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#21
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Minimum gauge wire for connection to ground rod
On Aug 3, 1:52*pm, "KJ4NTS" wrote:
On *3-Aug-2009, Michael Coslo wrote: http://tinyurl.com/ns7jjd Is a nice station grounding pdf from Polyphasor. Sorry for the tinyurl, the original url is a mile long. Everyone should have a copy, and the writer should get an award for his or her clarity. That's a good article, but it is focused on grounding for lightning protection. *I am interested in that, but I am also interested in having an effective RF ground. I've been able to get a total of 16 feet of ground rod down in the form of 4 four-foot rods, but I have a 50 foot run from my office to the ground rods, and I'm concerned about RF impedance in that connection. I ran attic mounted antennas for a period time and here is my experience. First, don't bother running any ground to the antenna, especially at 50 feet long. It will act more like a radiating element than a ground, then you will get to meet Mr. RFI as you will discover all types items your RF can affect. Since your antennas are in the attic, static and lightning should not be an issue, unless you or a close neighbor takes a direct hit. I keep everything disconnected unless I am operating, it is the only way I know of to keep lightning from destroying your station. If you or your neighbor takes a direct hit, there is not much you can do to prevent damage, unless you have everything disconnected, AC/Ground/ Antennas/Control lines/etc. Second, your four foot ground rods are about useless for both RF and DC ground. Minimal is eight feet. My current station has 7 eight foot ground rods, spaced no more than eight feet apart, and all grounds are tied together, including the panel box ground. This is done to make sure all grounds are at the same potential. You did not describe your station so the information someone can provide is limited. In my case, my station was on the second floor, so instead of having a ground, I used a tunable counterpoise system as my RF ground. MFJ makes the tune boxes, and Ten-Tec may still sell them in kit from too. I used the one from Ten-Tec, it worked well at my location, but it still was a compromise, when compared to having a true ground system and outside antennas. 73 |
#22
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Minimum gauge wire for connection to ground rod
"Art Unwin" wrote in message ... On Aug 3, 4:51 pm, "Dave" wrote: "Art Unwin" wrote in message i don't know where you get your radios, but in all of them here the coax connector is connected to the transmitter chassis which also has a lug for going to the safety ground. By golly, I believe you are correct! I thought that was a bad practice! Are all electronics at the same potential in the U.S. as its container? So with a ground connection at the electrical supply point and a provided connection for another generated "ground", is that not an invitation for ground loops? yes, it is. that is why your 'shack' ground has to be connected to the service entrance ground. Lightning tries to get to the true ground by the shortest route and the requirement is for you to not be part of it's route, even if your radio has to be the sacrificial lamb if you haven't pulled the plug. there ain't no such thing as 'true ground', tis all relative. H'mm, the ground point with the lowest potential relative to other ground potentials in the same system is the "true" ground of the system. you have to define the 'system'. but there is always the possibility that someone outside the 'system' can reach in and measure your 'ground ' connection relative to something else and measure a potential. potential is what is relative, hook a 12v battery to a meter and set it on a table, connect another 12v battery between a ground rod and one terminal of the first battery... does the meter reading change? what is the potential of the positive terminal of the battery? if you were sitting on the table and couldn't reach the ground rod or 2nd battery terminals could you tell what the voltage of the 2nd battery was? Now can you tell me which "ground" is protected and from what? 'ground' is not protected from anything. 'ground' is a convenient point to connect everything and to make measurements from. And RF ground; is that protected from lightning? there is no such thing as an 'rf ground'. there is at best a low impedance path for return currents to get back to the feedpoint... if that happens to be through the earth (note for you, earth means dirt, not earth as in electrical ground that we call it in the states) then we may want to call it 'ground' but it really isn't. now, why do i say there is no such thing as an rf ground? consider what happens around a 10m 1/4 wave vertical mounted on the ground connected to the center conductor of the coax, with a single ground rod as the connection for the shield of the coax. The current going into the 1/4 wave vertical and the current coming out of the ground rod have to be equal at the coax connection... so, current flows up the vertical, back through the ground, up the ground rod and back to the coax... very simple... until you take your rf voltmeter and connect it to the ground rod and stick the other probe in the ground some distance from the rod, lets say about 1/4 wave away (more or less, propagation velocity through the ground is slower so it would be physically less than 1/4 wave in