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.

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

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")

On Oct 16, 5:13*pm, Jim Lux wrote:
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")

'sort of grounded' is as good as grounded. The current that builds up
the charge to initiate the upward streamer is relatively small, that
can be seen in the use of the high value resistors commonly used to
'bleed off' charge from antennas. instead of bleeding it off they are
actually just equalizing it with the local ground potential, the same
as happens on a 'properly' grounded tower/vertical. in most cases you
won't find an amateur tower that isn't grounded one way or another
anyway, even if a specific ground rod or other system isn't supplied
at the base there is still a decent ground through the foundation.
and if not then there is through any cable going up the tower that
connects to a rotor or most antennas. it takes real work to really
insulate a tower from ground in such a way that it would not allow
charge attracted to the area under a downward leader from moving up
the tower... the large rf isolators and tower lighting isolators used
on base insulated AM broadcast towers are a good example... and even
on those types of towers you will hear reports of 'lots of snapping
and popping' before a stroke, those sounds are small flashovers of the
guy wire insulators as the charge flows up them toward the tower. So
to do it right would mean using insulating guys also. Then, even if
you got that far a tower of any height would end up going into corona
at the bottom and top anyway due to the high vertical electric fields
under a storm cloud... even if the corona didn't reach nearby ground
conductors it would still collect/dissipate charge on the conducting
vertical section which may still initiate a streamer. So the short
answer is that i have not seen any comparison between grounded and
ungrounded vertical structures.

On Oct 16, 12:13*pm, Jim Lux wrote:
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.

That's what I'm thinking. I know my well grounded mast is not a
lightning magnet. Some years I get no strikes to it. In fact, I think
it's been a few since the last one. Well, unless it was hit when I
was not at home.
I'm still of the opinion that the ability of an object to stream means
more than if it's grounded or not.
Airplanes are not grounded, and they get popped all the time.
Trees are poorly grounded, and they get popped all the time.
I've known quite a few people over the years that stuck a vertical
on their roof and didn't ground it, and they got popped. Blew
the ungrounded antenna to smithereens too..
Many golfers get popped on the golf course and they are not
really grounded too well unless they are wearing spikes on their
shoes. :/
I think the height of the object relative to it's surroundings, and
it's ability to stream mean more than anything.
Trees are a target lightning pick probably as much as anything,
and most are fairly poorly grounded due to being wood with some
moisture to helps things a bit. I think the trees ability to stream
well is why they get picked on so often. The pointy ends of a
leaf stream well, much like the pointy hairs on the head of a
golfer. And when you have hundreds of leaves on a tree...

But lets say a grounded mast does get struck slightly more than
an ungrounded mast. That would not a logical reason to avoid
grounding it, when the act of grounding the mast pretty much
negates the likelihood of a strike doing much damage to the mast
or the building next to, or under it.
A wooden mast would act much the same as a tree if it did not
have any kind of ground wire running along it's length.
That's why I never use wood masts here.
I'd rather have a grounded mast struck every 5 years with no damage,
vs an ungrounded mast struck every 10 years that led to heavy
damage or even burned the house down.
So worrying about that is kind of silly I think, when you know
an ungrounded mast is big trouble if it ever does get hit.

wrote:
I know my well grounded mast is not a lightning magnet.

At my last QTH, lightning passed up my 40' grounded
tower, my 40' vertical antenna, and all the surrounding
power poles. It instead struck a nearby 6' tall live oak
tree that, on a sunny day, was in the shadow of the tower,
antenna, and power poles. Half of the tree died but the
other half survives.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com