Well you are thinking like a Californian. Direct strikes maybe rare in
California, but in other places they are very common. I now live in a
much lower lightning probability area than my Florida location, and here
my tower got three direct hits in the first 6 months after erecting it.
It's difficult to say what the voltage on the coax shield could be at
the top of the tower during a strike. There are several possible
situations. A direct strike can even hit the antenna, or maybe it hits
the mast and only induces voltage into the antenna. Either way the coax
shield voltage could easily exceed the breakdown of the coax jacket.
That doesn't mean it will do that every time. You may only get a
secondary finger from the strike. Also everyone's tower is slightly
different, different antennas, different cable routing.
As for the other question of the coax connected to the tower at the top
but not at the bottom, without some analysis I don't think I could
accurately guess. (My guess would be large.) An interesting question
academically, but not something I would suggest doing. You don't want
those currents directed toward the shack. You want them dumped into the
ground at the base of the tower, so you want the coax tied to ground at
the tower base.
There are several contributors for cable currents. There are conducted
currents and there are induced currents, and of course the source has
frequency components ranging from close to DC to close to daylight.
(Come to think of it, it does have daylight components.) An analysis
considering all of those factors for a typical tower configuration is a
formidable task. That's why we just use a set of rules instead of
trying to analyze a particular situation.
Jerry, K4SAV
Jim Lux wrote:
> K4SAV wrote:
>
>> ...."Is it because of some overvoltage issue on the shield? Maybe
>> letting the coax be the sacrificial fuse (i.e. let it arc
>> through the jacket) might be a good approach?".....
>>
>> That is exactly what you are trying to avoid by grounding the coax to
>> the tower. Replacing all the coax on a tower after a lightning
>> strike is a difficult way to implement a fuse. Pin holes in the coax
>> jacket means death of the coax shortly after due to water entrance.
>>
>> Jerry, K4SAV
>>
>
> Indeed.. if the goal is to protect the coax, you're right. If the
> goal is to protect the equipment inside the shack, and you're willing
> to sacrifice the coax, then...
>
> But an interesting question arises. Say you have a piece of coax
> running down the tower, and it's connected to the tower at the top
> (where you could put a bulkhead connector where you transition to a
> jumper to the antenna, for instance). Just how high would the
> voltage get between shield and tower at the bottom of the tower (or at
> some point along the way).
>
> Would it get over, say, 1kV? (which would be a guess as to the
> breakdown voltage of the jacket)
>
> If there's a direct strike to the tower, I can see the voltage
> differential being pretty high. But, for a lot of people, the direct
> strike isn't the concern.. they're relatively rare. The concern is
> often the induced voltage from a nearby strike. Then that gets into a
> discussion of how near, etc.
>
>
> But, for discussion, say you have the shield also grounded at the
> entrance to the house, and that the coax runs on the ground, and is
> taped or fastened to the tower. It's not a very large loop (area
> wise) that's going to pickup the magnetic field from the nearby
> stroke. If the shield resistance is, say, no more than 10 ohms, and
> the tower resistance is the same, then it would take a induced current
> of 50 Amps to get the voltage difference to be 1kV between tower and
> shield.
>
> Obviously, if you bring your coax from the tower to the house on an
> overhead line 10 feet off the ground over a span of 20 feet, it's a
> different matter. Then you have a huge single turn coil to pick up
> the magnetic field.
>
> Maybe all this guestimation is totally out of the ballpark.
> I know that if I were doing it, I'd probably just put grounded
> bulkhead connectors at the base, if only for convenience. But, I'd be
> interested if someone can point to an actual analysis or measurement
> of the system.
>
> Jim, W6RMK
>
>
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