Lightning protection on ship antennas - RF Cafe Forums
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?
Posted: Tue Sep 27, 2005 8:34 pm
Joined: Fri Sep 02, 2005
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
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.
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
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.
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..