[BC] AM transmitter lightning sensitivity issue

[Ron Nott]

To solve the mystery (paragraph 2, below) of distant storms, be aware 
that the electric field gradient may extend horizontally outward for 
50 miles or more.  (Imagine iso-electric lines parallel to the 
surface of the earth and over the stuff mounted on it).  The field 
voltage can be measured with an electric field mill or meter which 
measures the vertical field in volts/meter of elevation.  When a 
lightning strike occurs at a distance, the DC field voltage will 
change instantaneously.  During the field buildup that occurs prior 
to a thunderstorm, everything isolated from ground such as guy wire 
segments will take on this charge partly from free ions in the air. 
Then when the sudden voltage change occurs, they instantaneously 
discharge into the nearest electrical conductor.  The sequence is 
like this:  The tower is DC ground, but the uppermost guy segments 
charge from the electric field to several thousand volts.  Then when 
the distant strike occurs and the voltage shifts downward, the highly 
charged uppermost guys dump into the tower.  Each succeeding segment 
below then follows suit, allowing all of them to dump into the 
tower.  This appears at the antenna input as a sudden change in 
impedance which is reflected back to the transmitter.  Its protection 
circuitry sees it and dumps the transmitter as it was designed to 
do.  A sensitive protection circuit can sense a relatively small 
discharge causing only a small impedance change for a few 
milliseconds, but its job is to protect that transmitter from a bad 
load (mismatch) and it does it. (Impedance mismatch?  DUH - 
yes.  Thats how they work).

Concerning the Lightning Protection Institute, on the surface it 
appears to be a worthwhile organization with an impressive 
website.  However, who are they really?  Under their membership list, 
you will only find companies that are in business to install NFPA 
type lightning rods.  In other words, they represent the 
manufacturers of this method and people who sell them.  The NFPA has 
settled on the system that includes a bunch of pointed copper rods 
over rooftops connected together with heavy conductors assuming that 
if a rod takes a lightning strike, the energy will be conducted to 
ground.  It is apparently a good system, but will not fit on top of a 
tower.  You may assume that tests have been conducted on this system 
to certify it, but not so.

Several years ago, a manufacturer of a multiple point dissipation 
system applied to NFPA to have their equipment listed.  The NFPA 
lightning protection committee is composed of representatives of the 
manufacturers of the NFPA system.  So they denied the listing based 
on the fact that the multi-point system had not been scientifically 
tested.  The manufacturer replied by saying that the conventional 
NFPA system also has not been scientifically tested.  The NFPA 
committee had to acknowledge that this was true and there was quite a 
haggle.  However, in the end they continue to endorse the 
conventional method and deny the multipoint method based on two 
factors:  Money and petty politics.  To sum up, I suggest that you 
take info from the Lightning Protection Institute with a big dose of salt.

Without scientific testing (which will be prevented from happening) 
the only proof is in the pudding.  Talk with fellow engineers and 
ask, does it work? Reach your conclusions this way rather than 
depending on an organization that is merely a pimp for the NFPA 
system manufacturers.

Ron Nott

----- Original Message ----- From: "Gregory Muir" <gmuir at cherrycreekradio.com>


>On the other hand, transient - and I stress transient - events are 
>the real killer here.  The normal situation places the actual 
>lightining discharge points miles away from the site.  Usually 
>storms within the range of 15 miles distant will start the 
>problem.  That's where the mystery begins.