John,
I don't know if I can help, but I'll tell you what I did. I laid 265 feet of
1/0 stranded bare copper wire at the bottom of a 4-foot deep conduit trench
and used it to connect the tower ground system to a ground rod at the house
that's connect to my single point ground.
The tower ground system consists of 12 ground rods connected to the base of
my US Tower MD-770MDP motorized rotating tubular tower. Four 36-foot runs of
1/0 wire radiate from the four base bolts at the bottom of the tower. Three
cadwelded ground rods are spaced about 12 feet apart along the wires (3 x 4
= 12.) The system is connected by 1/0 wire to the ground panel inside a
metal utility cabinet. Coax runs enter from the antennas enter the utility
cabinet via Polyphaser bulkhead coax lightning suppressors. The ground panel
inside has Polyphaser rotor suppressors for each rotor and switch cable.
There is also a run of 1/0 to a separate ground rod used to connect the
ground systems of other antennas near the tower. The long 265' run of 1/0 is
connected to the ground panel inside the metal utility cabinet. All the coax
and control cables exit the utility cabinet into PVC conduit runs (the grey
stuff used by electrical contractors, not the white schedule 40 used by
plumbers.) Conduit diameters are 4" (coax runs), 2.5" (rotor and control
cables) and 1" (220VAC for tower.)
The single-point ground is a matching metal utility cabinet mounted on the
side of the house. The coax, rotor and switch cables enter the cabinet
through the conduit runs and are attached to Polyphaser coax and rotor
suppressors. The cabinet has two large four-inch holes cut in the back to
pass the cables into the house. A short run of 1/0 connects the ground panel
in the utility cabinet to a nearby ground rod. The long 265' run from the
tower connects to this rod as well. A run of 1/0 inside the cabinet goes
through the wall to a copper bar mounted behind the radio desk in the shack.
All shack equipment is connected to this bar. Another 1/0 run goes through
the basement to the utility company ground at the main AC panel. I would
have preferred to make this connection outside, but the run around the house
would have required several hundred feet of 1/0 and many ground rods make
the connection. I figured there were so many shorter ground connections
between the shack and AC panel through the house wiring that an outside
connection would be ineffective and it would be better to make the
connection through a shorter, heavier wire through the basement.
Note that I used single and dual Cadweld one-shots on most of the
connections, but the multi-antenna rod at the tower and the single-point
ground rod at the tower required more than two connections. For this, I had
my Cadweld dealer order a custom reusable mold hat accomodates up to four
connections.
All that said, I have read (in Polyphaser's literature, I think) that ground
systems more than 75-feet apart do not "see each other" in a lightning
situation. This implies that my 265' run is not necessary. However, it may
serve some purpose as a long, horizontal ground rod! At least, that's the
way I like to think of it...
73, Dick WC1M
> -----Original Message-----
> From: John Dooley [mailto:w6zip@charter.net]
> Sent: Saturday, August 09, 2003 7:19 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Single point ground system connection to
> tower(??????)
>
>
> At the Polyphaser web site Engineering notes:
> "The tower ground system and the single point ground system
> must be interconnected. This interconnection should be below
> grade and with a bare low inductance conductor. The coax
> cable shield must not be the only interconnection between
> ground systems." My question is Does this imply I run a
> ground lead directly from my grounding grid at the tower 120
> ft. to the entrance panel and connect that with my electrical
> single point ground, then additional ground connection to the
> ground rod located at the back of my equipment? Very confused
> and can't seem to know if I will create a loop where all
> potentials will not rise and fall at the same time if I have
> a direct hit. Right now I'm installing my US Tower TMM-433
> approximately 120 feet from the garage entrance and see a
> need to create an entrance panel for the 1/2 hard line
> feeding the antenna at the tower. I plan to ground the hard
> line at the top of the first tower section and at the base,
> this will be tied to three ground rods around the outside of
> the tower form all interconnected by cadweld one shots. But
> does the technical article call for an additional ground line
> run over 100' to my entrance panel also. The plan is to
> ground the entrance panel to a ground rod at the feed line
> entrance to the shack and have this tied to the electrical
> single point grounding grid in the home. Somewhere I feel I'm
> missing the validity of having a "single point ground"
> because of all the measures I'm planning on taking, or just
> not experienced with a solid reliable system that I can draw
> designs from. Can anyone help me with this??? Thanks, John
> W6ZIP Victorville, Ca.
>
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