[BC] More thoughts about AM transmitter lightning

[RichardBJohnson at comcast.net]

There are several problems about lightning protection
that should have been addressed when the tower(s)
were first installed. If you have a directional array, and
the towers were not properly installed for lightning
protection, it is going to cost somebody some time
and money to redo everything. assuming a conventional
series excited tower, the following things need to be
done BEFORE antenna measurements.

(1) The tower tuning box or its chassis inside
a tower tuning house, needs to be properly
grounded to the straps which are supposed
to have been installed around the tower.
There should be two such straps connected
to different points to minimize the inductance
of the connections by forming a shorted turn
of any loop so created.

(2) A proper ball-gap needs to be installed
across the base insulator. It needs to be
adjusted for about an inch of spacing, slightly
greater if there are nuisance arcs. Note,
dry air at sea level holds off 30 kV per centimeter.

(3) A one-turn loop of copper tubing of
a diameter sufficient to handle the tower
current , about 10 inches in diameter, should
be incorporated into  the connection between
the tower base to the the tuning box. Since
the inductance of this loop becomes part
of the tower impedance, it needs to be
installed in a very solid manner so things
don't change. In other words, you don't
use some "super-flex" battery cable,
for this connection as I saw in one
installation!

(4) At the point at which this copper tubing
conductor enters the tuning house or tuning
box, probably through a bowl insulator, a
Jacobs ladder arc gap should be installed.
This needs to be in free air, never inside
anything.

(5) Just inside the tuning box, one needs a
static-drain choke. This can be part of a
tower lighting choke if, and only if, the lighting
choke has three conductors.

(6) The transmission line running to the
tuning box should follow the path of one
of the ground-straps and be as close
to the ground (preferably buried) as
possible. Cables from monitor-loops
need to follow this same installation
method.

(7) If the tower has a beacon on its top,
it needs lightning protection. Most "lightning-
proof" beacons (they have a metal structure
on top) will not survive a direct hit. The
glass gets broken. You need at least a
diverter (it looks like a lightning rod) or
a static dissipation device that sticks
above the tower light. The tower guys
sell these things --and you do need
them even in Alaska.

After you make a new installation, while
the transmitter is still in warranty, you should
deliberately create arcs at the two arc gaps
with the transmitter modulated and running
full power. You can use a long screwdriver
to create the arcs. Both should self-extinguish
and, ideally, the transmitter should stay on-the
air. In any event, no damage to anything should
occur.

If your tower lives in an area of low humidity and
wind-blown dust, sand, or snow, you may need
resistors installed across each of the guy
insulators to discharge the voltage developed
in the insulated sections by triboelectric effects.

--
Cheers,
Richard B. Johnson
Read about my book
http://www.LymanSchool.org


 -------------- Original message ----------------------
From: cld at admin.umass.edu
> I don't know if this has been mentioned, but I understand that it is important
> that the balls not be painted. Check to see if some tower company painted them
> in the past. Please feel free to correct me if this isn't a concern.
> 
> Chuck Dube
>