lower the potential per unit area given the same number of charges.

In the case of precipitation charge in storms we know that charges accumulate individually. Unlike normal current flow where the potential determines the volume of charge flow in a conductor, here the number of static (non-flowing) charges that accumulate determines a pure potential. Like shuffling your sox across the carpet you accumulate a set number of charges per shuffle. A larger person would have a net lower potential over his body than a small person (given the same sox, carpet, foot area, etc). But when you touch the light switch the current flow is equivalent to the number of charges on your surface regardless of what size area they are distributed over.
But I digress... back to our shapes.
If we consider the sharp corner of the cube we have three planes adding charge to a dimensionless point and subsequently still higher potential.

Now let's take this to an extreme case of a thin needle, or even a less than thin lightning rod, which comes to a sharp point. Here, we have a minimum surface area, the thinnest of needles would approach the theoretical dimensionless line. If we took our charged sphere and squeezed it into a thin needle we would have an extremely high potential produced by the high concentration of charges on a minimum surface area. At the very point where the dimension truly approaches zero the potentials can be incredibly higher than the rest of the surface.

So that's basically how surface charges accumulate and behave. Of course this is idealized and in a vacuum. In the real world we have the insulating/conducting properties of air and whatever it may contain, moisture, various gasses, etc. These tend to provide places for the charges to bleed off and equalize with the rest of its environment (coronal discharge). The worst case of course being a lightning strike through a fully ionized path in the atmosphere. The tendrils provide the ionized path which the lightning then follows like a wire.

In the case of a sharp lightning rod, or small antenna tip, the charge concentration readily exceeds the breakdown voltage of the surrounding air and we have corona. (I love it when my mobile antenna sizzles in humid weather. The blue cone at the tip is very impressive). But the undesired effect obviously is the increased probability that a tendril with form resulting in a lightning strike.

The reason barns and tall buildings have sharp lightning rods, which are heavily grounded, are to prevent lightning from striking other things which may ignite or explode. This is the safest way to protect valuable structures and their contents. In Manhattan where I have done some repeater work on tall buildings we often mount antennae near and higher than lightning rods. As long as we locate the antenna so its highest point does not include the lightning rod within it's 60 degree cone of protection the antenna is pretty safe and the lightning rod will almost always be struck first, even though it's much lower. The corona and subsequent tendril will draw the strike much more readily than the smooth tipped antenna. Seeing is believing. My club thought I was crazy when I mounted four lightning rods on the corners of the building when we moved our repeater. Four repeaters were hit in the area that year, ours wasn't one of them. Our controller did go stupid a few times from the strikes, but a phone call to a hard reset circuit fixed that in no time. Seeing is believing.

Before I conclude I'd like to give you one example where you can readily see this surface charge phenomenon first hand. Next time you have the opportunity to work on your car, lawnmower, chainsaw, etc... Remove the spark plug so you can watch if fire. You'll note that the center electrode is a cylinder that terminates in a clean circular edge. The sparks will ALWAYS emanate from this edge for exactly the reason I described above. A very old plug will have a rounded electrode and will require higher voltages to produce the arc, sometimes high enough to cause the contaminated ceramic insulator to break down first, and hence, no ignition. Filing an edge back on a plug with a thin diamond nail file can extend the life of a plug considerably. I have often gotten my two stroke dirt bike back home that way in the days before clean TCW-3 oil and income for new plugs :).

One last thing regarding flagpoles; on cheap flagpoles you'll often see a thin crimped seam extending 3/16" to 1/4" around the "equator" of the static ball. Care to guess what effect that has on its ability to prevent corona? You guessed it. Good poles have smooth surfaced, and even highly polished, balls on top. Some even go so far as to use non-corrosive gold coatings. At a microscopic level even the sharp grains of corroded oxides can contribute to irregular surface charges. Something to think about when putting your antennas together ehh?

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