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Lightning protection on ship antennas - RF Cafe Forums

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 Post subject: Lightning protection on ship antennas
Posted: Tue Sep 27, 2005 4:21 pm 
Does anyone know whether lightning protection is required on ship antennas/masts? If so, where does the energy go - to the hull?

What about for fiberglass ships, how do they protect equipment against lightning strikes? I guess the same goes for airplanes that are fiberglass or composite fiber. Anyone have any experience in these matters?

Thanks. :P


 
  
 
Graham
 Post subject:
Posted: Tue Sep 27, 2005 8:34 pm 
 
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Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
This is a really very good question! I don't actually know all the answer, but I can offer a few things closely related.

Ships get struck all the time, as do aircraft. Being metal, folk within are protected (Faraday shield effect) even if there is damage to the structure.

Lightning protection devices as used on buildings, connected to a grounding strap have an array of sharp points, designed to cause the air to reach corona breakdown point very quickly when a charged cloud is over. Well before any strike, the discharge glow starts, and dissipates the local field by generating ozone and other gas ions. I think the idea is that if there is to be a strike, it will hit someone elses highly charged boat!

I have always wondered if these sharp points building up a cloud of ions in the air above actually amounts to a lightning attraction device, but I am assured that a sharp point like that immediately and dramatically reduces the potential of the rest of the connected bits.

A fat grounding strap is a safety device designed more to ensure that a major strike will track a reasonably safe route over ceramic, brick and other fireproof surfaces, than to survive the strike itself. That said, most minor strikes may not do more than bend it a bit , or melt a few spots.

The actual design of safety strap route, its location, and what might connect to it where is not trivial. There are specialist outfits doing just that.

Protection of the ship is one thing, protection of any RF kit onboard is another. GRP vessels are going to be prone to fire. My guess is that a highest point set of ioniser spikes connecting to a thick copper strap, secured over a path of ceramic tiles, down over the side, might be the main feature.

Choke baluns, or even a few turns coiled into the coax can provide enough impedance on the coax outer that a strike transient produces a voltage pulse enough to break down a intentional spark gap in purpose-designed connectors on the antenna side. Thus a strike can be made to track elsewhere, maybe destroying the upstream cable in the process, but behind the impedance, the kit can survive.

Lightning is a great destroyer of RF kit connected to antennas, but by using spark gap protectors, chokes, coupled with overvoltage protection semiconductors at the kit end, is maybe what is needed.

I do admit that all this is gathered over time from discussion among working pals. I may even be misinformed. If there is anyone out there who *really knows*, able to speak with authority, and is willing to share some knowledge for free, please do..




Posted  11/12/2012