Was talking with someone the other day and mentioned that I had just
grounded the antenna (the mast, actually) to protect against lightning
strikes, and they said that was not such a good idea because lightning is
more likely to strike a path that goes straight to ground. Now I am not
sure what to do. Anyone have any input on this topic? Ideas are gratefully
received...

Thanks,

Dave

On Thu, 15 Oct 2009 08:42:05 -0500, "Dave" wrote:

they said that was not such a good idea because lightning is
more likely to strike a path that goes straight to ground.

Hi Dave,

Did they happen to mention what a great deal it was for that lightning
to go directly to your radio instead?

I suppose their next argument is that lightning won't strike an
"insulated" antenna....

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
On Thu, 15 Oct 2009 08:42:05 -0500, "Dave" wrote:

they said that was not such a good idea because lightning is
more likely to strike a path that goes straight to ground.

Hi Dave,

Did they happen to mention what a great deal it was for that lightning
to go directly to your radio instead?

I suppose their next argument is that lightning won't strike an
"insulated" antenna....

One reason for lightning conductors (and for grounding elevated
conductors, like radio antennas) is that it helps to stop a high
electrostatic charge from accumulating in the air immediately above
them. The intention is to PREVENT a direct lightning strike, rather than
conduct a strike to ground. Of course, if a direct strike DOES occur, an
antenna (and even a stout lightning conductor) may be seriously damaged.
--
Ian

On Oct 15, 10:54*am, Ian Jackson
wrote:


One reason for lightning conductors (and for grounding elevated
conductors, like radio antennas) is that it helps to stop a high
electrostatic charge from accumulating in the air immediately above
them. The intention is to PREVENT a direct lightning strike, rather than
conduct a strike to ground. Of course, if a direct strike DOES occur, an
antenna (and even a stout lightning conductor) may be seriously damaged.
--
Ian

The only problem with that is that the charge is so quickly
replenished
that I think trying to bleed off the charge is a waste of time.
Note the "brush" type conductors that supposedly dissipate the
charge to avoid a strike. They don't really work too well, and when
they do get struck, often you have blobs of molten metal flying
about.
The way I see it, the only way to really prevent a strike, and even
then I think it's iffy, is to provide a shape that does not stream
well.
And then have a lightning rod which does stream well to take the
strike vs the object that does not stream well.
IE: a rounded ball on top of a mast does not stream as well as
a sharp tip. This is why most flag poles have a ball on top.
To reduce the likelihood of streaming. And even those can still be
struck under some conditions. So I think trying to avoid a strike
using such methods is more wishful thinking than anything.
I look at it in a different way. I expect it to be struck, and it will
if it's any decent amount of distance above the surrounding objects.
It might take 6 months, or it might take 6 years, but it will be
struck some day. Count on it.
A strike does not know where it's going to hit until about the
last 150 yards or so. Then the streamers from the ground
are all pointing towards the down leader trying to entice it.
Usually the strongest streamer in the area will attract the
strike. And even that is never a sure thing. :/
So the best way to deal with lightning is to expect it to
strike sooner or later, and to provide the most efficient
path to ground when it happens.
A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location.
That's horse caca, and the OP can tell his friend I said so.
They will stream the same, and I consider the chances
of a direct strike about equal.
It's when you actually take a strike is when the differences
in grounding show up.
The well grounded mast will take the charge to ground
in an orderly military manner, and in most cases no
damage will occur.
The non grounded mast does not provide an orderly
path to ground, so the strike takes whatever path
has the least resistance. And even that path is likely
to be fairly high resistance and fireworks are likely
to occur. Houston, we have a problem.. :/
The OP did the proper thing by grounding his mast.
If properly grounded, if he does take a strike, the
mast will direct the charge to ground and little
damage should occur. This does not mean you
want a radio hooked up though. That takes extra
measures like suppressors, ground window, star
grounding, etc..
But at least he won't have to worry about burning the
house down.
I've had my mast directly struck more than once, and
I had no damage at all. Period. But my feed lines
were all grounded to my bulkhead outside the shack.
I don't leave them connected as I use no suppressors.
Two of those strikes were with me sitting in this chair
15 feet away from the base of the mast.
You can actually hear the difference between a strike
to a well grounded mast, and a strike to a poorly
grounded object.
The strike to the well grounded mast will be very quiet.
Just an arc sound like throwing a light bulb on the ground.
A strike to something poorly grounded like say a tree
will be much louder. More like a crack from a shotgun.
This is not to be confused with the overhead sonic boom
which both types of strikes will make. I'm just talking
about the local sound.
Anyway, that's my $12.47 worth from someone that lives
in lightning country and has taken multiple direct strikes
though the years.
If the OP wants some good lightning info, try searching for
Gary Coffman and lightning on google. I don't know if his
past posts are still archived, but they should be.
He had a lot of good posts on various aspects of lightning
protection.

A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location.

actually it does. both the ieee and cigre have been using lightning
statistics data collected mostly from tall masts for many years.
There are well known formulas used to calculate the number of strokes
to a pole or power line, both include the height, and as height
increases so does the number of strokes to the object. The height
also skews the current distribution with higher structures more likely
to get more high current strokes.

Dave wrote:
A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location.

actually it does. both the ieee and cigre have been using lightning
statistics data collected mostly from tall masts for many years.
There are well known formulas used to calculate the number of strokes
to a pole or power line, both include the height, and as height
increases so does the number of strokes to the object. The height
also skews the current distribution with higher structures more likely
to get more high current strokes.

