Tuesday, September 3, 2019

Post Mortem on my Lightning Strike

A little over a month ago, we were blasted awake by an extremely loud blast of thunder and within a couple of hours realized we had the worst damage we'd ever had from lightning and a lot was damaged.  As the weeks have gone by we discovered more of what was damaged and now think we've found everything.  We've have been able to fix some things, replace some, get some serviced and are still working on a few others.  This is still going to take a while. 

While we experienced damage to a few things elsewhere in the house, the most damage was in the ham shack.  The control box for my antenna rotator blew out, taking out the computer it was attached to, and the computer took out the radio it was attached to (at least on that one function).  One antenna showed up as open, the other two are fine.  Finally, the big thing, I have a linear power amplifier that won't turn on.  I've taken that apart to pull out the power supply for more troubleshooting.  When I found the part that I think is blown in the radio, I realized it's in a specialized package I'm not really set up to work on.  (It's in a 36 pin QFN, drjim)  I opted to send the radio for factory service.  Those guys wince when they have to replace those. 

Since then, I've been trying to figure out exactly what happened.  I have a small tower by many ham's standards, just 20 feet tall with a mast that gets that up to 26.  Many people would assume the tower was struck, since the damage focused on things in or on the tower, yet there's no damage apparent to it. 

Off to the tower's northwest is an oak tree that's now in the range of 35 to 40 feet tall, so quite a bit taller than the tower.  To its south are a couple of palm trees that are close to 30 feet tall and to its southwest is a maple tree that 's also in that 35 to 40' tall range, too. 

I've seen pictures of trees that have been hit by lightning and I've looked at the oak for signs it was hit - the bark splitting or blown off the trunk.  The tree showed no apparent damage.  Sunday, while putting up the shutters for Dorian, I found myself standing over near those two palm trees.  That's when I noticed interesting, telltale signs on the bark.

There are four or five trails from the top to the bottom of the tree.  More or less straight along the entire trunk, each looking blown out from the inside.  It wasn't until we were done with the shutters that I paid more attention and looked up to where the palm fronds grow out vertically (fronds would be called leaves on any other tree); I noticed one was bent over nearly double near the top of the tree.

The stem portion of the frond (circled in red) appears broken and even blown apart.  It looks like the palm took the lightning strike, and induced a strong current in the tower, which is about 25' away. 

I had taken down my antennas on Saturday and inspected the damaged antenna, finding no evidence of damage.  On the other hand, I hadn't taken apart the part most likely to be damaged, a transformer.  I was more interested in getting it sealed well so that when the storm is over I could look into it.  I capped the connector on the cable that attaches to the antenna and sealed it against the rain.  Today, with the storm looking less and less like a threat and with time available, I attached the cable to the bad antenna to my analyzer.  My reasoning is that I know the cable is open now, so if disconnecting the cable from the antenna didn't change the results I must have substituted one open for another.  Today's plot looked exactly like my last plot of the blown antenna. 

The antenna analyzer has a function that will tell you the distance to an open or a short on a bad length of cable.  That function told me the trouble was 8.9 feet from the instrument.  I realized that meant it was inside the room, and I quickly realized what it was.  I had a surge protector where the cable enters the house, attached to my heavy ground wire.  It was blown.

Replacing the Alpha Delta Transi-Trap with a dual female connector made the instrument tell me the open circuit was now 56 feet away, which is close enough to how long the cable is.  As luck would have it, I have a replacement surge protector (probably better quality than this one) from a company that got out of that business. 

Lightning causing damage by inducing current in a nearby conductor might well be the most common type of damage there is.  Lightning strikes vary in their current, but currents ranging from tens of thousands of amps to well over a hundred thousand amps have been studied.  Like an electrical transformer, current surging through a conductor (of sorts) like a tree induces current to flow in other conductors nearby.  How large that current is depends largely on the distance.  It can be coupled by the magnetic or electric field.  I think the current flowing in my radio tower induced currents in power lines in the walls which flowed around the house.  We had concluded earlier in the aftermath that things with long wires attached were more likely to be damaged.  It all adds up.

I'll know within the next couple of months if the palm will survive the strike or if it's already dead and just doesn't know it.


  1. Good job, SiG!

    You took a direct hit on your property, 25' from your tower, and had collateral damage from what Mother Nature did to that palm tree.

    From my readings and talking to People Who Know These Things, most lightning damage is caused by the induced currents from a nearby strike. The induced currents come from the main discharge path current, and raise havoc in any electrical conductors in the vicinity of the strike path.

    Most people think "Voltage!" when they think of lightning, but it's the tremendous currents in the main discharge path that do the damage, caused by inducing currents in other conductors, which gives rise to uncontrolled voltages being developed.

