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Ground conductivity's effect on vertical
Hi, Everybody,
In the process of modeling a vertical antenna (specifically, I am using EZNEC 5.0) I am noticing an effect I did not expect which could be the result of a modeling error on my part. The antenna is a 34-foot vertical above (12) 34-foot radials, making it a 1/4 wave on 40 and a 1/2 wave on 20. On 40, the antenna works as I expected; as the ground conductivity goes up, the gain and efficiency of the antenna both increase, too. But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008 S/m, the max gain and efficiency *decrease*! This is counter-intuitive to me. Can anyone point to something I'm doing wrong? Thanks, Al W6LX |
Ground conductivity's effect on vertical
Al Lorona wrote:
Hi, Everybody, In the process of modeling a vertical antenna (specifically, I am using EZNEC 5.0) I am noticing an effect I did not expect which could be the result of a modeling error on my part. The antenna is a 34-foot vertical above (12) 34-foot radials, making it a 1/4 wave on 40 and a 1/2 wave on 20. On 40, the antenna works as I expected; as the ground conductivity goes up, the gain and efficiency of the antenna both increase, too. But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008 S/m, the max gain and efficiency *decrease*! This is counter-intuitive to me. Can anyone point to something I'm doing wrong? Thanks, Al W6LX Ground loss is a sort of impedance matching problem. If you have perfectly conducting ground, there is no ground loss. If you have perfectly insulating ground, there is no ground loss. There's always some ground conductivity in between those extremes at which the loss is maximum. This value depends on the frequency among other things. Try a wider range of conductivities and you'll find this point. You should also be aware that if you have radials which are above but close to the ground, half wavelength ones can be considerably less efficient than quarter wavelength ones. One reason is that the points of maximum current are out near the centers of the radials, where they induce current into the lossy ground. When the radials are a quarter wavelength or shorter, the current maxima are near the center, so their fields nearly cancel. Another often-overlooked fact is that radials very close to the ground are electrically considerably longer than when more elevated. So radials which are a quarter wavelength in free space can have their current maxima well out from the center which results in lower efficiency. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
Very cool, Roy. Thank you for this, this is good information that makes
sense to me. There is a lot in there to think about and apply to the application for which I'm analyzing this antenna. When you talk about the current maximum and loss of a radial, you bring up another question. My 1/2 wave vertical is going to have a much higher feedpoint impedance than a standard 1/4 wave vertical. Let's say it's somewhere around 2000 ohms as compared to 40 or so ohms for a 1/4 wave. What effect does that higher feedpoint impedance have on the radial system? Does it relax the requirements for the number or radials or the length of them? Does it mask the loss that you'd have normally? Or do the "rules" of adding radials apply no matter what the vertical's input impedance? Thanks very much. Al W6LX "Roy Lewallen" wrote in message news:p4CdnVoHG_g30pXVnZ2dnUVZ_vudnZ2d@easystreeton line... Al Lorona wrote: Hi, Everybody, In the process of modeling a vertical antenna (specifically, I am using EZNEC 5.0) I am noticing an effect I did not expect which could be the result of a modeling error on my part. The antenna is a 34-foot vertical above (12) 34-foot radials, making it a 1/4 wave on 40 and a 1/2 wave on 20. On 40, the antenna works as I expected; as the ground conductivity goes up, the gain and efficiency of the antenna both increase, too. But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008 S/m, the max gain and efficiency *decrease*! This is counter-intuitive to me. Can anyone point to something I'm doing wrong? Thanks, Al W6LX Ground loss is a sort of impedance matching problem. If you have perfectly conducting ground, there is no ground loss. If you have perfectly insulating ground, there is no ground loss. There's always some ground conductivity in between those extremes at which the loss is maximum. This value depends on the frequency among other things. Try a wider range of conductivities and you'll find this point. You should also be aware that if you have radials which are above but close to the ground, half wavelength ones can be considerably less efficient than quarter wavelength ones. One reason is that the points of maximum current are out near the centers of the radials, where they induce current into the lossy ground. When the radials are a quarter wavelength or shorter, the current maxima are near the center, so their fields nearly cancel. Another often-overlooked fact is that radials very close to the ground are electrically considerably longer than when more elevated. So radials which are a quarter wavelength in free space can have their current maxima well out from the center which results in lower efficiency. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
Roy,
