On Fri, 19 Nov 2004 07:44:50 -0500, "Floyd Sense"
wrote:


"Jack Painter" wrote in message
news:zbbnd.1127$wa1.571@lakeread04...
snip
The coax shields grounded at the tower (min. at the bottom, best top and
bottom), and again at the basement entrance single point ground. Shields
must be grounded before connection to an arrestor.


Jack - regarding your comment on grounding the shields BEFORE connection to
an arrestor: My arrestors are mounted on the common ground panel and the
coax is grounded via the coax connector to those arrestors. What is the
reason for a separate ground prior to that one. Maybe I misunderstood
something, but it seems redundant to have a separate ground a few inches
from that one.

In response to another's comments regarding protection of the SteppIR,
rotor, and other control lines: I use MOVs and .01 bypass caps on all those
lines in a box at the base of the tower and have another set of the same at
the entry panel box. Those components are mounted via European-style screw
terminal strips (12 positions per strip) obtained from Jameco via the Web.
Much cheaper than the same from Radio Shack or other sources. MOVs came
from Mouser. A lightning strike last year entered my shack via relay
control lines which were unprotected at that time. Hopefully, the new
arrangement will help.

73, Floyd - K8AC




Floyd,

The real reason for grounding the coax shield at the entrance panel in
addition to having the protection device grounded is the voltage drop
between the connector and the coax line. The cable to connector
connection is usually not a good low resistance - high current joint
as far as lightning is considered. During a lightning strike you may
have considerable voltage drop across that junction.
Sometimes connectors are found to have been welded to the cable or
their threads welded due to lightning strikes because of the poor
connection at the connector. Other times the junction may get burned
open.
It can also melt the solder quickly in a soldered on connector which
would provide for an immediately poor connection.
However, lots of people do not do the extra grounding of the cable at
that point. Most are lucky if they get some sort of protection device
installed and a ground connected.

As Jack mentioned grounding the cable "at the bottom of the tower like
is used nation wide in tower designs" is ideal. But unfortunately that
is not how it usually gets done. Often the lines come off the tower at
6 to 10 feet above the ground to go to the building in a cable tray.
But it would indeed be best if they were taken all the way to ground
level before exiting the tower.

The reason being that during a strike the tower and associated lines
on it develop considerable voltage drop due to the high current being
conducted. Coming off the tower above ground is like taping a resistor
part way up from the ground end. Allowing more voltage to exit on the
lines rather than the potential at the base of the tower where it is
closer to ground. The tower usually has considerable inductance for
voltage to develop across.

Ideally lines should be grounded to the tower not only at the top and
bottom but at distances along the tower length as well. This is to
avoid flashovers that may puncture the line.

Lightning protection schemes are all about voltage drop. Most due to
inductance of the tower or other conductors carrying the current.
All conductors will have inductance and resistance and therefore
voltage drop if they are asked to carry lightning current. Keeping
things out of the middle of that path is the trick.

73
Gary K4FMX