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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 |
#2
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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 |
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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 |
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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 |
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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 |
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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 |
#7
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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 |
#8
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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 |
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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 |
#10
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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 |
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