[TowerTalk] Lightnig Arrestors at tower base or house entrance?

Tue Feb 26 15:12:00 EST 2008

How tall is your tower? My family lives in NE Florida and are always trying to get me to move there...

-----Original Message-----
>From: K4SAV <RadioIR at charter.net>
>Sent: Feb 26, 2008 11:51 AM
>To: towertalk at contesting.com
>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.
>
>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
>>
>>
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk