Has anyone had experience, or knows from first principles, whether installing a G5RV or dipole BELOW a "lightning-rod" in such a manner would serve as a means of discouraging direct lightning strikes onto your dipole:

  • A "lightning rod" is some distance above the highest point of the G5RV, and has a ground wire that avoids coming near to the dipole.

  • Such that the "lightning rod" is connected to a ground wire which goes directly to its own ground-rod which has been attached to its own dedicated ground-spike.

  • With some hope that lightning, should it strike above your QTH, would hit the "rod" above the G5RV and most of the current would travel down the ground wire.

  • The lightning rod is about 5 feet above the dipole, and is supported from below by fiberglass pole.

  • There are no other antenna towers or tall metal objects within 100 feet of the above.

  • The antenna wire and antenna coax do not come within 4 feet of the ground wire, the air gap is intended to discourage lightning jumping from the lightning rod to the antenna. The ground wire is installed with a sloping section to avoid coming near to the G5RV or dipole.

Is the above a reasonable thing? Is it worth doing? Assume that the G5RV or dipole has a lightning arrestor attached to its coax feed line in the usual manner, and that this "lightning rod" is a secondary protective feature.

Here's a sample scenario for the sake of criticism/critique:

enter image description here

  • I have read that there is a "cone effect" for lightning protection, which really is a mathematical way of expressing a path of least-resistance, given that air-resistance is much greater than the resistance of objects like lightning rods, wet or dry trees, buildings nearby, etc. – Warren VE3WPX Jul 30 '14 at 16:19
up vote 2 down vote accepted

I suspect this scheme will do little or nothing to decrease the chance of damage to your equipment in the case of a lighting strike.

For the sake of argument, let's just presume that it does work as you expect: lighting always strikes the rod, which has a dedicated conductor to a dedicated ground rod, and all of that is sufficiently isolated from everything else that no arcing occurs. Your situation is something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

We've made a bunch of assumptions but you still have a problem. Lighting happens when the charge imbalance between Earth and a cloud becomes so great that the voltage difference is enough to ionize the air and establish a conductive channel. Electric charge then travels over this channel to equalize the charge imbalance. That electric charge doesn't just need to get to the ground: it needs to spread out over the whole Earth. If it didn't, then there would be a spot on the ground where the lighting struck that would have a severe excess or lack of charged particles. It's like dumping a huge bucket into a pool: the water spreads out over the entire pool to minimize the gravitational potential of the water.

Now, Earth is anything but a great electrical conductor. That's why we need radials for ground plane antennas if we don't want them to be lossy. The impedance of the Earth between your connections to ground is represented by R2, R3, and R4.

Now, you have many kiloamperes of current flowing through an Earth that is a little bit resistive. Do you see the problem? Do you think most of that electric charge will go through R3, or your radio?

So, one important aspect of effective lighting protection: have exactly one ground. You can have multiple ground rods, but you should tie them together with a low impedance path, but any equipment you want to protect should have exactly one connection to this system of grounds. Remember to consider non-obvious paths through your electrical outlet, feedline, Ethernet cable, etc.

Also, those assumptions we made are probably not good ones. Having a lighting rod up high reduces, but does not eliminate the odds of a strike on things below it, and it's very likely lighting will arc to anything and everything even many feet away from your lighting rod system.

I suggest you check out How can I protect equipment against a lightning strike? for other important aspects of effective lightning protection.

  • Ground loops will also form in "Power Strips", from the magnetic pulse. A Faraday shield worked for Tesla, but he didn't deal with real lightning. A good post Phil. – Optionparty Jul 31 '14 at 13:16
  • Wow. I love this answer because of the circuit diagram. Thanks. – Warren VE3WPX Aug 1 '14 at 0:35
  • A single ground connection that has multiple closely-spaced copper ground spikes would be better than a single copper spike in the ground, right? – Warren VE3WPX Aug 1 '14 at 0:36
  • @WarrenVE3WPX the multiple ground rods aren't as much of a problem as it is that the radio is connected between two of them, and shorts R3. Pick just one point and run all your grounds (feedline, electrical outlet, network/speaker cables, everything to that one point. No current can use your equipment to cut around an impedance (R3 here) if there is only one way in or out. Of course, you must make sure that lightning doesn't make a path by arcing, which is where bonding the grounds together comes in play, but if you are going to do only one thing, eliminate the loop. – Phil Frost - W8II Aug 1 '14 at 1:58
  • 1
    @WarrenVE3WPX Also, if you want a better connection to Earth, then spacing your ground rods closer together doesn't yield much improvement. You need to contact a large area of Earth to be able to effectively move the electric charge of a strike. Radial fields work great for this purpose, and if you do ground rods, the rule of thumb is to put them as far apart as they are long (8 or 10 feet, usually), then connect them together with 2" copper strap that's brazed or welded to the rods and buried. – Phil Frost - W8II Aug 1 '14 at 12:59

That will not help enough. Lightning spreads out as it gets near to the ground. It also doesn't really conduct along the ground rod, it can jump back and forth to other objects. So you would get a lot of energy into the antenna. It is no problem for lightning to jump five feet. It has already jumped all the way from the clouds!

My neighbor growing up was a ham and worked on lightning arrestors for GE, so this information is really from him. He said the lightning pulse is so fast that the impedance of a straight wire is too much. You could see a pattern of scorch marks where lightning had jumped from a lightning rod grounding strap to the house and back to the strap every few feet.

  • Agree, arcing is unavoidable, especially when all the Earth connections are not tied together by a very low impedance path. Check out this example where lighting went to ground via a tree, then came back out of the ground to travel down a long beverage antenna which had at the far end another ground. – Phil Frost - W8II Jul 30 '14 at 21:48
  • That makes a certain amount of sense now that I think about it from an Impedance point of view. It's extremely high voltages, and so normal rules for AC/DC low-voltages are not the same as these ultra-high voltage situations. – Warren VE3WPX Aug 1 '14 at 0:34
  • @WarrenVE3WPX The problem isn't so much the high voltage, but the very high rate of change of current. Consider, you might go from 0A to 30kA in 5μs. Then consider the definition of inductance: $v = L (\mathrm di / \mathrm dt)$. So if $v = L (30\:\mathrm{kA}/5\:\mathrm{\mu s})$, then $L$ need not be very big to make a very large $v$, which will have no problem arcing through 10 feet of air. – Phil Frost - W8II Aug 1 '14 at 12:53
  • Yes, I should have said "inductance of a straight wire is too much". – Walter Underwood K6WRU Aug 2 '14 at 1:17

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