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Lightning protection

N

Nostalgia

A tale of two Tx sites: One has a new Nautel J1000 (< 2years old) that I installed and has never been off the air. 26 miles away another Tx site with an Armstrong X1000B about 4-5 years old that came with the station that goes into cardiac arrest whenever there is a thunderstorm anywhere in the area!

Both have grounded towers with folded dipole antennas and both transmitters are located in shacks at the base of the towers. Standard engineering practice grounding systems in place at both sites. Only the Nautel site has their SPU-1 Interface Protection Unit installed. For those unfamiliar with this, both the power, antenna, and control wires pass through this device before entering the transmitter. Contains MOV's, torroids, other neat stuff.

Looking at it I see no reason that the SPU-1 could not be used with the Armstrong with a little rewiring. Just curious if anyone has ever done this and did you notice a reduction in transmitter kick offs.
 
At the Nautel site, yes. And if you are familiar with the SPU-1 it has a place to tie into the station ground since both the AC and RF pass through it.

At the Armstrong site, not sure. This is a multiuser site and we are in a small outbulding next to the main building housing the AC mains and the FM transmitters. I will look into that the next trip up there.

What is your thoughts on this?
 
Even with a grounded tower, a lightning hit will cause quite an induced 'kick' into the antenna.
Make sure that the tower ground is good. A static drain choke from the transmitting antenna and a good earth ground might help too.
 
The earth is not a perfect conductor like a metal ball. Soil resistance could be anywhere from a few ohms to hundreds of ohms per meter depending on water and mineral content. At the moment of a lightning strike the soil around the tower (including your tower ground system) becomes charged - taking some amount of time to dissipate. At that instant the utility company's ground (which consists of many many grounded power poles tied together) is a better ground than your local (tower) grounding system. If your local ground is not tied to the utility ground the electric charge from the lightning strike will attempt to flow through your equipment to the utility company's ground. This usually has a deleterious effect on your equipment.
 
I have seen many sites that eat transmitter after transmitter, while other sites in the same town never suffer damage. It's not the equipment's fault in most cases (but, there are exceptions). It's usually the design of the plant that's at fault.

I think the general consensus is that discharging the energy from a strike to the earth is not as important as making sure that everything in the transmitting plant is at the same potential. Damage happens when some part of the plant is at a much higher voltage than another during a strike. A few Ohms of difference is enough to produce damage because of the enormous potentials in lightning. When that happens, current flows across the difference and equipment gets blown up. Aside from anything static sensitive, the equipment that ties two, or more, systems together often takes a lot of damage. It's also important to remember that lightning has a substantial high frequency componant to it. DC grounding/bonding won't help very much. The bonding has to have a low impedance to lightning in order to be effective. That is why copper strap is common in transmitter sites and why the strap has to be carefully routed, folded and seams soldered so as to avoid creating inductance in it. Ground loops should also be avoided whenever possible.

So, there are at least four commonly used tactics to try to mitigate lightning. In order of importance (IMHO) they are:

1. Bond everything together so that it is at the same lightning potential.

2. Apply series chokes/transformers and parallel supressors where practical. This can include power lines, coax, and phone lines as well as individual equipment I/O.

3. Discharge the energy to ground as effectively as possible.

4. Lighning avoidance systems, like the static discharge systems that are sometimes placed at the tops of towers. Although these remain somewhat popular, I believe that recent research has demonstrated that these systems are generally ineffective because they cannot discharge energy fast enough to avoid the formation of step-leaders that result in lightning strikes. Never-the-less, there are those that swear by them.

There are probably other methods, too, but 1 and 2 have proven the most effective in my experience.
 
Good points Kyle. Since you are in Florida - where you can just about set your clocks by the daily arrival of afternoon thunderstorms - I know you speak from experience. One thing we used to do was separate the individual conductors of telephone lines coming in from the outside and wrap each conductor 20 or more times around a big stove bolt to act as an inductor and reduce the flow of lightning induced spikes. You are right when you say that lightning acts a lot like AC. The rise time of a lightning pulse is just as fast as the leading edge of a high frequency wave - hence the AC characteristics. The incoming lightning is like a speeding freight train that doesn't like to make turns. Anywhere you can introduce a loop (inductance) in front of the incoming lightning pulse can only help to diminish its rise time enough that your conventional surge protection can handle it.
 
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