At 75 feet from the entrance, my tower is close enough that it's all one
big ground system. I used to put arrestors at the tower base and house
entrance. I had to replace those at the tower often, but not the house.
Then I asked, Why? They both do the same thing (protect the equipment in
the station), but with a much higher voltage at the tower. The series
of pulses sees the coax as a big inductor and capacitor, automatically
reducing the voltage between the center conductor and shield. What
useful purpose do the extra arrestors at the base of the tower serve?
They certainly do not protect the coax as the voltage across the
conductors is highest at the top of the tower with a reduction by the
time it reaches the bottom. If it held up to that, the coax from the
tower to the house will see even less and fail less while still
protecting the equipment. The same for the rotator and control box.
I took out the arrestors at the base of the tower, but I rarely have to
replace ones at the house entrance/SPG. The cables run underground from
the tower to the house. There is inductance in those lines, but also a
large capacitance to ground. The capacitance is essentially a series of
parallel capacitors to ground so the voltage is lowered as you move
toward the house end. The coax jacket is grounded to the tower, top and
bottom. At some point the distance from the tower to the house becomes
great enough that they can be treated as separate entities. I believe
Polyphaser says >= 200 feet you no longer need to maintain a continuous
ground. I believe it said, needed, not that they must be seperate, but
if not needed, why go to the additional work and expense of doing it?
None that I can think of
It's only a single installation, but it appears to work for me.
73
Roger (K8RI)
On 1/13/2016 Wednesday 1:30 PM, Gary Schafer wrote:
Lightning can be thought of as a constant current source. A strike will
contain X amount of amps and it will conduct that amount of current to
earth. If there is resistance/inductance in the path, the voltage will rise
to whatever is needed to maintain the X amount of current.
So yes, leaving ungrounded coax cables laying on the floor is a bad idea.
The voltage will rise until it is high enough to create an arc to a path to
earth to dissipate the charge.
You most always should have two ground systems unless your tower is right at
the shack entrance.
The tower should have its ground system at the tower base, ground rods
radials etc.
Then you need a similar ground system at the shack entrance where your
single point shack ground system gets grounded. This needs ground rods,
radials etc.
You should always tie both ground systems together and to the power ground
for safety. The ground lead between tower and shack will dissipate some of
the lightning energy and help equalize voltages in the area but it should
not be a substitute for a ground system at either end.
Both ground systems are important as you want to discharge as much energy to
earth right at the tower as possible if the tower is struck or energy is
induced into it.
The ground system at the shack will take up a lot of what is left over that
the tower ground system did not take care of and it will dissipate a large
part of energy if a strike comes in on the power line or phone/TV etc.
lines.
Having all the equipment, power, coax etc tied together at the single point
ground panel keeps the potential the same on all lines as they enter the
shack. The fact that the single point panel has a ground lead a few feet
long to its ground system will allow some voltage rise there but all lines
to the equipment will have near equal voltage.
If the power comes into the shack from a ways away of the other side of the
house a good thing to do is run an ac line (from an outlet in the shack)
over to the single point ground panel where AC protectors are placed on that
line and then power all of your equipment only from that AC outlet on the
single point ground panel. That will place the AC power at the same level as
the coax lines.
73
Gary K4FMX
<snip>
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