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#2
December 1st 03, 12:42 AM
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Roger Halstead wrote in message . ..
On 29 Nov 2003 02:35:22 -0800, (Mark Keith) wrote: (K9SQG) wrote in message ... That is not a wise idea. If your mast takes a direct hit, or if there is one nearby, there is a possibility that some of the energy will go into the house via that ground rod. Antenna grounds should always be isolated from housewiring grounds. If all grounds are at the same potential, no current can flow between them. Not to mention the NEC requires it in the U.S. MK When you figure the rise time there can easily be several thousand volts between the base of the antenna mast and the electrical system ground. IE the voltage could be 50,000 at the tower and not even have started to rise at the electrical ground. In my case, those are pretty much the same place. All my grounds tend to rise in potential at the same time, being they are all tied together at the base of the mast. Nothing is perfect of course, but you sure don't want any large potential differences between grounds. It's not just a simple CD circuit. I've seen tower strikes where there was such a strong current and fast rise time, the magnetic field quenched the current flow and the lightening got off part way down and jumped sideways to something else. If I remember right, the original poster was going to mount an antenna on a roof. That has the potential for even more serious problems if the antenna acts as a lightning rod. He'd want as few turns or sharp bends in the ground wire to earth. This is why I much prefer using a metal mast on the side of the house to support verticals or other high risk antennas, rather than a mast attached to the roof of the house. And then hoping a ground wire will safely direct the charge to earth...It usually will, at least much better than the house itself, but it's kind of scary if it has turns or bends. The important part at the house is making sure that everything rises together so there's no lower resistance path for the charge to take through the shack or house. At the mast, I always tape all coaxes or wires to the mast, and run them all the way down to earth to try to avoid flashing problems. So far in two strikes to the mast, I haven't noticed any, and I have a breaker box, elevated power lead in, phone lines, cable lines, all within 5 ft of that mast. MK |
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#3
November 29th 03, 03:03 PM
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