This is blowing my mind. What you said about ground loss peaking at some certain conductivity makes perfect sense but I never thought about it before as applied to a vertical antenna system like this. Sure enough, I found a maximum. This is wild. We always think, "The better the ground, the better the antenna system," but it's not that simple. It's funny to think that really terrible ground can have an advantage over pretty good ground. Next I'm going to play with the radial length so I can see the other effect you described for me. Regards, Al W6LX "Roy Lewallen" wrote in message news:p4CdnVoHG_g30pXVnZ2dnUVZ_vudnZ2d@easystreeton line... Al Lorona wrote: Hi, Everybody, In the process of modeling a vertical antenna (specifically, I am using EZNEC 5.0) I am noticing an effect I did not expect which could be the result of a modeling error on my part. The antenna is a 34-foot vertical above (12) 34-foot radials, making it a 1/4 wave on 40 and a 1/2 wave on 20. On 40, the antenna works as I expected; as the ground conductivity goes up, the gain and efficiency of the antenna both increase, too. But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008 S/m, the max gain and efficiency *decrease*! This is counter-intuitive to me. Can anyone point to something I'm doing wrong? Thanks, Al W6LX Ground loss is a sort of impedance matching problem. If you have perfectly conducting ground, there is no ground loss. If you have perfectly insulating ground, there is no ground loss. There's always some ground conductivity in between those extremes at which the loss is maximum. This value depends on the frequency among other things. Try a wider range of conductivities and you'll find this point. You should also be aware that if you have radials which are above but close to the ground, half wavelength ones can be considerably less efficient than quarter wavelength ones. One reason is that the points of maximum current are out near the centers of the radials, where they induce current into the lossy ground. When the radials are a quarter wavelength or shorter, the current maxima are near the center, so their fields nearly cancel. Another often-overlooked fact is that radials very close to the ground are electrically considerably longer than when more elevated. So radials which are a quarter wavelength in free space can have their current maxima well out from the center which results in lower efficiency. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
On Fri, 18 Apr 2008 10:26:50 -0700, "Al Lorona"
wrote: This is blowing my mind. What you said about ground loss peaking at some certain conductivity makes perfect sense but I never thought about it before as applied to a vertical antenna system like this. Sure enough, I found a maximum. This is wild. We always think, "The better the ground, the better the antenna system," but it's not that simple. It's funny to think that really terrible ground can have an advantage over pretty good ground. Hi Al, There are two sides to this coin and you are only looking at one. Let's take for example the vaunted superlatives that are often tied to swamp land, or even planting your antenna farm offshore (ah that seawater!). There is more loss there than if you planted your antenna farm on a 100 foot sand dune (think glass). So, why do so many eyes mist up with the opportunity of being in a swap or in the ocean? Launch angle. That same high (lossy) conductivity is also responsible for what is called the Brewster angle when it comes to vertical antennas. The higher the conductivity, the lower the Brewster angle, the lower the launch angle possibilities, the longer out to the first skip - DX! Longer radials will not lower that Brewster angle unless you extend them out very far with a lot of them (think of carpeting the soil out with a copper mesh for several 10s of wavelengths). So, returning to your comment: "The better the ground, the better the antenna system," Better ground may subtract power in the near proximity, but better ground may also boost relative power at low angles. So, you need to define what is meant by "better" because this is a choice of opposite alternatives. 73's Richard Clark, KB7QHC |
Ground conductivity's effect on vertical
Al Lorona wrote:
It's funny to think that really terrible ground can have an advantage over pretty good ground. Free space is just about the most terrible "ground" that one can imagine. :-) -- 73, Cecil http://www.w5dxp.com |
Ground conductivity's effect on vertical
On 18 abr, 19:02, "Al Lorona" wrote:
Very cool, Roy. Thank you for this, this is good information that makes sense to me. There is a lot in there to think about and apply to the application for which I'm analyzing this antenna. When you talk about the current maximum and loss of a radial, you bring up another question. My 1/2 wave vertical is going to have a much higher feedpoint impedance than a standard 1/4 wave vertical. Let's say it's somewhere around 2000 ohms as compared to 40 or so ohms for a 1/4 wave. What effect does that higher feedpoint impedance have on the radial system? Does it relax the requirements for the number or radials or the length of them? Does it mask the loss that you'd have normally? Or do the "rules" of adding radials apply no matter what the vertical's input impedance? Thanks very much. Al W6LX "Roy Lewallen" wrote in message news:p4CdnVoHG_g30pXVnZ2dnUVZ_vudnZ2d@easystreeton line... Al Lorona wrote: Hi, Everybody, In the process of modeling a vertical antenna (specifically, I am using EZNEC 5.0) I am noticing an effect I did not expect which could be the result of a modeling error on my part. The antenna is a 34-foot vertical above (12) 34-foot radials, making it a 1/4 wave on 40 and a 1/2 wave on 20. On 40, the antenna works as I expected; as the ground conductivity goes up, the gain and efficiency of the antenna both increase, too. But on 20, if I increase the ground conductivity from, say, 0.005 to 0.008 S/m, the max gain and efficiency *decrease*! This is counter-intuitive to me. Can anyone point to something I'm doing wrong? Thanks, Al W6LX Ground loss is a sort of impedance matching problem. If you have perfectly conducting ground, there is no ground loss. If you have perfectly insulating ground, there is no ground loss. There's always some ground conductivity in between those extremes at which the loss is maximum. This value depends on the frequency among other things. Try a wider range of conductivities and you'll find this point. You should also be aware that if you have radials which are above but close to the ground, half wavelength