free space, etc, etc). what voltage do you read on your meter?? it is difficult to calculate because of all the interfaces and bulk conductivities, the sizes of the ground rod and probe, etc... but suffice it to say, it ain't zero. why not, aren't both probes connected to 'ground'?? well, yes, and no. because there is current flowing through the earth trying to get back to that coax connection there has to be a voltage difference. current in earth * resistivity of earth = voltage between points on the earth, give or take a few units. now lets say we add a radial wire to connect the ground rod at the feed point of the vertical to the probe where you measure the voltage of the earth... does the voltage go away? after all you just shorted out your voltmeter didn't you?? but no! there is still voltage! why you ask?? well, even though you have made a lower impedance path there is still current on the wire, and since the wire has inductance and capacitance the propagation time is not zero, so there is still a voltage difference causing that current to flow along the radial wire... why is it necessary to insulate the ends of raised radials if they are a 'ground' plane under a vertical antenna? lightning 'ground' is even worse because the currents are bigger... consider a 100ka bolt with a rise time of 1usec hitting the ground and spreading out at about .3c (approximately what i measured in one experiment for a buried radial wire)... the potential difference between points just a few feet apart can be huge, that is a common cause of lightning deaths, current hits tree, goes through ground, up one leg, down the other, or up one leg and out an arm that is closer to something conductive away from the tree... like a golf club. the key with designing lightning 'ground' systems is to realize that this potential exists and make sure you can handle it. the 'single point ground' is the classic way because it makes the distance zero which takes the rise time and propagation speed out of the equation. unfortunately that is not always possible, so that is where other methods are needed to keep voltages across protected equipment equalized. Lightning is multi frequency oriented so it would appear to me it would gyrate towards a radiator. i have never known lightning to gyrate. it takes all sorts of convoluted paths, but none that i would call gyrating. What about just one wire to the antenna and let the earth be the return line to close the circuit! certainly, and many have done that, and some have gotten burned. consider this case... sit on your table with the 12v battery again, but instead of the meter hook up a transmitter to it. assume there is one so-239 output jack mounted on the radio's metal case and no other connections, just 12v in, rf out. run a wire straight out from the so-239 center conductor then touch the radio's metal case... what happens? if the power is high enough you get burned... why? you are touching the radio case, isn't that 'ground'? you aren't touching the earth? so what is happening??? well, you are now part of the return path for current that has to find a way back to the chassis side of the so-239, and since you are likely much larger than the case of the radio you make a more efficient collector. change the setup, now run a wire from the case of the radio to the ground rod where the 2nd battery used to be connected... now you have another path for current to get back to the chassis side of the connector, from the 'earth' through the ground rod up the wire and to the radio... now touch the case and what happens? well, if the earth to radio circuit is lower impedance than your body to radio, more current flows on that side and you don't get burned... not good enough? still getting burned??? get one of those counterpoise resonator thingies... they are really just a small match box for a long wire... run the wire around the floor and tune it up. why does it work better? it provides a lower impedance path for collecting up that return current and getting it back to the radio. no magic, it is just providing the return current instead of letting your body collect it. Seems like the definitions have gone awry without involvement of a chassis ground ah, chassis ground, yet another type of ground. but then it is you that is electrically educated and the better judge. one of the few things you have said that i agree with |
#23
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Minimum gauge wire for connection to ground rod
"KJ4NTS" wrote in message
... On 3-Aug-2009, "JB" wrote: Primarily an RF ground since the antennas will be in my attic. However, I would appreciate the information for both cases. I think code calls for #6 solid Copper or Aluminum for lightning protection. Consider the 50 ft as part of the antenna though. Rather than even trying to have an RF "ground" try to make a counterpoise instead. Read up on the difference between RF ground, lightning ground, and electrical ground. You don't say what kind of antenna. I could put a surface wire counterpoise in the crawl space under my office where the radio is located. If I did that, there would be only about a 5 or 6 foot run to the center point of the counterpoise. Assuming the counterpoise wires are laying on the dirt floor of the crawl space, how many wires would I need, and how long do they need to be. I want to be able to work 6 through 40 meters. If I install a wire counterpoise like this, is there any problem also connecting the earth ground connection to the central ground bar in my office? The attic antenna is an Alpha Delta DX-EE. This is a balanced antenna and if the antenna is stretched out in an area where there is no metal it should do it's best. You probably won't benefit from a counterpoise at all. In fact, any RFI you might encounter will have to be specifically addressed because of the antenna is within the living quarters. Your antenna will be too close to the ground and partly shielded by roofing materials as it is for optimal performance, but we do what we can just to get on the air when there is no good solution. Having said that, You should be able to load up with an outboard tuner and make some contacts. |
