Inductance in lightning protection

[CALNNC]

A high frequency wire dipole antenna installed at 80 feet is fed with a balanced line, at the transmitter feed point is a 1:1 balun that the balanced feeline hooks to on one side, a tuner on the other. The 1:1 balun is 12 parallel turns of #12 around a ferrite core. The input to the balun has one leg going direct to ground, grounding one side of the antenna from input to antenna wire. The other side goes to the tuner that is above ground. A 100uh coil is attached to the baluns input that does not go to ground, its function to act as a way for the ungrounded leg to have a discharge path for static buildup, and to look invisible to the RF going across it. Does it have any effect on the Murphy's Law inclination of things that are grounded that seem to attract a strike, meaning, does having that inductor installed really make a difference?

 

[Baluncore]

Does it have any effect on the Murphy's Law inclination of things that are grounded that seem to attract a strike, meaning, does having that inductor installed really make a difference?

Your essay on the schematic is not so easy to follow. There are too many interpretations. Perhaps you can attach a schematic diagram to your next post.

A static electric charge will build up on the antenna until something breaks down to ground. There needs to be an earth return path for DC from all exposed antenna components. The inductor is such a DC path. I prefer a link across balun centre taps.

A direct lightning strike to the antenna will arc across the inductor before it can build up current through it, so it is wise to provide a couple of external points that can steer the arc away from the components before they are burned in the flash over. A gas filled lightning arrester, (as used on telephone lines), will protect most antenna systems from strikes.

The support structure for the dipole antenna can include some form of lightning conduction protection. A strike will prefer a bare metal tower to thin insulated antenna cables.

If you carefully monitor the ground discharge current from the antenna using an oscilloscope with a 50 ohm input, you will see individual raindrops or pollen particles dumping their charge to ground. The static noise is most evident when the first drops of rain start to fall. The discharge frequency will rise to a crescendo, just as the first lightning strikes, when it all goes silent.

A RX-only wire dipole antenna on HF should use insulated wire to reduce the static discharge noise by lengthening the time constant, but a TX or TRX antenna should be bare metal, which reduces the fire hazard.

 

[tech99]

A high frequency wire dipole antenna installed at 80 feet is fed with a balanced line, at the transmitter feed point is a 1:1 balun that the balanced feeline hooks to on one side, a tuner on the other. The 1:1 balun is 12 parallel turns of #12 around a ferrite core. The input to the balun has one leg going direct to ground, grounding one side of the antenna from input to antenna wire. The other side goes to the tuner that is above ground. A 100uh coil is attached to the baluns input that does not go to ground, its function to act as a way for the ungrounded leg to have a discharge path for static buildup, and to look invisible to the RF going across it. Does it have any effect on the Murphy's Law inclination of things that are grounded that seem to attract a strike, meaning, does having that inductor installed really make a difference?

It is very difficult to protect the antenna against lightning unless very heavy conductors are used, with a low resistance earth connection. Maybe the the downcoming open wire feeders can be directly earthed - that would be a good step. Maybe you can arrange them to form a quarter wave stub to give a good DC path to earth. In addition, as in post #2, obviously earth the support masts.

Protection of the radio equipment is best handled in stages, rather than relying on on just a single path to earth. Why not use an ATU circuit using a DC path to earth? Why do you have such a large inductor, it needs only be just enough for RF purposes and needs to be heavily built. Maybe a discharge device is a better proposition. I am not sure that the radio equipment will be safe with a direct strike whatever you do, because the current is possibly 10^4 Amps. Commercial lightning protection systems use conductors of 50 mm^2 cross section.

As a further comment, in the UK we have to be very careful to check the electricity regs if wanting to use an earth connection on mains powered equipment, as we generally now use Protective Multiple Earthing, which creates a risk in the event of a broken neutral.

Some information about the European Standard for Lightning Protection, EN62305, here:
http://www-public.tnb.com/eel/docs/furse/BS_EN_IEC_62305_standard_series.pdf

Finally, it is thought that earthing an object does not increase the chance of a lightning strike, but it just helps avoid damage if it happens. My own approach to lightning protection of antennas has been to simply disconnect the equipment from the wall socket when not in use, as we do for TV antennas.

 

[Baluncore]

Commercial lightning protection systems use conductors of 50 mm^2 cross section.

Conductors should not be round or stranded, but be made from flat bar or strap, which has a lower inductance than round.

The conductors should not be insulated. Insulation will slow down the velocity of propagation, which causes a strike to flow outside the insulation. Insulation can also trap moisture, causing a steam explosion that removes the insulation when struck.

The lightning conductors should not have sharp bends. Lightning will cut a corner, jumping 400 mm through air to avoid a right-angle bend.

Antennas are generally protected from a direct strike because they avoid the above. They have round, insulated wires, that go around sharp corners and pass through bulkheads. The lightning discharge will prefer a more direct route down the surface of a mast.

I have seen neon lamps (Ne2 indicator) used to eliminate static from antennas. They appear to work well to prevent insulation breakdown in the baluns, coax and termination resistors, but when a real strike occurs they are totally destroyed. They cannot be protected with a spark gap because they restrict the voltage needed to strike out until after they become an expanding ball of plasma. Always provide a spark gap to the metal structure, and across any component in the static conductive path.

 

[CALNNC]

Gas discharge tubes work fine to dissipate static, but they are nothing but vapor in a direct strike, unless there are giant ones out there, and not just the little thimble sized. We had hundreds of them on control lines and they would light up nicely in a storm with lightning strikes as far as 7 miles away.

My Murphy's Law comment was due to an experience at a facility where $30,000 was expended for a lighting protection system, installed on a 50 ft steel tower, 2 air terminals at the top, each on a separate Thompson Lightning woven cable, bonded every so many feet to the tower, and attached to ground rounds, all connected by 500MCM with ground rods every 10 feet, including the perimeter of the building, all exothermically welded, a ground system resistance of 4 ohms (took a lot of rods and pretty good for rocky soil) and a direct strike turned the equipment inside the shelter into a $180,000 charcoal briquette.

My one on one with a direct strike on a facility I was in that was also fully 'protected', was the room took on a blue caste and the air in the room became warm instantly. The schematic of the circuit is attached. Basically a 100uh (approx) coil to ground, a 100 turns #16 enameled wire on a 1/1/4inch form and the XL high enough that the coil is invisible to the 1.8 to 30Mhz energy going across it. The spark gap recommendation was noted, and now has been implemented. Started with .020 inch gap, we'll see what happens.

 

[tech99]

I believe that the rise time of a lightning curent is traditionaly taken as approx 1 uS, and the current might be 10^4 Amps. So the rate of increase of current is 10^4 / 10^-6 = 10^10 Amps/second. For a 100uH inductor the volt drop is then -L dI/dt = - 10^-4 x 10^10 = -10^6 volts. So the inductor will flash over and be destroyed.