Hi, Jason;
Independently grounding each tower leg provides parallel paths to
dissipate the lightning charge and thus reduces the impedance and the
maximum voltage developed. (V = I Z)
Wide straps offer less inductance than round wires, the less impedance.
Fortunately, the inductance can also be reduced by parallel wires,
spaced to the width of the strap. The combination of resistance and
inductance of the parallel wires can be calculated, as can the impedance
of a wide, thin strap conductor. As a rule of thumb, put together 3 to
5 wires as wide as the strap and with equivalent cross sectional area.
Don't worry too much about not achieving a full peripheral ground. Seek
the least impedance paths, several of them, leading to distributed
ground rods. PolyPhasor is correct in citing the advantage of full
peripheral ground, but leaving a gap is seldom fatal. DO TIE ALL GROUND
SYSTEMS TOGETHER, INCLUDING ALL UTILITY GROUNDS, so that you maintain
low voltage between them. Don't let your transceiver be between two
grounds (AC power ground and antenna ground) that are hundreds of volts
apart.
The need for multiple ground rods results from the characteristics of
earth; think of the earth surrounding each ground rod as a lossy or
leaky capacitor (determined by the permitivity and conductivity of the
earth). As lightning charge is conducted to that capacitor, its voltage
increases (V = Q/C) until the charge is leaked away by the limited
ground conductivity. The key to protection is to limit the voltage.
Therefore, use more capacitors, more ground rods. Think of each rod as
a certain capacity, paralleled by resistance that slowly leaks off the
charge. As recommended by PolyPhasor, space the ground rods at a
distance about twice their depth.
Think of the protection this way: If you could connect your tower and
antenna to an infinite capacity through zero impedance, the greatest
lightning charge in the world could not develop any voltage between your
antenna and ground. Keep the capacity high and the impedance low and
you keep the voltage low. Don't worry that the lightning charge comes
from a very high voltage; the voltage divides between circuit components
according to the impedance of those components. If you can achieve zero
impedance from the antenna down, all the voltage will be dissipated in
the path from the charged cloud to the antenna, leaving zero voltage on
the antenna.
Incidentally, Uman reports that #12 copper wire will conduct the charge
from 90% of lightning strikes. I don't care to live with the remaining
10%, but the several #6 copper wires in my lightning ground system bring
the percentage way down to a fraction of a % of lightning strikes that
might damage the system. To learn more about lightning, read Uman's
books on the subject. They are available in today's cooperative library
systems.
Equally important to preventing damage to your station equipment is to
block the low frequency components of the remaining voltage rise with
suppressers like those from PolyPhasor or ICE. Each manufacturer offers
advantages. Get details on each and choose one. Don't depend on spark
gaps and simple gas tubes alone to protect modern transistorized
equipment, although those devices remain useful for some purposes,
including as components of the modern suppressers.
73 de WOØW
Jason Hissong wrote:
>I have been reading Polyphasor's site about lightning protection. And it
>seems that a roof tower is probably the worst type of mounting you can have
>(great!). However, it mentions what can be done about it.
>
>It seems that the best thing to do is have multiple connections to a ground
>perimeter around the house. My planned installation is to ground each leg
>of the GM roof tower with #2 solid copper wire to 4 ground rods
>interconnected to each other. Polyphasor recommends using copper straps
>(which I would be willing to do). Where would one get some locally
>(hardware stores?)
>
>My primary concern is not my equipment (although it would be nice) but my
>home. The tower is going on my garage. My lot is surrounded by trees and I
>have electrical poles about 40ft behind my house. I would be unable to have
>a ground perimeter around the house due to the fact that the ground strap
>would have to go over things like a driveway and a porch. Any ideas on how
>to get around that? (The strap would be running over the top of the
>driveway and porch). I hear you want to bury the copper strap and the
>ground rods. I would be able to basically make a "U" around the house. The
>shack ground would be connected to the perimeter ground.
>
>Anyways... maybe I am being overly worried. I live in Columbus, OH which I
>would consider a moderate frequency for lightning. Of course, it only takes
>one.....
>
>Initially, the four legs of the tower will be grounded when I install the
>tower in the next couple of weeks. Over the winter, I will add additional
>ground rods around the house and connect them together. So by spring, I
>hope to have enough protection. What type of compound should I buy to make
>sure the corrosion is minimized where the copper strap/wire and ground rods
>meet?
>
>I guess if there was a direct strike I am screwed anyway :)
>
>Thanks
>
>Jason
>
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