#24
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Minimum gauge wire for connection to ground rod
"Dave" wrote in message ... "Art Unwin" wrote in message ... On Aug 3, 4:51 pm, "Dave" wrote: "Art Unwin" wrote in message i don't know where you get your radios, but in all of them here the coax connector is connected to the transmitter chassis which also has a lug for going to the safety ground. By golly, I believe you are correct! I thought that was a bad practice! Are all electronics at the same potential in the U.S. as its container? So with a ground connection at the electrical supply point and a provided connection for another generated "ground", is that not an invitation for ground loops? yes, it is. that is why your 'shack' ground has to be connected to the service entrance ground. Lightning tries to get to the true ground by the shortest route and the requirement is for you to not be part of it's route, even if your radio has to be the sacrificial lamb if you haven't pulled the plug. there ain't no such thing as 'true ground', tis all relative. H'mm, the ground point with the lowest potential relative to other ground potentials in the same system is the "true" ground of the system. you have to define the 'system'. but there is always the possibility that someone outside the 'system' can reach in and measure your 'ground ' connection relative to something else and measure a potential. potential is what is relative, hook a 12v battery to a meter and set it on a table, connect another 12v battery between a ground rod and one terminal of the first battery... does the meter reading change? what is the potential of the positive terminal of the battery? if you were sitting on the table and couldn't reach the ground rod or 2nd battery terminals could you tell what the voltage of the 2nd battery was? Now can you tell me which "ground" is protected and from what? 'ground' is not protected from anything. 'ground' is a convenient point to connect everything and to make measurements from. And RF ground; is that protected from lightning? there is no such thing as an 'rf ground'. there is at best a low impedance path for return currents to get back to the feedpoint... if that happens to be through the earth (note for you, earth means dirt, not earth as in electrical ground that we call it in the states) then we may want to call it 'ground' but it really isn't. now, why do i say there is no such thing as an rf ground? consider what happens around a 10m 1/4 wave vertical mounted on the ground connected to the center conductor of the coax, with a single ground rod as the connection for the shield of the coax. The current going into the 1/4 wave vertical and the current coming out of the ground rod have to be equal at the coax connection... so, current flows up the vertical, back through the ground, up the ground rod and back to the coax... very simple... until you take your rf voltmeter and connect it to the ground rod and stick the other probe in the ground some distance from the rod, lets say about 1/4 wave away (more or less, propagation velocity through the ground is slower so it would be physically less than 1/4 wave in free space, etc, etc). what voltage do you read on your meter?? it is difficult to calculate because of all the interfaces and bulk conductivities, the sizes of the ground rod and probe, etc... but suffice it to say, it ain't zero. why not, aren't both probes connected to 'ground'?? well, yes, and no. because there is current flowing through the earth trying to get back to that coax connection there has to be a voltage difference. current in earth * resistivity of earth = voltage between points on the earth, give or take a few units. now lets say we add a radial wire to connect the ground rod at the feed point of the vertical to the probe where you measure the voltage of the earth... does the voltage go away? after all you just shorted out your voltmeter didn't you?? but no! there is still voltage! why you ask?? well, even though you have made a lower impedance path there is still current on the wire, and since the wire has inductance and capacitance the propagation time is not zero, so there is still a voltage difference causing that current to flow along the radial wire... why is it necessary to insulate the ends of raised radials if they are a 'ground' plane under a vertical antenna? lightning 'ground' is even worse because the currents are bigger... consider a 100ka bolt with a rise time of 1usec hitting the ground and spreading out at about .3c (approximately what i measured in one experiment for a buried radial wire)... the potential difference between points just a few feet apart can be huge, that is a common cause of lightning deaths, current hits tree, goes through