ones can be considerably less efficient than quarter wavelength ones. One reason is that the points of maximum current are out near the centers of the radials, where they induce current into the lossy ground. When the radials are a quarter wavelength or shorter, the current maxima are near the center, so their fields nearly cancel. Another often-overlooked fact is that radials very close to the ground are electrically considerably longer than when more elevated. So radials which are a quarter wavelength in free space can have their current maxima well out from the center which results in lower efficiency. Roy Lewallen, W7EL Hi Al, From my experience, and simulation, a halve wave vertical has less stringent radial requirements. The current reduces with a factor of about sqrt(2000/50)= 6 (16 dB). The result is less loss in the vicinity (near field) zone of the antenna. Far field maximum of radiation (elevation) will virtually not change because of canceling effect of reradiated field from ground below the (pseudo) Brewster angle. Because of the somewhat higher radiation center with respect to a 1/4 wave, the elevation of maximum radiation will be somewhat less. Off course when you have high ground conductivity (for example fertilized wet soil), you will have maximum radiation under lower elevation. It is likely that in the end the produced field under relative low elevation will be higher than that from a horizontal dipole. Elevated radials give less ground loss. At high frequency, the relative epsilon becomes important. A High dielectric constant and low conductivity may result in less loss. At high frequency a larger part of the current goes to the capacitance of the ground instead of the loss resistance (in a parallel equivalent circuit). See it as having a low loss dielectric under your antenna. Hope this help you a bit. Best regards, Wim PA3DJS www.tetech.nl please remove abc when replying. |
Ground conductivity's effect on vertical
"Cecil Moore" wrote in message ... Al Lorona wrote: It's funny to think that really terrible ground can have an advantage over pretty good ground. Free space is just about the most terrible "ground" that one can imagine. :-) -- 73, Cecil http://www.w5dxp.com So much disinformation by W8JI School of DC circuitry :-) Modeling various configurations shows benefits of good ground, especially for taller than 1/4 wave radiators. Myth that half wave radiators do not need ground is spreading like snake oil wild fire. They need it but "looking" for it further out, not just at the base. I will anytime trade good ground (mirror) for lossy (RF sponge) ground. Its just where the radiator is "looking" for the mirror, taller one - further out, enhancing signals at lower angles. 3/8 vertical with some 3/8 physical length radials start morphing into far field. Yuri, K3BU.us |
Ground conductivity's effect on vertical
There are two quite separate ways which ground affects a vertical
antenna's performance. The first is loss due to current returning to the antenna base when the antenna is grounded, or induced in the ground under an elevated radial system. To minimize loss, you want as much of the current to flow through radial wires as you can. The power loss is I^2 * R. For a given power input, I is much lower for a half wave bottom fed vertical than a quarter wave bottom fed vertical. So the loss due to the conducted or induced current is much less, and you can get by with a much simpler ground system with the half wave vertical and still have low loss. This ground loss is usually the chief determining factor of a vertical's efficiency. The other effect of ground is that the field from the antenna reflects from it some distance from the antenna. The reflected field adds to the directly radiated field to form a net field which is different at each elevation angle. This is a major factor in determining the antenna's elevation pattern. The conductivity and permittivity (dielectric constant) of the ground affect the magnitude and phase of the the reflected field, so the pattern changes with ground quality. In general, the more conductive the ground the better the low angle radiation. However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. The only way to improve the situation is to move the antenna to a location where the ground is better, which usually isn't possible or practical. Because of the two separate effects, the overall field strength might be better or worse as the ground conductivity improves, and it might even be better at some elevation angles and worse at others. Roy Lewallen, W7EL Yuri Blanarovich wrote: "Cecil Moore" wrote in message ... Al Lorona wrote: It's funny to think that really terrible ground can have an advantage over pretty good ground. Free space is just about the most terrible "ground" that one can imagine. :-) -- 73, Cecil http://www.w5dxp.com So much disinformation by W8JI School of DC circuitry :-) Modeling various configurations shows benefits of good ground, especially for taller than 1/4 wave radiators. Myth that half wave radiators do not need ground is spreading like snake oil wild fire. They need it but "looking" for it further out, not just at the base. I will anytime trade good ground (mirror) for lossy (RF sponge) ground. Its just where the radiator is "looking" for the mirror, taller one - further out, enhancing signals at lower angles. 3/8 vertical with some 3/8 physical length radials start morphing into far field. Yuri, K3BU.us |
Ground conductivity's effect on vertical
Very nicely put, Roy. Although I "knew" this in the recesses of memory, the refresher will stick with my memory more, now. Thanks. In my case, I am considering the use of a vertical at a new residence built on sand. Since I am not concerned about low angle radiation characteristics, the Half Wave may be something to consider..... giving me a fairly efficient vertical operation with some NVIS characteristics. Ed K7AAT |
Ground conductivity's effect on vertical
