Hi, Jeff,
A direct strike can easily overwhelm a single ground rod, not so much
because of the ground rod, but because of the resistivity of the
ground that is touching it. The condition of the total interface of
rod touching dirt limits it's current carrying capacity. If
sufficiently spread, a network of ground rods multiplies the current
carrying capacity, and divides the resistance of the total ground rods
to dirt interface. You also have the interface of the tower base to
ground, perhaps useful depending on exactly how that was done.
#18 can easily be popped by a strike, if left as the only other
interface to ground around an insufficient ground rod or rod array. If
this has been that way for a long period of years, you may in fact
have a lot of popped #18, where the discontinuity is actually out in
the wires. This is not helped by moisture induced surface corrosion of
the individual strands.
There is no guarantee that the pop would take place at the radial
connection ring. You may have places on the #18 where both wire and
insulation are evaporated into the soil. When strike current is moving
out a wire, you get a counter-EMF which starts to raise the voltage
across the insulation. The pop at the ring likely was current induced,
and pops out in wire would be voltage induced since the wire is
insulated.
A lot of folks poke at the FCC specification of bare #4 for commercial
radials, but as #4 is rated for bus to ground rods, it also cannot be
popped by diverted lightning. Pretty sure (!!) that hams are not
willing to put down 80 times 1/4 wave #4 for a ground field. That
leaves a quality ground rod field as the way to protect the #18 in the
radials.
73, Guy K2AV
On Thu, Sep 5, 2019 at 1:00 PM Jeff Blaine <KeepWalking188@ac0c.com> wrote:
>
> I have a full size vertical built from 25G running against about 80 1/4
> WL radials - #18 insulated wire sewn into the ground an inch or two
> below ground. There is a single ground rod next to the base but no
> other ground rods on this tower.
>
> We had a lot of lightning here last week and the tower took a hit which
> popped a wire off the matching net. I am going to put a spark gap of
> some sort on that tower to help address that but it's got me wondering
> if I should also add in a set of ground rod.
>
> The other towers on the site have extensive ground rod structures. The
> nearest tower to the 160m, for example, has 27 rods mounted along 9 bus
> runs.
>
> If rods are needed, does the existing ground radial screen offer me a
> discount on what is needed? I'm assuming that the existing radial
> field would provide some level of ground coupling for lightning and to
> get an equivalent level compared to the other tower. Meaning somewhat
> less than 27 rods - when combined with the existing 80 1/4WL radial
> field - would be functionally the same as the other tower has.
>
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
>
>
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