[NJARC] question regarding long wire antennas

Sat Sep 2 01:34:58 EDT 2017

Lightning Current
Williams says that a typical lightning bolt may transfer 1020 electrons in a
fraction of a second, developing a peak current of up to 10 kiloamperes. 
According to Uman, the German scientist Pockels discovered that basalt rock
in the vicinity of lightning strikes was magnetized and deduced currents on
the order of 10,000 amps in 1897. Ampere's law allows you to deduce the
current in a wire from the measurement of the magnetic field at some radius
from the wire. Pockels presumably had measured the magnetizing effects of
large currents on basalt and was able to scale those experiments to estimate
the current associated with the lightning. Based on that principle, magnetic
links are widely used for the measurement of the lightning currents. Most
measurements have been in the range 5,000 to 20,000 amps but a famous strike
just before the Apollo 15 launch in 1971 was measured at 100,000 amperes by
magnetic links attached to the umbilical tower. Currents over 200,000 amps
have been reported. 
One could envision a magnetic detector based on both Ampere's law and
Faraday's law which could give you an estimate of lightning current provided
you had a measurement of the distance from the detector to the lightning
strike point. If you set up a coil of wire in a vertical plane, then the
rate of change of magnetic field through the coil would generate a voltage.
If you could sum (integrate) the current generated by that voltage, you
could calculate the charge transferred in the lightning strike. With several
such detectors in an area, you could model the location as well as the
charge associated with the strike. 
Most commonly, the lightning current ceases in about a millisecond for a
given stroke, but sometimes there is a continuing current on the order of
100 amps following one or more of the strokes. This is called "hot
lightning" and it is the cause of lightning fires according to Uman. The
temperatures of lightning are 15,000-60,000°F for both "cold" and "hot"
lightning - it is the continuing current that starts some 10,000 fires per
year in the U.S. in the estimation of Uman.

The above is both in one of my text books and at one time I also found it
online.

I am an electric utility engineer and have worked in EHV electric
transmission design and in utility operations.  When lightning strikes
hardware on poles or houses in electric distribution, it usually rips the
conductors to pieces or at least rips it loose from its mountings on the
poles or buildings, leaving a charred track where the cables had been.
Utilities employ expulsion arrestors (which operate differently than gap
static arrestors) to protect such facilities and they do some of the time.
However, let's think about even #10 or #12 wire in the face of the amperages
involved.  These conductors don't stand a chance with that kind of energy,
even for milliseconds.

I did experience one case out on the farm where I had a radio and an antenna
which was hit with a strike.  I had previously lost a couple of antenna
coils in radios and was unable to explain that the radio that experienced
the hit was OK after the strike.  I am guessing that the strike was at the
windmill end (175 feet away) and the antenna wire simply melted or vaporized
before the currents could travel the length of the antenna to the radio.

The one thing of which I am certain, however, is that in 45 years in the
utility business, I have run across many, many unexplainable incidents
involving lightning.  The most fun to watch was seeing a lightning strike to
ground where the lightning actually crawled along the surface of the ground
for a couple hundred feet before it disappeared into the ground.  I was
sitting on a tractor about 1/8 mile away at about age 16.

I am not familiar with the unit you pictured.  The arrestors I used were the
old porcelain gap type from Allied, BA, McGee and others back in the 1960s.
However just from the physical size, these units are not dealing with the
currents of the scale of lightning strikes.

Gerry

-----Original Message-----
From: Chuck [mailto:cpaci1 at verizon.net] 
Sent: Friday, September 1, 2017 9:45 PM
To: Gerry Steffens; 'Joe Giliberti'; 'New Jersey Antique Radio Club'
Subject: Re: [NJARC] question regarding long wire antennas

Hi Gerry,
I'm not an electrical engineer like yourself, but my experience of the
extent of the spark arrestor's capabilities went a little further.
I am a software engineer and my knowledge of the subject comes only from my
limited experience.
My antenna took a direct strike from a HUGE bolt of lightning, according to
my neighbor who saw it happen. It felt like a truck hit my house!
The antenna is mounted on a 25 foot aluminum flagpole mounted at the top of
the gable of my house and is the highest object in the area at 48 feet high.
So I guess it's a prime target for a lightning strike!
Somehow the spark arrestor saved it as well as the radio, even though the
radio was connected to the antenna at the time.
The antenna was not trashed and the radio was still intact. Go figure??? I
would think it should have all been burnt to a crisp.
I only had damage to the spark arrester itself! After changing it out for a
new one, I found, to my surprise, everything still works fine???
I guess I just got a really good spark arrestor - - - or I was really lucky!
I attached a picture of the spark arrestor I had (and replaced it with), in
case anyone one would like to get the same one for their antenna.
Now days, I keep a spare in the shop - lol

Chuck

-----Original Message-----
From: Gerry Steffens
Sent: Friday, September 01, 2017 5:09 PM
To: 'Joe Giliberti' ; 'New Jersey Antique Radio Club'
Subject: Re: [NJARC] question regarding long wire antennas

Just remember
Reply = Poster
Reply All = Everyone

_________________________________________________________
The analogy to a spark plug is a good one.  However, maybe I am just a
wording junky but, if lightning strikes your antenna, the antenna is trashed
and so is the radio, TV or what have you.  The function of a (miss-named)
lightning arrestor is to bleed off static charges which can develop by wind,
rain, snow, etc. blowing or falling by the antenna.  The thinking, by some,
was that the charged antenna attracts lightning, maybe so maybe not, I am an
electrical engineer, not a meteorologist and more.  But, static charges and
sparks can and do wreck antenna coils in radios and more.

My first time, taking arrestors seriously was in the early 1950s as a kid
with a shortwave radio and antenna (Allied supplied an arrestor with the
antenna kit).  I was sitting in the living room and I kept hearing a snap
every few seconds.  After much searching for the cause, I found it.  The
night before we had experienced the beginning of a pretty wild storm (snow
storm - ice crystals and high wind).  Dad had disconnected the TV twin-lead
from the TV set and even unplugged it 'cause up front there had been a
couple of lightning flashes.  The snap was a static spark between the
exposed wires of the 300 ohm twin-lead of the TV antenna.  The next time at
the auto garage where dad worked I got to talk with the radio repairman
employed there.  He explained the definition of "snow static".  I went home
and installed the arrestor with a good ground!

-----Original Message-----
From: njarc-bounces at mailman.qth.net [mailto:njarc-bounces at mailman.qth.net]
On Behalf Of Joe Giliberti
Sent: Friday, September 1, 2017 7:44 AM
To: New Jersey Antique Radio Club
Subject: [NJARC] question regarding long wire antennas

Just remember
Reply = Poster
Reply All = Everyone

_________________________________________________________

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