On 19 Apr 2008 21:35:28 GMT, "Ed_G"
wrote: In my case, I am considering the use of a vertical at a new residence built on sand. Since I am not concerned about low angle radiation characteristics, the Half Wave may be something to consider..... giving me a fairly efficient vertical operation with some NVIS characteristics. Hi Ed, Efficient? A vertical has almost no Near Vertical radiation for Near Vertical Incidence Skywave. You can get along with "almost no," or you can simply use a low horizontal which would exhibit "a lot of" Near Vertical Incidence Skywave. Good ground, bad ground, radials, no radials won't change efficiency much for the vertical's incidence overhead (there's a hole in that pattern). 73's Richard Clark, KB7QHC |
Ground conductivity's effect on vertical
Richard Clark wrote in
: On 19 Apr 2008 21:35:28 GMT, "Ed_G" wrote: In my case, I am considering the use of a vertical at a new residence built on sand. Since I am not concerned about low angle radiation characteristics, the Half Wave may be something to consider..... giving me a fairly efficient vertical operation with some NVIS characteristics. Hi Ed, Efficient? A vertical has almost no Near Vertical radiation for Near Vertical Incidence Skywave. You can get along with "almost no," or you can simply use a low horizontal which would exhibit "a lot of" Near Vertical Incidence Skywave. Good ground, bad ground, radials, no radials won't change efficiency much for the vertical's incidence overhead (there's a hole in that pattern). 73's Richard Clark, KB7QHC Richard, By "efficient" I was referring to the transfer of power.... to a presumed 50 ohm antenna input, not to any radiation characteristics ! As I understood it, a half wave vertical can give me this, with a little effort. I also understood it to have a fairly high take off angle.... which will certainly give me better in-state coverage than a good low angle takeoff would..... wouldn't it? Yes, I know a proper NVIS antenna would be far better than this.... that is why I used the term "some NVIS" characteristics. TNX Ed |
Ground conductivity's effect on vertical
Ed,
It won't be suitable for NVIS, as you can see from a model. Roy Lewallen, W7EL Ed_G wrote: Very nicely put, Roy. Although I "knew" this in the recesses of memory, the refresher will stick with my memory more, now. Thanks. In my case, I am considering the use of a vertical at a new residence built on sand. Since I am not concerned about low angle radiation characteristics, the Half Wave may be something to consider..... giving me a fairly efficient vertical operation with some NVIS characteristics. Ed K7AAT |
Ground conductivity's effect on vertical
Ed_G wrote:
Richard, By "efficient" I was referring to the transfer of power.... to a presumed 50 ohm antenna input, not to any radiation characteristics ! As I understood it, a half wave vertical can give me this, with a little effort. I also understood it to have a fairly high take off angle.... which will certainly give me better in-state coverage than a good low angle takeoff would..... wouldn't it? Yes, I know a proper NVIS antenna would be far better than this.... that is why I used the term "some NVIS" characteristics. TNX Ed All the radiation from an antenna isn't concentrated at some "takeoff angle", but radiates at all angles at various amounts. That distribution is known as the "elevation pattern" and trying to replace it with a single "takeoff angle" value loses nearly all the information about how and where the antenna radiates. The half wavelength vertical radiates very little above about 60 degrees elevation angle regardless of the ground characteristics. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
"Roy Lewallen"
However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. ___________ Roy, didn't the experiments of Brown, Lewis & Epstein of RCA in ~1937 show that the h-plane field measured 3/10 mile from a vertical monopole of about 60 to 88 degrees in height, over a set of 113 buried radials each 0.41 WL, was within several percent of the theoretical maximum for the applied power as radiated by a perfect monopole over a perfect ground plane? And conductivity at the NJ test site was poor -- 4 mS/m or less. That tends to show that the fields radiated at very low elevation angles also will be close to their theoretical values when measured at this radial distance, even though ground conductivity at the antenna site is poor. The relative field (E/Emax) for radiators of these heights and propagation paths approximately equals the cosine of the elevation angle. The greatest radiated fields always will be directed in or near the horizontal plane when measured/calculated for such conditions. This also will be true for any monopole from infinitesimal to 5/8 wavelength in height, although the elevation pattern of monopoles from /4- to 5/8-WL no longer are described by the cosine function (see http://i62.photobucket.com/albums/h8...omparison.jpg). Elevation patterns show maximum relative field centered at various elevation angles above the horizon, when those fields are measured at progressively longer radial distances from the monopole, due to the propagation loss for the surface wave over other than a perfect, flat, infinite ground for those ranges. Earth curvature and terrain diffraction add to those losses for longer surface wave paths over real earth, and for very great distances the h-plane relative fields falls to ~zero. But that pattern shape is not the pattern shape originally radiated by the monopole, it also includes the effects of the propagation environment at the range where it was measured (or calculated). If this were not true then MW broadcast stations would have essentially zero coverage area for their groundwave signals. RF |
Ground conductivity's effect on vertical
On Sat, 19 Apr 2008 14:20:33 -0700, Roy Lewallen
