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On Sun, 21 Nov 2004 23:06:45 -0500, "Jack Painter"
wrote: "Brian Kelly" wrote Gary Schafer wrote No that won't do much good. If you ground the coax shield to the tower where it bends away from the tower you will have a much better (lower inductance) to ground with the tower than what the wire would provide. The wire would do almost no good at all when compared to the much larger tower in parallel. This is correct, and why I mentioned even 6" was "too much". It can be significant. Especially on a smaller tower. It took a few seconds to get your point but yes, it's a matter of how far up the tower the coax departs the tower as a percentage of the tower height. The last was not a correct assumption. The distance across a conductor (and in this case it is also the distance to ground) is what allows inductance to create deadly voltage potentials. Any conductor in series with a lightning strike will exhibit the same characteristics. 6" above ground near the base of a tower can translate to as much as 9800v above ground, with just modest assumptions of a very average return stroke current of 25Ka with a rise time of 40Ka/usec. It has no bearing whatsoever how tall or short the tower is. It's not long (or high above ground) before you could see over 100,000v potential develop where coax leaves any tower too soon. A little clarification here. When I said "smaller tower" I was not necessarily referring to a shorter tower but one that has less surface area. (smaller face) The main consideration is the distance up from ground that the cables leave the tower. A lightning strike is a constant current source. If it is a 20ka strike the voltage across whatever it hits is going to raise high enough to conduct 20ka. If you have a low impedance conductor (tower) the voltage developed across it will be less than it would be on a high impedance tower (smaller face tower). That is why large communication towers have less problems with lines coming off at higher points on the tower. More of the strike current makes it to ground via the tower with the larger surface it has. Leaving the tower at some height above ground with the cables is still a division of the voltage like a voltage divider. The higher up you are the higher the voltage you will see with respect to ground. But what determines what that actual voltage goes to is the amount of strike current and the amount of inductance between the cable exit point and ground. Of course the cables leaving the tower will also carry part of the current too. Bury it along with the cables. That will give you more contact with the earth as well as tying the grounds together. The wire will be there but I doubt that I'll be able to bury it. Burying a grounding electrode conductor is normally a code requirement. But that is not what you have in connecting the tower ground system to the station ground, AC mains ground, etc. Those are bonding conductors, and they are in many cases required to be insulated. Not in this case, but I want you to understand the difference between grounding, voltage division from many grounds, and a bonding conductor between your station and the tower. The latter is to maintain equipotential, and will not carry more than just equalizing currents. It will be well within the capability of a #6 insulated wire, should you choose to use that. Personally I would go a little larger but #6 is as largest that NEC or NFPA recommend for a bond in *most* cases. So burying the bonding conductor is not a requirement, although to protect it that is exactly what most facilities do. Neither will burying coax feedlines help in lightning protection, unless you are counting on them by design to be grounding electrode conductors! Pretty foolish but heh, if someone tosses feedlines out a window, they may as well short them to a ground rod and "bring it on". In that case any more than about 5,000v will breakdown the dialectric both inside and outside the coax, and anything nearby may be the next target before it ever reaches the ground rod. It doesn't matter what you want to call a ground conductor. The point here is if it can carry any lightning current you are much better off with it buried in the ground. A bare ground conductor making contact with the soil acts like additional ground rods. Why would you not want that? Burying coax feed lines will help with lightning protection. It greatly increases the inductance of the lines to lightning. It also helps to dissipate the energy to ground by the coupling provided. (ie you get less at the other end) You can't help but view them as "grounding electrode conductors" as you may want to call them. After all they are connected to the tower. They are going to carry lightning current if you want them to or not. Might as well let them dissipate part of the energy to earth. A large part of the lightning is RF. You have to treat it as such. A good lightning ground also makes a very good antenna ground system. ( buried radial system) Think in those terms. 73 Gary K4FMX The good news is that the soil is eternally damp highly conductive dark loam . . Gary K4FMX That is very good news, and it makes your job easier. But good soil or poor soil, understanding what bonding provides is equally if not more important than having a ground rod at all. To rest on the laurels of highly conductive soil and ignore bonding, would be inviting disaster. Yes commercial tower design does require many shield "bonds" along the height of towers, but as I said, I applied a reasonable approach which the average short tower or mast-only owner could and would be likely to comply with - bonding at the top, bottom and station entrance. I suspect few go even that far. You may or may not be interested in all the surge protection diatribe in my website, but it's there because so many unfortunate souls were mislead in this area. I do think you might benefit from it's coverage of what bonding does to protect both you and your station, and it is a lot harder for most to get a hold of then simple mast or tower grounding. It doesn't have to be. http://members.cox.net/pc-usa/station/ground0.htm 73, Jack Painter Virginia Beach, VA |
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