ground, up one leg, down the other, or up one leg and out an arm that is closer to something conductive away from the tree... like a golf club. the key with designing lightning 'ground' systems is to realize that this potential exists and make sure you can handle it. the 'single point ground' is the classic way because it makes the distance zero which takes the rise time and propagation speed out of the equation. unfortunately that is not always possible, so that is where other methods are needed to keep voltages across protected equipment equalized. Lightning is multi frequency oriented so it would appear to me it would gyrate towards a radiator. i have never known lightning to gyrate. it takes all sorts of convoluted paths, but none that i would call gyrating. What about just one wire to the antenna and let the earth be the return line to close the circuit! certainly, and many have done that, and some have gotten burned. consider this case... sit on your table with the 12v battery again, but instead of the meter hook up a transmitter to it. assume there is one so-239 output jack mounted on the radio's metal case and no other connections, just 12v in, rf out. run a wire straight out from the so-239 center conductor then touch the radio's metal case... what happens? if the power is high enough you get burned... why? you are touching the radio case, isn't that 'ground'? you aren't touching the earth? so what is happening??? well, you are now part of the return path for current that has to find a way back to the chassis side of the so-239, and since you are likely much larger than the case of the radio you make a more efficient collector. change the setup, now run a wire from the case of the radio to the ground rod where the 2nd battery used to be connected... now you have another path for current to get back to the chassis side of the connector, from the 'earth' through the ground rod up the wire and to the radio... now touch the case and what happens? well, if the earth to radio circuit is lower impedance than your body to radio, more current flows on that side and you don't get burned... not good enough? still getting burned??? get one of those counterpoise resonator thingies... they are really just a small match box for a long wire... run the wire around the floor and tune it up. why does it work better? it provides a lower impedance path for collecting up that return current and getting it back to the radio. no magic, it is just providing the return current instead of letting your body collect it. Seems like the definitions have gone awry without involvement of a chassis ground ah, chassis ground, yet another type of ground. but then it is you that is electrically educated and the better judge. one of the few things you have said that i agree with The electrical ground is Neutral at the entrance so that any wires shorting to the boxes or metal cabinets will pull the breaker and so that none of the cabinets can have AC potential on them. The lightning ground should be there at the entrance because that is the common tie point of all grounds, and any lightning currents flowing on the ground wires throughout the house will induce massive voltages into the house wiring. If you have multiple ground rods, they should tie together with heavy cable to the panel before all else. There is a problem with a good RF ground at the antenna, because lightning strikes to the power pole or house will want to flow through your station to the good ground in addition to the panel ground. So it is a good Idea to at least bring the coax and all wires to the electrical entrance for protection before entering the house and preferable to site the tower outside the electrical box and bond all grounds with heavy gauge conductors. The man is putting a dipole in the attic, so this talk about ground is a moot point. |
#25
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Minimum gauge wire for connection to ground rod
On Wed, 05 Aug 2009 17:08:15 GMT, "JB" wrote:
The man is putting a dipole in the attic, so this talk about ground is a moot point. The "balanced" antenna is connected to an "unbalanced" system if the rig has AC power through any device or has a powered mike connection, or has a computer control line, or has a powered speaker, or has a ... and so on. It takes supreme effort to maintain "balance" but most are happy to ignore analysis and put a rubber stamp to it. That an antenna is in a attic space is, no doubt, a cry that it is insulated from the sky. If that were all that was needed, every plastic covered wire in the air would be lightning proof. That is a superstition. If it were as effective as supposed, no one could use the antenna for its purpose. The only bright point of there being so few reports of lightning strikes to attic antennas is that they present so little individual risk that low within a neighborhood of equally available targets - you could as easily use a closeline. Every home in a community over 30 years old has a vertical radiator penetrating the roof line, the plumbing system vent that dives solidly straight into ground. How often is a house struck there? 73's Richard Clark, KB7QHC |
#26
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Minimum gauge wire for connection to ground rod