wrote: There are two quite separate ways which ground affects a vertical antenna's performance. The first is loss due to current returning to the antenna base when the antenna is grounded, or induced in the ground under an elevated radial system. To minimize loss, you want as much of the current to flow through radial wires as you can. The power loss is I^2 * R. For a given power input, I is much lower for a half wave bottom fed vertical than a quarter wave bottom fed vertical. So the loss due to the conducted or induced current is much less, and you can get by with a much simpler ground system with the half wave vertical and still have low loss. This ground loss is usually the chief determining factor of a vertical's efficiency. The other effect of ground is that the field from the antenna reflects from it some distance from the antenna. The reflected field adds to the directly radiated field to form a net field which is different at each elevation angle. This is a major factor in determining the antenna's elevation pattern. The conductivity and permittivity (dielectric constant) of the ground affect the magnitude and phase of the the reflected field, so the pattern changes with ground quality. In general, the more conductive the ground the better the low angle radiation. However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. The only way to improve the situation is to move the antenna to a location where the ground is better, which usually isn't possible or practical. Because of the two separate effects, the overall field strength might be better or worse as the ground conductivity improves, and it might even be better at some elevation angles and worse at others. Roy Lewallen, W7EL Maybe I should change the subject line, but here goes. First of all, i am fishing for information, not challenging anyone's intelligence. I understand from books I have read, that a ground mounted vertical antenna needs many radials. IIRC, the point of diminishing returns on adding radials falls somewhere between 64-128 radials. I imagine the best radial-based ground I could have for 20 meters would be a solid copper disk with about 16 feet radius, give or take. However, I recall in the ARRL Antenna handbook, not the latest version, but one prior to this one, there is no noticeable difference between a raised ground plane antenna with 4 elements as opposed to 128. (From here, or another antenna forum, I heard for the first time that it holds true for two radials.) I am still trying to figure out why so many radials are needed on the ground and a few feet higher so few are needed. Actually, more important than the why, is how high is high enough to reduce the optimum number of radials? For example, i want to build a 20 meter vertical. I understand the best place for it is on top of a 100 foot+ tower, but somewhere in between, there has to be a place where 4 radials above ground is noticeably better than the same 4 radials on the ground. Another point I have heard in the forums, but not confirmed, is that a reduced size vertical element doesn't gain much by adding radials longer than the antenna is high. 73 for now, N4PGW Buck -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
Ground conductivity's effect on vertical
I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on adding radials falls somewhere between 64-128 radials. I imagine the best radial-based ground I could have for 20 meters would be a solid copper disk with about 16 feet radius, give or take. However, I recall in the ARRL Antenna handbook, not the latest version, but one prior to this one, there is no noticeable difference between a raised ground plane antenna with 4 elements as opposed to 128. (From here, or another antenna forum, I heard for the first time that it holds true for two radials.) ================================================== = Instead of an approx 16 ft radius disc ,you might consider an area covered with chicken wire mesh with the antenna in its centre.If that's laid on a lawn the grass will quickly grow over it . In a few weeks time the wire mesh will no longer be visible and the lawnmower can just run over . A Butternut (vertical)antenna with a mesh ground plane compared with none gave an improvement of 1 to 2 S points on 20 ,15 and 10 m. Frank GM0CSZ / KN6WH |
Ground conductivity's effect on vertical
Richard Fry wrote:
"Roy Lewallen" However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. ___________ Roy, didn't the experiments of Brown, Lewis & Epstein of RCA in ~1937 show that the h-plane field measured 3/10 mile from a vertical monopole of about 60 to 88 degrees in height, over a set of 113 buried radials each 0.41 WL, was within several percent of the theoretical maximum for the applied power as radiated by a perfect monopole over a perfect ground plane? And conductivity at the NJ test site was poor -- 4 mS/m or less. That tends to show that the fields radiated at very low elevation angles also will be close to their theoretical values when measured at this radial distance, even though ground conductivity at the antenna site is poor. The relative field (E/Emax) for radiators of these heights and propagation paths approximately equals the cosine of the elevation angle. I believe we've discussed this before, so I'll be brief. Their calculation of the field at the receiving site when the radial system is perfect was adjusted for the effect of ground wave attenuation caused by the imperfect ground conductivity. If the ground between the antenna and receiving site were perfect, the field strength would have been greater. Also, I'm speaking of sky wave. Ground reflection isn't a factor in determining surface wave, which is what they measured and which isn't of interest to most amateurs. The greatest radiated fields always will be directed in or near the horizontal plane when measured/calculated for such conditions. This also will be true for any monopole from infinitesimal to 5/8 wavelength in height, although the elevation pattern of monopoles from /4- to 5/8-WL