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A good RF ground
On Fri, 20 Apr 2007 07:58:16 +0100, "Richard"
wrote: More like the radials make the ground below your feet more like say a pool of salt water than the high resistivity ground that it normally is. What do you think? Hi Richard, Actually salt water sucks as a local ground - it is as poor a "good" conductor as you could imagine. Carbon is a far better conductor than salt water, but no one yet has suggested building on top of a coal seam. You would be better off filling your yard with sand to the depth of 30 feet or so (yeah, sure). The testimonials attributed to salt water comes with its far field qualities of a tremendous mismatch to air and offering spectacularly low radiation launch angles. So, copper replaces a very poor conductor (as a first pass approximation). Invest your copper in close proximity to the base of the antenna. That is, a lot of short radials, and a fair number of medium size ones, and a few long ones. Two things to consider. The ground closest to the antenna is responsible for efficiency in loading. The ground further out (between 5 and 10 wavelengths, or more) is responsible for launch efficiency (offering lower launch angles). 73's Richard Clark, KB7QHC |
#2
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A good RF ground
"Richard Clark" wrote in message ... On Fri, 20 Apr 2007 07:58:16 +0100, "Richard" wrote: More like the radials make the ground below your feet more like say a pool of salt water than the high resistivity ground that it normally is. What do you think? Hi Richard, Actually salt water sucks as a local ground - it is as poor a "good" conductor as you could imagine. Carbon is a far better conductor than salt water, but no one yet has suggested building on top of a coal seam. You would be better off filling your yard with sand to the depth of 30 feet or so (yeah, sure). The testimonials attributed to salt water comes with its far field qualities of a tremendous mismatch to air and offering spectacularly low radiation launch angles. So, copper replaces a very poor conductor (as a first pass approximation). Invest your copper in close proximity to the base of the antenna. That is, a lot of short radials, and a fair number of medium size ones, and a few long ones. Two things to consider. The ground closest to the antenna is responsible for efficiency in loading. The ground further out (between 5 and 10 wavelengths, or more) is responsible for launch efficiency (offering lower launch angles). Hmm, a new concept introduced. I was thinking on these lines. The first thing to do is to prepare the ground, that is, alter the nature of the ground in the vicinity of the antenna from an electrical point of view. To improve from "poor ground" to "good ground", in an electrical sense. That can be done by laying wires in the ground (radials) or a wire mesh. Once that is done, one is in a better postion to have the best RF ground possible. I think this is what the radial/wires do, they simply alter the nature of the ground where they are placed. This is like making poor ground, good ground or good ground, excellent ground by laying wires in the ground. Now what seems to be the case is that there is an advantage in making the ground nearest the antenna the very best ground that you can. So, if you are going to use wires to better the ground, make sure that most radials go in near the base of the antenna. Of course, wires improve the ground, it's conductivity, but in practice, you tend to connect the antenna to the radials/wires rather than make for seperate arrangements. |
#3
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A good RF ground
On Fri, 20 Apr 2007 09:02:34 +0100, "Richard"
wrote: The first thing to do is to prepare the ground, that is, alter the nature of the ground in the vicinity of the antenna from an electrical point of view. To improve from "poor ground" to "good ground", in an electrical sense. That can be done by laying wires in the ground (radials) or a wire mesh. Once that is done, one is in a better postion to have the best RF ground possible. I think this is what the radial/wires do, they simply alter the nature of the ground where they are placed. This is like making poor ground, good ground or good ground, excellent ground by laying wires in the ground. Hi Richard, Yes, this is a good analogy. It fails quickly, however. That is, you do not gain better ground characteristics in proportion to the number of radials. So, as general rules go, the common advice is to make your radials as long as the radiator is high, and to lay out as many as you feel comfortable doing. If you need a hard number, then a dozen to sixteen is a good starting place from which others can argue ceaselessly to offer you to gain only 1 more dB of performance (maybe 2). Now what seems to be the case is that there is an advantage in making the ground nearest the antenna the very best ground that you can. So, if you are going to use wires to better the ground, make sure that most radials go in near the base of the antenna. Of course, wires improve the ground, it's conductivity, but in practice, you tend to connect the antenna to the radials/wires rather than make for seperate arrangements. True, that is the sense of a counterpoise. However, even a ground field beneath a dipole overhead improves the dipole's performance and there is no direct connection there. Some would argue this only describes a yagi pointing straight up and the gain only comes to more power poured into the clouds. Yes, this is an appealing argument, and yet if you consider how much the gain rises at a low angle of launch; then you find it is beneficial in that respect as well. The alternative view of radials is that they operate as a shield against loss. 73's Richard Clark, KB7QHC |
#4
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A good RF ground
Richard Clark wrote:
On Fri, 20 Apr 2007 07:58:16 +0100, "Richard" wrote: More like the radials make the ground below your feet more like say a pool of salt water than the high resistivity ground that it normally is. What do you think? Hi Richard, Actually salt water sucks as a local ground - it is as poor a "good" conductor as you could imagine. Carbon is a far better conductor than salt water, but no one yet has suggested building on top of a coal seam. You would be better off filling your yard with sand to the depth of 30 feet or so (yeah, sure). The testimonials attributed to salt water comes with its far field qualities of a tremendous mismatch to air and offering spectacularly low radiation launch angles. mostly due to that epsilon of 80, more than the conductivity So, copper replaces a very poor conductor (as a first pass approximation). Invest your copper in close proximity to the base of the antenna. That is, a lot of short radials, and a fair number of medium size ones, and a few long ones. Exactly... In fact, one can do some numerical analysis to figure out an optimum strategy, based on minimizing IR losses in the soil. The current density is higher close to the base of a vertical, so, at first glance, it would appear that improvements in conductivity there would have more "value". I imagine there's some nice integration that covers it all. All that analysis and spreadsheets out there all make the assumption that you want all radials the same length, which isn't necessarily so. What also throws a "wrench into the gear train" is that if you start looking at verticals with nonuniform current distributions, especially if they aren't representable by a "simple" form such as linear or cos(h), an analytical approach gets tricky (hence the suggestion for numerical methods). Consider, for instance, a half wave dipole with the center, say, 3/8 wavelength above the ground. Or some sort of asymmetric vertical with an elevated feedpoint, or loading coils. The optimum radial layout gets a bit trickier to figure out. Two things to consider. The ground closest to the antenna is responsible for efficiency in loading. The ground further out (between 5 and 10 wavelengths, or more) is responsible for launch efficiency (offering lower launch angles). 73's Richard Clark, KB7QHC |
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