On 5-Aug-2009, "JB" wrote: This is a balanced antenna and if the antenna is stretched out in an area where there is no metal it should do it's best. You probably won't benefit from a counterpoise at all. The idea behind the counterpoise was to provide an RF ground for the radio, not as a part of the antenna. Your antenna will be too close to the ground and partly shielded by roofing materials as it is for optimal performance, but we do what we can just to get on the air when there is no good solution. Having said that, You should be able to load up with an outboard tuner and make some contacts. I have a friend with an Alpha Delta DX-EE in his attic, and he has achieved DX CC -- contacts with 100 countries. I don't know if it is a one or two story house. I'm going over to look at his installation his weekend. |
#27
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Minimum gauge wire for connection to ground rod
On 5-Aug-2009, Richard Clark wrote: The only bright point of there being so few reports of lightning strikes to attic antennas is that they present so little individual risk that low within a neighborhood of equally available targets - you could as easily use a closeline. Every home in a community over 30 years old has a vertical radiator penetrating the roof line, the plumbing system vent that dives solidly straight into ground. How often is a house struck there? As far as I know, we've never had a lightning strike. From a lightning perspective, we are fortune to have a hill at least 4 times as high as the house immediately behind us. Pity the house on the top of that hill. |
#28
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Minimum gauge wire for connection to ground rod
KJ4NTS wrote:
On 5-Aug-2009, "JB" wrote: This is a balanced antenna and if the antenna is stretched out in an area where there is no metal it should do it's best. You probably won't benefit from a counterpoise at all. The idea behind the counterpoise was to provide an RF ground for the radio, not as a part of the antenna. Ah, I'd never mind the counterpoise if you are using the balanced antenna. Just put up the antenna, run the coax or ladder line, get yourself a good station ground and some lightning protection. You are not likely to get struck directly, but could be the victim of a large static buildup. Polyphaser makes very good ones, and there is a simpler one that uses a spark gap, which also provides protection. Don't know the brand of that one. Disconnect the antenna and ground it when not in use. I'd say with this setup, you could do DXCC. DXCC and other awards have been achieved on more modest setups than what you are looking at building. You won't be the loudest signal on the band, but there can only be one of those anyhow. Now get to work! 8^) - 73 de Mike N3LI - |
#29
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Minimum gauge wire for connection to ground rod
"Michael Coslo" wrote in message
... KJ4NTS wrote: On 5-Aug-2009, "JB" wrote: This is a balanced antenna and if the antenna is stretched out in an area where there is no metal it should do it's best. You probably won't benefit from a counterpoise at all. The idea behind the counterpoise was to provide an RF ground for the radio, not as a part of the antenna. There is no need for the "radio" to have an RF ground. If you are feeding the radio with 50 ohm coax that is matched in a 50 ohm non-reactive load (ie tuner or dummy load) the RF will stay where it belongs. The only concern for the balanced antenna, is lightning protection and shock hazard from the power supply. The best lightning protection depends on your specific situation so I will leave you to read up on that. Ah, I'd never mind the counterpoise if you are using the balanced antenna. Just put up the antenna, run the coax or ladder line, get yourself a good station ground and some lightning protection. You are not likely to get struck directly, but could be the victim of a large static buildup. Polyphaser makes very good ones, and there is a simpler one that uses a spark gap, which also provides protection. Don't know the brand of that one. Disconnect the antenna and ground it when not in use. I'd say with this setup, you could do DXCC. DXCC and other awards have been achieved on more modest setups than what you are looking at building. You won't be the loudest signal on the band, but there can only be one of those anyhow. I don't have DXCC but I do have a wall of cards from around the world. Many of them were logged when I was mobile to and from work or school. It will sharpen your skills to steal them from the big guns. I had a regular daily lunch break sked with an Aussie from my truck in the parking lot.when the sunspots were working. Conditions can even favor the well placed weak station over the big guns. That is the sport of it all. Now get to work! 8^) - 73 de Mike N3LI - |
#30
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Minimum gauge wire for connection to ground rod
JB wrote:
I don't have DXCC but I do have a wall of cards from around the world. Many of them were logged when I was mobile to and from work or school. It will sharpen your skills to steal them from the big guns. I had a regular daily lunch break sked with an Aussie from my truck in the parking lot.when the sunspots were working. Conditions can even favor the well placed weak station over the big guns. That is the sport of it all. Concur 100 percent, JB. I know that new guys love the high power and high gain antenna setups, but NTS will learn a lot on how to be a better operator with a modest setup such as he proposes. - 73 de Mike N3LI - |
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