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Ground antenna?
Was talking with someone the other day and mentioned that I had just
grounded the antenna (the mast, actually) to protect against lightning strikes, and they said that was not such a good idea because lightning is more likely to strike a path that goes straight to ground. Now I am not sure what to do. Anyone have any input on this topic? Ideas are gratefully received... Thanks, Dave |
Ground antenna?
On Thu, 15 Oct 2009 08:42:05 -0500, "Dave" wrote:
they said that was not such a good idea because lightning is more likely to strike a path that goes straight to ground. Hi Dave, Did they happen to mention what a great deal it was for that lightning to go directly to your radio instead? I suppose their next argument is that lightning won't strike an "insulated" antenna.... 73's Richard Clark, KB7QHC |
Ground antenna?
In message , Richard Clark
writes On Thu, 15 Oct 2009 08:42:05 -0500, "Dave" wrote: they said that was not such a good idea because lightning is more likely to strike a path that goes straight to ground. Hi Dave, Did they happen to mention what a great deal it was for that lightning to go directly to your radio instead? I suppose their next argument is that lightning won't strike an "insulated" antenna.... One reason for lightning conductors (and for grounding elevated conductors, like radio antennas) is that it helps to stop a high electrostatic charge from accumulating in the air immediately above them. The intention is to PREVENT a direct lightning strike, rather than conduct a strike to ground. Of course, if a direct strike DOES occur, an antenna (and even a stout lightning conductor) may be seriously damaged. -- Ian |
Ground antenna?
On Oct 15, 10:54*am, Ian Jackson
wrote: One reason for lightning conductors (and for grounding elevated conductors, like radio antennas) is that it helps to stop a high electrostatic charge from accumulating in the air immediately above them. The intention is to PREVENT a direct lightning strike, rather than conduct a strike to ground. Of course, if a direct strike DOES occur, an antenna (and even a stout lightning conductor) may be seriously damaged. -- Ian The only problem with that is that the charge is so quickly replenished that I think trying to bleed off the charge is a waste of time. Note the "brush" type conductors that supposedly dissipate the charge to avoid a strike. They don't really work too well, and when they do get struck, often you have blobs of molten metal flying about. The way I see it, the only way to really prevent a strike, and even then I think it's iffy, is to provide a shape that does not stream well. And then have a lightning rod which does stream well to take the strike vs the object that does not stream well. IE: a rounded ball on top of a mast does not stream as well as a sharp tip. This is why most flag poles have a ball on top. To reduce the likelihood of streaming. And even those can still be struck under some conditions. So I think trying to avoid a strike using such methods is more wishful thinking than anything. I look at it in a different way. I expect it to be struck, and it will if it's any decent amount of distance above the surrounding objects. It might take 6 months, or it might take 6 years, but it will be struck some day. Count on it. A strike does not know where it's going to hit until about the last 150 yards or so. Then the streamers from the ground are all pointing towards the down leader trying to entice it. Usually the strongest streamer in the area will attract the strike. And even that is never a sure thing. :/ So the best way to deal with lightning is to expect it to strike sooner or later, and to provide the most efficient path to ground when it happens. A well grounded mast DOES NOT attract lightning any better than a non grounded mast at the same location. That's horse caca, and the OP can tell his friend I said so. They will stream the same, and I consider the chances of a direct strike about equal. It's when you actually take a strike is when the differences in grounding show up. The well grounded mast will take the charge to ground in an orderly military manner, and in most cases no damage will occur. The non grounded mast does not provide an orderly path to ground, so the strike takes whatever path has the least resistance. And even that path is likely to be fairly high resistance and fireworks are likely to occur. Houston, we have a problem.. :/ The OP did the proper thing by grounding his mast. If properly grounded, if he does take a strike, the mast will direct the charge to ground and little damage should occur. This does not mean you want a radio hooked up though. That takes extra measures like suppressors, ground window, star grounding, etc.. But at least he won't have to worry about burning the house down. I've had my mast directly struck more than once, and I had no damage at all. Period. But my feed lines were all grounded to my bulkhead outside the shack. I don't leave them connected as I use no suppressors. Two of those strikes were with me sitting in this chair 15 feet away from the base of the mast. You can actually hear the difference between a strike to a well grounded mast, and a strike to a poorly grounded object. The strike to the well grounded mast will be very quiet. Just an arc sound like throwing a light bulb on the ground. A strike to something poorly grounded like say a tree will be much louder. More like a crack from a shotgun. :( This is not to be confused with the overhead sonic boom which both types of strikes will make. I'm just talking about the local sound. Anyway, that's my $12.47 worth from someone that lives in lightning country and has taken multiple direct strikes though the years. If the OP wants some good lightning info, try searching for Gary Coffman and lightning on google. I don't know if his past posts are still archived, but they should be. He had a lot of good posts on various aspects of lightning protection. |
Ground antenna?
Dave wrote:
Was talking with someone the other day and mentioned that I had just grounded the antenna (the mast, actually) to protect against lightning strikes, and they said that was not such a good idea because lightning is more likely to strike a path that goes straight to ground. Like many things, "it depends".. what else is near the antenna? From an electric field standpoint, whether it's actually physically connected to the ground isn't all that important. It's that it's a conductor sticking up, and that causes the field to change. As the surrounding E-field changes over a time span of milliseconds, a mast connected to the ground vs isolated will have a slightly different voltage distribution over time, but over long times, it's not much different. Now I am not sure what to do. Anyone have any input on this topic? Ideas are gratefully received... Thanks, Dave |
Ground antenna?