  2. When you described using the antenna analyzer to find the distance to the problem, the tiny part of my brain that is in charge of remembering odd things woke up and reminded me that I had once read a story about telegraphy and there was mention of a Wheatstone Bridge to find wheere there were breaks in wires.

    Any relation to the instrument you used? Or did I just remember wrong?

    1. Any relation to the instrument you used?

      It's rough to see in a schematic but I think a Wheatstone Bridge is its great grandfather. This version is referred to as a return loss bridge. If I got a schematic it would be hard to see the similarity because a Wheatstone Bridges use resistors and a DC source, while this one uses radio frequency sources (single ICs) and balances impedance instead of resistance.

      I believe what it's doing is sweeping the frequency and looking for places where the impedance goes to zero or infinite. An open cable(infinite resistance) looks like a short circuit (zero resistance) when seen from a quarter wavelength away. Once it finds the lowest frequency that can be seen at, it converts that to electrical length and then the physical length in that cable. A short circuited cable would look like an open circuit a quarter wavelength away, so same concept, just looking for high impedance, not low.

  3. Dunno if your property is big enough, but if I ever get into radioing. Immana set up a "sacrificial anode" back on the treeline about 100' away from housing.

    Bolt a pole near the top of the tallest tree and run a ground wire down to a ground rod hammered in.

    Replacing that kind of hardware is a lot cheaper than fiddly bits of electronics..

    1. If I put up a big enough metal rod, the 60 degree Cone of Protection can include everything. (45 degree?) I don't think there's a legal limit to how tall a pole I could put up.

      I see the cone of protection is referred to as a myth because things inside the cone can still be damaged by the dissipating strike, and I'd still wonder about the induced strike, which is what I had. In my case, the palm tree seemed to act as that tall sacrificial anode in taking the worst of it, but what coupled out was good enough to blow out lots of stuff.

      The cone of protection is still referred to in real lightning protection classes, though.

  4. The house I lived in had a medium sized pine tree about 15 feet from the corner of the house. In ten years lightning hit that tree three times, killing one phone and two TVs. The fourth time lighting split the tree and it hit the ground. But after that tree was gone I never had lightning hit again. Just something about that tree...

  5. transitraps and surge protector power boxes saved me large amounts or downtime and cash. before transitraps, I lost and satellite antenna LNAs and associated receivers and cabling. It was a mess. the cost of the transitraps was far less then the equipment the eddy currents destroyed. we now have surge suppressor in the mains coming into the breaker panel.
    the only fault with transitraps is their one shot usefulness. but, that is a good thing. gallium arsenide transistors switch very fast to ground and the current blows them away. better that than my gear.
    still, physically disconnecting the rf connections outside the home/shop if a storm is forecast is the best way to go. there is nothing I have to say during a storm that I cannot say later.
    some years ago, I observed sparking at the 300 ohm twin lead and 52 ohm coax connectors while lightning strikes were occurring some miles away from the antenna array. I found it to be an excellent learning experience however late it came to me.

    1. My three antenna coax lines are routed through an Alpha Delta DELTA-4B, surge-protected switch and the output goes through a duplicate switch to select the radio. Neither of those gas discharge tubes fired. Those radios run on 12VDC through a linear power supply which is undamaged. The radios are all fine on the RF ports. Everything transmits and receives. The only problem is that on my main radio, one arguably minor function (its USB port) seems blown out. That's because it was attached to my PC which was blown out by its RS-232 port, and that was blown out by a surge on the antenna rotator control cable coming down the tower.

      The pattern appeared to be that long cables acted as pickup loops. Both of our computers, about 50 feet from that end of the house, had our Ethernet ports blown. Both were connected to the WiFi router through coiled up Ethernet cables. That was connected to a cable modem, which was blown. All of that hardware was plugged into surge protected strips. Nothing came over the power lines that I can tell. Elsewhere in the house, things plugged into surge protected strips were mostly OK, but a wall wart supply plugged into an extension cord in a surge protected strip blew out. That's as far away from the strike as it can be and still be in my house. It's also the longest power line loop in the house. My cable company's internet feed into my house, farther away still, showed surge damage.

      If I had disconnected the antenna cables outside, this all would have happened the same way. The surge didn't come down the RF path. It came into the house by induced voltages.

    2. Just curious, but how do you know the gas tubes didn't fire? They can 'fire' multiple times without damage, but unless they got walloped hard enough to destroy, which happens, I don't see how you can tell they 'fired' or not.

    3. My mistake, then. I thought they went one time and died, not that they're reusable. It says so pretty clearly in the instructions, though. RTFM isn't much good unless you RTFM - where the second one starts with Remember!