How tall is your tower? My family lives in NE Florida and are always trying to
get me to move there...
>From: K4SAV <RadioIR@charter.net>
>Sent: Feb 26, 2008 11:51 AM
>Subject: Re: [TowerTalk] Lightnig Arrestors at tower base or house entrance?
>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.
>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
>> 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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