no longer are described by the cosine function (see http://i62.photobucket.com/albums/h8...omparison.jpg). Elevation patterns show maximum relative field centered at various elevation angles above the horizon, when those fields are measured at progressively longer radial distances from the monopole, due to the propagation loss for the surface wave over other than a perfect, flat, infinite ground for those ranges. Earth curvature and terrain diffraction add to those losses for longer surface wave paths over real earth, and for very great distances the h-plane relative fields falls to ~zero. As I thought you were aware, the surface wave propagates considerably differently than the sky wave. But that pattern shape is not the pattern shape originally radiated by the monopole, it also includes the effects of the propagation environment at the range where it was measured (or calculated). If this were not true then MW broadcast stations would have essentially zero coverage area for their groundwave signals. It would be a mistake to design HF antenna systems based on optimizing surface wave propagation as AM broadcasters do, unless you desire communication for distances not exceeding a few miles. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
Buck wrote:
Maybe I should change the subject line, but here goes. First of all, i am fishing for information, not challenging anyone's intelligence. I understand from books I have read, that a ground mounted vertical antenna needs many radials. IIRC, the point of diminishing returns on adding radials falls somewhere between 64-128 radials. I imagine the best radial-based ground I could have for 20 meters would be a solid copper disk with about 16 feet radius, give or take. However, I recall in the ARRL Antenna handbook, not the latest version, but one prior to this one, there is no noticeable difference between a raised ground plane antenna with 4 elements as opposed to 128. (From here, or another antenna forum, I heard for the first time that it holds true for two radials.) When a ground system is a long distance above ground, only two radials are needed for high efficiency and a circular pattern at zero elevation angle. The pattern does become non-circular at higher angles, which can be prevented by adding two more radials. A ground system which is above but close to the ground requires fewer radials for good efficiency than one with radials which are buried or much closer to the ground. I am still trying to figure out why so many radials are needed on the ground and a few feet higher so few are needed. The ground is very lossy. When the radial system is on or in the ground, current flowing to the radials is forced to flow through the lossy ground. Actually, more important than the why, is how high is high enough to reduce the optimum number of radials? For example, i want to build a 20 meter vertical. I understand the best place for it is on top of a 100 foot+ tower, but somewhere in between, there has to be a place where 4 radials above ground is noticeably better than the same 4 radials on the ground. That's a good question without a single good answer. It depends at least on the ground conductivity and permittivity (down to a considerable depth), frequency, and radial length. Modeling can give you a good idea of the tradeoffs, although the very simple minded ground model might not be adequate to make a very accurate comparison. Another point I have heard in the forums, but not confirmed, is that a reduced size vertical element doesn't gain much by adding radials longer than the antenna is high. I don't believe that's true. I'll gladly consider any supporting evidence. Hearing something on forums is among the worst justification for believing it, in my opinion. Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
On Sun, 20 Apr 2008 14:26:26 +0100, Highland Ham
wrote: ================================================= == Instead of an approx 16 ft radius disc ,you might consider an area covered with chicken wire mesh with the antenna in its centre.If that's laid on a lawn the grass will quickly grow over it . In a few weeks time the wire mesh will no longer be visible and the lawnmower can just run over . Thanks, Frank, but the example was for illustrative purposes only. I'd hate to think of the cost of that solid sheet of copper hihi. -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
Ground conductivity's effect on vertical
"Roy Lewallen" wrote in message news:z8WdnThgM7y9_5fVnZ2dnUVZ_gWdnZ2d@easystreeton line... There are two quite separate ways which ground affects a vertical antenna's performance. The first is loss due to current returning to the antenna base when the antenna is grounded, or induced in the ground under an elevated radial system. To minimize loss, you want as much of the current to flow through radial wires as you can. The power loss is I^2 * R. For a given power input, I is much lower for a half wave bottom fed vertical than a quarter wave bottom fed vertical. So the loss due to the conducted or induced current is much less, and you can get by with a much simpler ground system with the half wave vertical and still have low loss. This ground loss is usually the chief determining factor of a vertical's efficiency. Here we are again forgetting that we are dealing with standing wave circuit and cos/sin current distribution along the elements. Half wave vertical might have low current at the base but quarter wave away it will be max (assuming half wave elevated electrical radial). The radiation pattern is formed between the radiator and radials (and how they are affected by ground under). Radials close to ground couple to it and depending on ground RF quality we are dealing with decent reflecting mirror or "RF eating sponge". The other effect of ground is that the field from the antenna reflects from it some distance from the antenna. The reflected field adds to the directly radiated field to form a net