A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location. actually it does. both the ieee and cigre have been using lightning statistics data collected mostly from tall masts for many years. There are well known formulas used to calculate the number of strokes to a pole or power line, both include the height, and as height increases so does the number of strokes to the object. The height also skews the current distribution with higher structures more likely to get more high current strokes. |
Ground antenna?
Dave wrote:
A well grounded mast DOES NOT attract lightning any better than a non grounded mast at the same location. actually it does. both the ieee and cigre have been using lightning statistics data collected mostly from tall masts for many years. There are well known formulas used to calculate the number of strokes to a pole or power line, both include the height, and as height increases so does the number of strokes to the object. The height also skews the current distribution with higher structures more likely to get more high current strokes. But, is there a difference in strike rate between grounded and ungrounded towers of the same height. I would think that the difference would be very small, and smaller as the height gets bigger. Since the vast majority of commercial masts,towers,buildings used to collect the stats are probably grounded (Because the code requires it...), it might be hard to find decent data for "ungrounded" things. (for one thing, the equipment used to collect the strike data, until recently, probably measured the current spike on the grounding wire.. these days, you could use the RF lightning detection systems, and match up strike locations against structure locations) Maybe wooden poles? (which are only "sort of grounded") |
Ground antenna?
On Oct 16, 5:13*pm, Jim Lux wrote:
Dave wrote: A well grounded mast DOES NOT attract lightning any better than a non grounded mast at the same location. actually it does. *both the ieee and cigre have been using lightning statistics data collected mostly from tall masts for many years. There are well known formulas used to calculate the number of strokes to a pole or power line, both include the height, and as height increases so does the number of strokes to the object. *The height also skews the current distribution with higher structures more likely to get more high current strokes. But, is there a difference in strike rate between grounded and ungrounded towers of the same height. *I would think that the difference would be very small, and smaller as the height gets bigger. Since the vast majority of commercial masts,towers,buildings used to collect the stats are probably grounded (Because the code requires it...), it might be hard to find decent data for "ungrounded" things. (for one thing, the equipment used to collect the strike data, until recently, probably measured the current spike on the grounding wire.. these days, you could use the RF lightning detection systems, and match up strike locations against structure locations) Maybe wooden poles? (which are only "sort of grounded") 'sort of grounded' is as good as grounded. The current that builds up the charge to initiate the upward streamer is relatively small, that can be seen in the use of the high value resistors commonly used to 'bleed off' charge from antennas. instead of bleeding it off they are actually just equalizing it with the local ground potential, the same as happens on a 'properly' grounded tower/vertical. in most cases you won't find an amateur tower that isn't grounded one way or another anyway, even if a specific ground rod or other system isn't supplied at the base there is still a decent ground through the foundation. and if not then there is through any cable going up the tower that connects to a rotor or most antennas. it takes real work to really insulate a tower from ground in such a way that it would not allow charge attracted to the area under a downward leader from moving up the tower... the large rf isolators and tower lighting isolators used on base insulated AM broadcast towers are a good example... and even on those types of towers you will hear reports of 'lots of snapping and popping' before a stroke, those sounds are small flashovers of the guy wire insulators as the charge flows up them toward the tower. So to do it right would mean using insulating guys also. Then, even if you got that far a tower of any height would end up going into corona at the bottom and top anyway due to the high vertical electric fields under a storm cloud... even if the corona didn't reach nearby ground conductors it would still collect/dissipate charge on the conducting vertical section which may still initiate a streamer. So the short answer is that i have not seen any comparison between grounded and ungrounded vertical structures. |
Ground antenna?
On Oct 16, 12:13*pm, Jim Lux wrote:
Dave wrote: A well grounded mast DOES NOT attract lightning any better than a non grounded mast at the same location. actually it does. *both the ieee and cigre have been using lightning statistics data collected mostly from tall masts for many years. There are well known formulas used to calculate the number of strokes to a pole or power line, both include the height, and as height increases so does the number of strokes to the object. *The height also skews the current distribution with higher structures more likely to get more high current strokes. But, is there a difference in strike rate between grounded and ungrounded towers of the same height. *I would think that the difference would be very small, and smaller as the height gets bigger. That's what I'm thinking. I know my well grounded mast is not a lightning magnet. Some years I get no strikes to it. In fact, I think it's been a few since the last one. Well, unless it was hit when I was not at home. I'm still of the opinion that the ability of an object to stream means more than if it's grounded or not. Airplanes are not grounded, and they get popped all the time. Trees are poorly grounded, and they get popped all the time. I've known quite a few people over the years that stuck a vertical on their roof and didn't ground it, and they got popped. Blew the ungrounded antenna to smithereens too.. Many golfers get popped on the golf course and they are not really grounded too well unless they are wearing spikes on their shoes. :/ I think the height of the object relative to it's surroundings, and it's ability to stream mean more than anything. Trees are a target lightning pick probably as much as anything, and most are fairly poorly grounded due to being wood with some moisture to helps things a bit. I think the trees ability to stream well is why they get picked on so often. The pointy ends of a leaf stream well, much like the pointy hairs on the head of a golfer. And when you have hundreds of leaves on a tree... :( But lets say a grounded mast does get struck slightly more than an ungrounded mast. That would not a logical reason to avoid grounding it, when the act of grounding the mast pretty much negates the likelihood of a strike doing much damage to the mast or the building next to, or under it. A wooden mast would act much the same as a tree if it did not have any kind of ground wire running along it's length. That's why I never use wood masts here. I'd rather have a grounded mast struck every 5 years with no damage, vs an ungrounded mast struck every 10 years that led to heavy damage or even burned the house down. So worrying about that is kind of silly I think, when you know an ungrounded mast is big trouble if it ever does get hit. |
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