On 8/7/2013 1:56 PM, Jim Brown wrote:
On 8/7/2013 9:59 AM, Earl Morse wrote:
Whether you run the conduit to the top of the tower and put your
polyphasers at that point OR run the conduit to the base of the tower
and place the polyphasers at that point OR run conduit through the
house and place the polyphasers at the entrance point its all about
providing that alternate path for lightning that doesn't pass through
your hamshack.
It DOES matter where Polyphasers are located -- their function is to
protect the equipment to which they are connected, and they work by
shorting the center conductor of the coax to the shield. For that
reason, they should be as close to the equipment as practical.
That is VERY separate from the issue of grounding, which is the most
critical aspect of lightning protection. I think most experts would
advise that the cables go down the tower all the way to its base, be
bonded to the tower at the top and bottom, and go along or in the ground
to the house, where they should be bonded to the building ground, then
come up to the shack. There must also be a bond from your shack ground
to the building ground.
Another aspect of grounding the shield at the top and bottom of the
tower as well as at your SPG at the base of the house is the
considerable capacitance from the center conductor to ground/shield in
that length of coax. I believe this tends to bypass much of the pulse
coming down the line "before" it reaches the rigs, so theoretically
it's not only better for the rigs but reduces the work the Polyphasers
have to do.
73
Roger (K8RI)
The tower itself must be grounded at its base, both to it's own concrete
base (a Ufer ground) and to at least three rods, usually each bonded to
a leg. This provides a short (low inductance) path to earth for a
strike, which minimizes what is coupled to your home.
Remember -- lightning is not a DC event, it is an RF event, with its
energy broadly centered around 1 MHz, and extending for at least one
decade of frequency above and below. (that is, 100 kHz to 10 MHz). So
what matters most is the inductance and the routing of the grounding
conductors, not their resistance. And remember also that lightning comes
into our homes from the power system, the telephone system, CATV
systems, and on the wiring in our homes, so all of this stuff must be
bonded together and to that central ground, usually at the point were
power comes into the house.
Finally, radios do not need "an RF ground" to work, to be safe, or to
minimize noise or RFI. But they DO need proper bonding, as I've
described for lightning safety, and proper bonding can often help with
noise and RFI.
73, Jim K9YC
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