field which is different at each elevation angle. This is a major factor in determining the antenna's elevation pattern. The conductivity and permittivity (dielectric constant) of the ground affect the magnitude and phase of the the reflected field, so the pattern changes with ground quality. In general, the more conductive the ground the better the low angle radiation. However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. The only way to improve the situation is to move the antenna to a location where the ground is better, which usually isn't possible or practical. Dense radial field with electrical length of radials around wavelength has shown remarkable imrpovement in low angle performance over "regular"ground. Because of the two separate effects, the overall field strength might be better or worse as the ground conductivity improves, and it might even be better at some elevation angles and worse at others. Roy Lewallen, W7EL Yuri, K3BU.us Yuri Blanarovich wrote: "Cecil Moore" wrote in message ... Al Lorona wrote: It's funny to think that really terrible ground can have an advantage over pretty good ground. Free space is just about the most terrible "ground" that one can imagine. :-) -- 73, Cecil http://www.w5dxp.com So much disinformation by W8JI School of DC circuitry :-) Modeling various configurations shows benefits of good ground, especially for taller than 1/4 wave radiators. Myth that half wave radiators do not need ground is spreading like snake oil wild fire. They need it but "looking" for it further out, not just at the base. I will anytime trade good ground (mirror) for lossy (RF sponge) ground. Its just where the radiator is "looking" for the mirror, taller one - further out, enhancing signals at lower angles. 3/8 vertical with some 3/8 physical length radials start morphing into far field. Yuri, K3BU.us |
Ground conductivity's effect on vertical
Previously, about BL&E's 1937 measurements:
Their calculation of the field at the receiving site when the radial system is perfect was adjusted for the effect of ground wave attenuation caused by the imperfect ground conductivity. Anybody: Just wondering -- how does this conclusion flow from the findings published in the 1937 I.R.E.paper of BL&E? The theoretical (not measured) BL&E groundwave field at 1 mile for 1 kW radiated from a perfect monopole over a perfect ground plane as shown in the BL&E I.R.E. paper is not the equivalent/adjusted field they measured from the monopole heights they tested. But, as BL&E published, the groundwave fields they measured from these real monopoles over real earth was within several percent of that theoretical maximum, when working against 113 buried radials each of 0.41WL -- even for the poor conductivity at/near their antenna site. Also, I'm speaking of sky wave. Ground reflection isn't a factor in determining surface wave, ... But neither theory nor practice supports this, does it? If so, then the groundwave fields that BL&E measured at 3/10 of a mile would have been at least 29.3% less than that theoretical maximum field, which included a perfect (3 dB) ground reflection -- not just the several percent they measured. And this measured performance just beyond the near field radius has been re-proven in thousands of groundwave r.m.s. field strength measurements of AM broadcast stations over many decades since the BL&E work. It would be a mistake to design HF antenna systems based on optimizing surface wave propagation as AM broadcasters do, unless you desire communication for distances not exceeding a few miles. Just to note that since the 1930s (at least), AM broadcasters have been aware of the effects of the differing propagation characteristics of groundwaves and skywaves. This is evident in the fact that most 50 kW, fulltime, AM broadcast stations in the US use a radiator height that minimizes the self-interference of their skywave with their groundwave, so as to ~maximize their interference-free coverage areas when skywave propagation occurs. The great majority of these stations use a monopole radiator height of about 195 degrees. RF |
Ground conductivity's effect on vertical
On Apr 20, 8:05 am, Buck wrote:
Maybe I should change the subject line, but here goes. First of all, i am fishing for information, not challenging anyone's intelligence. I understand from books I have read, that a ground mounted vertical antenna needs many radials. IIRC, the point of diminishing returns on adding radials falls somewhere between 64-128 radials. I imagine the best radial-based ground I could have for 20 meters would be a solid copper disk with about 16 feet radius, give or take. However, I recall in the ARRL Antenna handbook, not the latest version, but one prior to this one, there is no noticeable difference between a raised ground plane antenna with 4 elements as opposed to 128. (From here, or another antenna forum, I heard for the first time that it holds true for two radials.) I am still trying to figure out why so many radials are needed on the ground and a few feet higher so few are needed. Actually, more important than the why, is how high is high enough to reduce the optimum number of radials? For example, i want to build a 20 meter vertical. I understand the best place for it is on top of a 100 foot+ tower, but somewhere in between, there has to be a place where 4 radials above ground is noticeably better than the same 4 radials on the ground. If you used 120 radials on the ground as optimum, even raising only 1/8 wave off the ground will enable one to reduce the number of radials to equal the same performance. But... You will still probably need at least 60 radials at 1/8 wave up to equal the 120 on the ground. So even at that low elevated height, 4 radials is better than 4 on the ground. But... Still not very good.. :/ At 1/4 wave in height, you will need about 8-10 radials to equal the 120 on the ground. Only when you approach 1/2 wave in height can you use 2-4 