On Wed, 24 Nov 2004 18:40:41 -0500, "Jack Painter"
wrote:

RADIALS
It is suggested that many shorter radials be used rather than a few
long ones in a lightning ground system. The reason is that the
inductance gets too high to do much good as the length gets long.

50' is the number generally used as the longest effective run of an
individual radial arm for lightning protection.


Also long ground rods suffer the same fate. Too long and the
inductance is so high. That is one reason why additional shorter rods
are recommended rather than very long rods.

Up to 100' lengths of 1" copper clad steel ground rod are often sunk in
order to obtain 25 ohm resistance.

Those kind of lengths are only "last resort" ground systems if there
is no other possible way to get any sort of ground. The inductance
will be so high on that length that it will do little for lightning.

5 ohm would be better but some soil
condtions (such as dry sand) may be 1000 ohms. Bentonite and other similar
soil-conditioners cannot be used effectively in sand, that is unless they
occupy a deep 3' diameter hole. Here's an interesting anamoly in the NEC,
which requires grounding electrode resistance to be 25 ohm at the service
entrance ground. If that cannot be accomplished, then a second ground
electrode must be used. But there is no requirement to test the results
after the second grounding electrode is added! It could easily be 400 ohms
after two rods sunk in terrible soil, and yet it meets the code. There are
few such glaring examples of short sightedness in the NEC, but engineers and
contractors both have to be aware of these problems to design and build safe
lightning protection systems.

According to both NEC and NFPA, the first grounding electrode coming off a
structure may be shorter if conditions require, but then must be followed
with a second electrode no shorter than 8' long and sunk to a depth of 10'.
This is brand new in the 2004 edition btw. The minimum 8' long rod sunk to
depth of 10' remains the code *minimum* in all other standards and
applications where grounding electrodes are required. Distance between
electrodes should be not less than sum of both their lengths. O.K. there is
another little anamoly, since this does not take into account having to
reach down 100' to find low resistance ground! Of course UFER grounds are
also well accepted and commonly specified. This relates to another FAVORITE
wives tale of hams, that is that tower foundations will explode if the
grounding system goes into its concrete. There is not one documented case of
this happening, yet the fable continues. It's true masonry and lightning do
not get along well at all, but bonded conductors inside concrete foundations
make it an excellent grounding system. It has been used world wide for
decades.

Unfortunately it is not just an old wives tale. Over the years I have
seen several tower bases cracked or exploded chunks out of them from
lightning strikes.

To effectively use the tower concrete as a ground is a good thing to
do. However it must never be the only ground. Ground rods also need to
be installed and connected to the tower or you will have the effects
that I have witnessed. The ones that I saw did not have any auxiliary
ground connections.



COAX LINES
Only talking about the energy on the OUTER SHIELD of coax here now.
With buried coax lines coming from the tower they serve the same as a
radial tied to the tower. They will dissipate some of the energy just
the same as a radial will. And they do not have to be bare copper
shield lines to do so either.

This was where I was going with the buried insulated single wire long
wire lead.

We do now agree with where you were going. But I hope I can show you it's
not a place you want to hold any faith in protecting equipment with.

By the time the energy on the shield of the cable reaches the shack,
part of it has dissipated by the coupling to earth. Also the impedance
seen at that end is much higher than it is at the tower end of the
coax.
Just like the radial. The ground is a choke if you will.
Add ferite beads on a transmission line and you greatly limit the
amount of RF current that gets past on the shield. That is what the
earth does to the coax line buried.

I understand what you are saying, thanks. Lets leave the ferrites alone
though, because they can't shunt energy into dirt that doesn't have them,
and if this was effective for lightning protection we would all use
ferrite-shielded coax, right?

Back to the coax shield now - are you aware what is the most voltage that
standard coax can carry? It's about 5,500v. At close to 4,500v coax
dielectric breaks down and inner and outer conductors become "one". Close to
5,500v, the outer insulator of coax fails (similar to house wiring at
6,000v). Above breakdown voltage there will be flashovers to any object of
lower potential, including the air in some cases. At anything less than
breakdown voltage, you could hold the coax in a gloved hand and not feel
anything. And neither would the earth, if that's where it was laying.

Let's review: It does not matter one bit whether you bleed off some RF
energy out of a coax underground, because up to 5,500v of DC energy is going
to get through with absolutely no affect from being near the dirt. And at
over 6,000v nothing is going anywhere through that coax any more, it has
reached breakdown and arcs over, melting and expelling the remains of its
conductive materials all over the place. I have several such samples in my
collection btw.

Finally, this will conclude my points about not burying coax for lightning
protection:
Unless coax feedlines are protected by shield grounding and lightning surge
protection devices in the proper manner and locations, there is no hope for
its survival in even a nearby strike, let alone a direct attachment. If they
are properly protected, it does not improve this protection to bury them.
Perhaps it would increase the longevity of some surge protection devices and
arrestors by limiting some RF energy they would otherwise be presented with.
You would have to ask them that question. I have never seen a manufacturer
of these devices require feedlines be buried, maybe they are missing this
too. They do however require the devices we have talked about.

Well now, what if the DC component of the strike is only 5495 volts?
Still useless to remove most of the RF component of the strike? The RF
part does contain significant energy you know. After all isn't the
whole idea here to keep as much energy out of the shack as possible?
The DC part of the strike is usually easier to suppress with a ground
connection. The AC part is dependent on the inductance of the
conductor to get it to ground. Why not use all available means at
hand.

Jack, without trying to sound sarcastic I would recommend that you do
some reading about transmission line theory . Also some basics on
inductance and capacitance and how energy is transferred in their
fields. Outside of what the lightning protection people publish. These
may help you in your quest to become a lightning guru. It is fine to
read all the specifications of the NEC and other agencies that write
specs. But you need to remember that they are using one glove size to
try and fit all. If you understand a little more about how things work
you can come up with your own fixes to fit the situation much better.
You obviously have an appetite for this stuff. Spend a little time on
the theory side.
You have to be able to adapt to and use the surroundings available to
best advantage. The perfect system can not always be built.

Well I think I have had my run on this thread.

Best Holidays
Gary K4FMX


It's of course fine if you want to hide coax, possibly reducing some
external RF affects *and* the manufacturer says it is designed for below
frost-line burying. Mine are buried too.

73 AND HAPPY THANKSGIVING

Jack Painter
Virginia Beach VA