But, is there a difference in strike rate between grounded and
ungrounded towers of the same height. I would think that the difference
would be very small, and smaller as the height gets bigger.

Since the vast majority of commercial masts,towers,buildings used to
collect the stats are probably grounded (Because the code requires
it...), it might be hard to find decent data for "ungrounded" things.
(for one thing, the equipment used to collect the strike data, until
recently, probably measured the current spike on the grounding wire..
these days, you could use the RF lightning detection systems, and match
up strike locations against structure locations)

Maybe wooden poles? (which are only "sort of grounded")

wrote in message
...
On Oct 15, 10:54 am, Ian Jackson
wrote:


One reason for lightning conductors (and for grounding elevated
conductors, like radio antennas) is that it helps to stop a high
electrostatic charge from accumulating in the air immediately above
them. The intention is to PREVENT a direct lightning strike, rather than
conduct a strike to ground. Of course, if a direct strike DOES occur, an
antenna (and even a stout lightning conductor) may be seriously damaged.
--
Ian

"The only problem with that is that the charge is so quickly
replenished
that I think trying to bleed off the charge is a waste of time."

The turn of the century genius, Testla, patented some lighting protection
devices based on having an insulated "cap" at the highest object on the
protected property. The "cap" would rise thousands of volts above the
protected structure and this would reduce the tendency of lightning to
strike.

Seems to me that the federal government has lots and lots of buildings and
would relatively inexpensively conduct definitive experiments to see what
works and what doesn't in the area of lightning protection.

So far as I know, the feds have done no such thing.

On Oct 16, 8:55*pm, "John Gilmer" wrote:
wrote in message

...
On Oct 15, 10:54 am, Ian Jackson

wrote:

One reason for lightning conductors (and for grounding elevated
conductors, like radio antennas) is that it helps to stop a high
electrostatic charge from accumulating in the air immediately above
them. The intention is to PREVENT a direct lightning strike, rather than
conduct a strike to ground. Of course, if a direct strike DOES occur, an
antenna (and even a stout lightning conductor) may be seriously damaged..
--
Ian

"The only problem with that is that the charge is so quickly
replenished
that I think trying to bleed off the charge is a waste of time."

The turn of the century genius, Testla, patented some lighting protection
devices based on having an insulated "cap" at the highest object on the
protected property. * The "cap" would rise thousands of volts above the
protected structure and this would reduce the tendency of lightning to
strike.

Seems to me that the federal government has lots and lots of buildings and
would relatively inexpensively conduct definitive experiments to see what
works and what doesn't in the area of lightning protection.

So far as I know, the feds have done no such thing.

yes, they have, not that i believe everything the government has said,
but this stuff i do.

this presentation lists several reports by the navy and faa among
other agencies: http://www.docstoc.com/docs/4177489/...-Abdul-M-Mousa

the nfpa and others are quoted here, even though the author is
skeptical: http://www.straightdope.com/columns/...-strikes-again

this one again quotes the onr, nasa, and air force studies as well as
others. http://lightning-protection-institut...ct-fallacy.htm

as far as tesla's stuff, i file it with most of the other experiments
with lightning protection and prevention, if it really worked it would
be used all over the place. the only system that has stood the test
of time is the old franklin rod system, it doesn't prevent strikes,
but it does try to provide a safe route to ground besides through the
protected structure. much of the success of it depends on the quality
of the installer, mostly how well they bond the various wires and how
well they take it to ground.

John Gilmer wrote:
wrote in message
...
On Oct 15, 10:54 am, Ian Jackson
wrote:

One reason for lightning conductors (and for grounding elevated
conductors, like radio antennas) is that it helps to stop a high
electrostatic charge from accumulating in the air immediately above
them. The intention is to PREVENT a direct lightning strike, rather than
conduct a strike to ground. Of course, if a direct strike DOES occur, an
antenna (and even a stout lightning conductor) may be seriously damaged.
--
Ian

"The only problem with that is that the charge is so quickly
replenished
that I think trying to bleed off the charge is a waste of time."

The turn of the century genius, Testla, patented some lighting protection
devices based on having an insulated "cap" at the highest object on the
protected property. The "cap" would rise thousands of volts above the
protected structure and this would reduce the tendency of lightning to
strike.


This is done in some HV test laboratories to avoid flashover to the
ceiling and to make the field more representative of "outdoors".. they
hang a semiconductive curtain in a horizontal plane above the apparatus
which charges up and makes what's above look less like "ground"


There's also the whole thing of surrounding a valuable structure (e.g.
ammunition storage bunker, rocket launch pad) with an array of high
towers with grounded wires from the tops of the towers. While no
guarantee that lightning won't strike elsewhere, it definitely ups the
odds of the protective structure taking the hit.

here's a pictu
http://commons.wikimedia.org/wiki/Fi...launch_pad.jpg

Dave wrote:
Was talking with someone the other day and mentioned that I had just
grounded the antenna (the mast, actually) to protect against lightning
strikes, and they said that was not such a good idea because lightning is
more likely to strike a path that goes straight to ground.

Like many things, "it depends".. what else is near the antenna? From an
electric field standpoint, whether it's actually physically connected to
the ground isn't all that important. It's that it's a conductor
sticking up, and that causes the field to change. As the surrounding
E-field changes over a time span of milliseconds, a mast connected to
the ground vs isolated will have a slightly different voltage
distribution over time, but over long times, it's not much different.





Now I am not
sure what to do. Anyone have any input on this topic? Ideas are gratefully
received...

Thanks,

Dave