radials and have the same appx ground losses as the 120 on the ground. You *must* think in terms of wavelength off the ground, not just feet in general. A 160m vertical will need to be about 250 ft off the ground to be able to use 2-4 radials with optimum results. A 10m vertical can be 16 ft off the ground with 2-4 radials and have the same performance. If you had the 20m vertical at 16 ft, "1/4 wave", and used 4 radials, it would be equal to a ground mount using about 60 radials. Pretty decent antenna. In the case of your 20m vertical, it will need to be 32 ft high to be able to use 2-4 radials and appx equal 120 on the ground. But a 20m vertical at 8 ft off the ground with 4 radials will be better than the same vertical on the ground with 4 radials. But if you want that 8 ft high vertical to equal 120 on the ground, you will need about 60 or so, being it is only 1/8 wave up at that frequency. This would give pretty decent performance. Much better than the 4 radials at 1/8 wave up where 4 radials is equal to about 8-10 on the ground. Neither one of those is going to set the woods on fire.. Obviously, an elevated multi band vertical with radials for each band will have varying degrees of ground loss depending on the band in use at the time. If you had a multi band 1/4 wave vertical "GP" at 32 ft, and had 4 radials for each band, you would have much less ground loss on 10m, than on 80m. On 10m, it's 1 wavelength, and just 1 radial will be enough to make an efficient antenna, except you have a dipole. If you use two radials 180 degrees apart, that should actually be a tad lower ground loss than 120 radials on the ground being it's at 2 wavelengths up. On 20m, it's at 1/2 wave up, and still very low loss. At this point the 4 radials should be very close to the 120 on the ground mount. On 40m, it's at 1/4 wave up, and the 4 radials will be equal to about 50-60 on the ground. The antenna will still work quite well. On 80m, it's at 1/8 wave up, and the 4 radials would be about equal to about 8-10 on the ground. A good bit of loss on that band. You will be able to operate, but not with the gusto of the higher bands. :( Maybe this will give you an idea of the appx level of loss for a given number of radials at certain heights, vs the 120 on the ground. The most important thing to remember is to think in terms of wavelength off the ground for the band to be used. Another point I have heard in the forums, but not confirmed, is that a reduced size vertical element doesn't gain much by adding radials longer than the antenna is high. Nope, I don't really agree. In fact, I think using the shorter radiator makes the use of the lower ground loss radial set even more important and worthwhile if you are trying to approach full size performance. You would see a difference I'm fairly sure. BTW, a lot of the info I just wrote a novel about came from the Bill Orr handbooks. He has sections on the subject, and also graphs that match the levels of loss I mentioned at the various heights. In testing various verticals, including a full size 40m ground plane which I could vary the height, I've never seen anything to show his data is incorrect. It's from one of those blasted books Art has problems with, but I happen to trust it as fairly accurate. All this pertains to the usual 1/4 wave elevated ground planes vs a ground mount. |
Ground conductivity's effect on vertical
On Apr 20, 10:32 pm, wrote:
All this pertains to the usual 1/4 wave elevated ground planes vs a ground mount. BTW, using a larger number of radials will not improve the ground conditions of the far field, but being that improving the efficiency of a given height vertical improves the gain equally in all directions, you should see an improvement for all types of propagation. The ground wave, space wave, and sky wave will all improve by increasing antenna efficiency. |
Ground conductivity's effect on vertical
On Apr 20, 10:32 pm, wrote:
If you use two radials 180 degrees apart, that should actually be a tad lower ground loss than 120 radials on the ground being it's at 2 wavelengths up. Make that one wave up for 10m.. :/ But the rest should still apply. Two should be slightly lower loss than the 120 on the ground, being 20m is the frequency where they should be about equal. I was rereading all that to see if I molested any numbers.. I knew I'd find one.. :( |
Ground conductivity's effect on vertical
Richard Fry wrote:
. . . I've discussed the difference between sky and ground wave, and Brown, Lewis, and Epstein's measurements a number of times on this newsgroup in response to pretty much the same questions by Richard, so there's no need to do it again. Anyone interested in my comments can do a search of my postings which include "ground wave" or "surface wave". Roy Lewallen, W7EL |
Ground conductivity's effect on vertical
On Sun, 20 Apr 2008 14:07:59 -0700, Roy Lewallen
wrote: Buck wrote: Another point I have heard in the forums, but not confirmed, is that a reduced size vertical element doesn't gain much by adding radials longer than the antenna is high. I don't believe that's true. I'll gladly consider any supporting evidence. Hearing something on forums is among the worst justification for believing it, in my opinion. Roy Lewallen, W7EL Long ago, I discovered that anyone can become an "expert" on the internet by making a statement and having someone else agree with them or back them up. This is, of course, why I included the statement "but not confirmed." Very recently I heard a conversation on the air between a ham who was "elmering" a new ham and another ham. The "elmer" was trying to help the new ham convert a "vertical dipole" so it would operate on HF. It was rather confusing for a bit until he better described it. It turned out to be some kind of VHF ground plane antenna. I understand ignorance.... but this worries me. -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
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