[TowerTalk] Dealing with Lightning and MCI

[Tom Rauch]

Excuse the stttering post yesterday. MCIONE does that if I post at 
busy times of the day, I have no idea why.


> Date:          Mon, 11 May 1998 23:46:39 -0700
> From:          Stephen Vinson <kd4wiw@ipass.net>

Hi Steve,

Let me ask a few questions.

> Mick ... I read your post with interest.  I have a 145' tower with 12'
> mast and 12' antenna mounted above it.  I have installed PolyPhasers on
> all feed lines and installed a PolyPhaser strike counter.  I even
> extended the leads longer than recommended which should allow me to
> measure near strikes.  To date (after several severe storms) I have no
> recordings on the counter.  

 I have a nearly 200 ft tall tower, and nothing on top or sides. The 
tower is insulated for dc except for a 100 k ohm HV bleeder 
resistor,  and the guy lines are insulated also. To date (after 
several severe storms) I have not had a single hit either, despite 
being on the highest area for miles (about 200 feet ASL higher than 
I-75 five miles away.

When we deal with random events, it is a statistical analysis 
problem. You would have to accumulate data for years, change systems, 
and accumulate data for many more years to prove any trend....all 
while maintaining the integrity of the test site from any other 
changes. 

Not getting hit, or getting hit, proves absolutely nothing unless you 
do the A-B test in a controlled environment. That's why the FDA 
requires fake pills in drug tests, and a control group.  

> I noticed that you have done the normal grounding setup by most amateurs
> but (and I might have missed it) I did not hear you say you had bonded
> all tower sections together.  That might be why your Indonesian friends 
> tower has not suffered a strike.  Try bonding the tower sections to
> gether at the joints.  Use stainless bands and straps and use a
> electrical antioxidant to treat the connections.  

Steve, what do you suppose the resistance of the leg joints are? Have 
you measured them? 

What about all that pressure on the bolts? If the downward force of 
the tower is 3000 lbs, and the bolt surface area supporting that 
sheer is 0.15 square inches, the pressure is 20,000 lbs per square 
inch.How much resistance remains in connection of that pressure, 
compared to the resistance of the air between the earth and the cloud 
overhead? 

> tower is struck due to the charge it holds.  Grounding lowers the
> potential due to static charge build up, but only if a low resistant
> path is present to bleed off the charge.  

You imply the tower "charges up", probably because you hear others 
say the same thing. 

Voltage drop is a function of resistance and current. E=I*R

How much current do you suppose flows into the tower? 

Let's assume the tower has a resistance of 1 ohm from end to end, and 
1000 ohms to earth. How much current has to flow in the path to hold 
the tip of the tower at 1000 volts potential more than the earth 
surrounding the tower?

The tower would require just under one ampere of current 
CONTINUOUSLY to have a 1000 volt charge!

Is such current possible except during an actual strike?

> I don't profess to know alot about grounding.  I my self am learning but
> one of our local repeaters will get very noisy (electrically) when the
> joints oxidize in the joints.  Bonding the sections reduces this build
> up.  

Repeaters get noisey from poor connections because they have 
transmitters. The transmitter excites high current levels in the 
conductors around it, and any moving or arcing metal to metal joints 
cause noise from the tiny sparks. You can hear this effect by shaking 
a set of keys next to a repeater antenna. If the transmitter is on, 
the receiver will hear noise from the keys arcing. If you shut the 
transmitter off, you will hear no noise at all.

That's why repeaters near crummy joints often come up, de-sense, 
chunk off, and come up again, on threshold-level  signals. Their own 
RF excites the metal around the antenna, and supplies the energy for 
the tiny poor connection arcs.

It is the worry of those poor connection RF induced arcs from the 
tower joint rocking and sliding that causes broadcast tower design 
engineers to have installers bond leg joints. It has nothing to do 
with power loss, or lightning. 

73, Tom W8JI
w8ji.tom@MCIONE.com

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