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#21
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| Sorry to disagree, Reg, but it appears you're overlooking an
important | point--the difference between the efficiency of the radiating system | itself, versus the efficiency of the ground area external to the | radiating system. | | BL&E shows that when 90 - 120 (actually 113) radials of 0,4 w/l form | the ground system for a 1/4 wl radiator, the efficiency is 98.7% | efficient, REGARDLESS OF THE SOIL RESISTIVITY UNDER THE RADIALS. This | is shown by obtaining the field strength of 192 mv/meter at 1 mile for | 1000 watts delivered to the antenna under the conditions described | above, compared to 194.5 mv/meter with a perfect ground having an | efficiency of 100% | | It is only the soil resistivity of the ground external to the radial | system that determines the field stength external to the radial | system. Consequently, the soil resistivity (or conductivity, if you | like) is significant only in the areas external to the radial system. | | Walt, W2DU Walt, What if the ground outside the radial system was comprised of 30-1000 feet of sand and rock overlying any conductive soil below? Would we then be able to measure 192 mv/meter at one mile with our 113 radials of 0.4 w/l? Ron, WA4IWN |
#22
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dansawyeror wrote:
All, A fundamental basic question, which is the primary purpose of radials: 1. is it to create a ground, that is a as close as possible to zero ohm virtual reference for the 'real' vertical half of the dipole? There is no such thing as "creating a ground". As for a "virtual reference", you can declare any point on any conductor a "reference" and for that matter "ground" that you wish. 2. Or are they to create a real resonant half of a dipole? There's no need to try to make a dipole. If the radials are on or very near the ground, their sole purpose is to reduce the amount of loss due to current returning to the base of the antenna. The current entering the antenna at the base equals the current flowing into the source through the ground. This ground current results in I^2 * R loss; radials reduce the R and therefore the loss. In the case of a "ground plane" antenna with highly elevated radials, the radials provide a path for the base current to flow (again, current out of the source -- into the antenna -- has to equal the current into it -- from the radials). Because of the physical configuration, it's sometimes more convenient to build an antenna this way instead of making a dipole. The radials radiate very little, and the vertical section radiates twice as much per unit length as a dipole, resulting in the same overall gain and pattern. If it is the first then what does the 'efficiency' curve look like for a shortened, loaded, vertical? That is if the vertical element is loaded to resonate at 1/5 of a half wave length what does the ground resistance profile look like for 120 radials at various lengths of 1/20 wave, 1/10 wave and 1/5 wave? The answer to this depends on the ground conductivity and frequency. But the radiation resistance of the shortened antenna will be less than that of a full-height one. Therefore, if the ground resistance is fixed and determined by the ground system (not completely true -- it does depend some on the antenna height -- but close enough for discussion), the efficiency of the short antenna will be less than for a full-height one. I recommend finding and reading "The W2FMI Ground-Mounted Short Vertical", by Jerry Sevick, W2FMI in March 1973 QST. He built several antennas very much like you describe and made extensive measurements. The question I am really driving at is if mesh is layed down at 100% coverage about what fraction of a wave length needs to be covered to create a 2.5, 5, and 10 ohm equivalent ground for the vertical above? Sorry, I don't know the answer to that one right off the bat. It could be determined with NEC-4 modeling, but I don't have time to do that. I suggest that you locate a copy of Brown, Lewis, and Epstein's paper "Ground Systems as a Factor in Antenna Efficiency", now posted on the web. You should be able to get a fairly good idea from their measurements of 113-radial systems. In the paragraph above is the mesh simulating a ground or is it fact operating as a ground. I really don't know what "simulating a ground" and "operating as a ground" means. But the radial field doesn't act like either real Earth or a perfect infinite plane, if that's what you mean. If sufficiently fine, a mesh will act like a solid conductor the size of the mesh. Roy Lewallen, W7EL |
#23
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"Walter Maxwell" wrote in message ... On Wed, 7 Sep 2005 14:07:39 +0000 (UTC), "Reg Edwards" wrote: MK, How satisfying it is to read your message, written in plain, easy to understand, well-punctuated English, without any undeciferable coded abbreviations. I agree with what you say although I am unfamiliar with exactly how the FCC fits into the scheme of things. Amateurs and commercial broadcasters have a common fundamental requirement. There is a service area to be covered with a given field strength. Depending on frequency, requirements then diverge. But the design methods used to satisfy requirements are all confined (or should be) to the principles of engineering economics. Inevitably, the Dollar, Pound, Frank, Mark, Rouble and the Yen rule the roost. Both commercial broadcasters and amateurs do a cost-befit analysis. The broadcaster takes into account the revenue acruing from selling the service. The amateur, whether he likes it or not, has to ask himself what the satisfaction of using the station is worth. Amateurs' bank accounts are not unlimited. Field strength at the limits of the service area depends on the power efficiency of the radiating system. If engineering economics dictate use of a set of buried ground radials then the peformance of the ground radials must be included. Considering the system as a whole, it may be economical NOT to achieve the maximum possible radiating efficiency. Indeed, the maximum is seldom the target. If there is an economical choice in the matter, once the location of the station is decided, everybody agrees that efficiency depends on soil resistivity at the site. To estimate efficiency it is necessary, at the very least, to make a guess at soil resistivity. Perhaps just by looking at the type of weeds growing in it. Or it can be measured. Depending on how far it enters into station economics, it is possible to numerically estimate efficiency from the number and length of radials AND FROM SOIL RESISTIVITY. B.L & E and the FCC don't enter into it. ---- Reg. Sorry to disagree, Reg, but it appears you're overlooking an important point--the difference between the efficiency of the radiating system itself, versus the efficiency of the ground area external to the radiating system. BL&E shows that when 90 - 120 (actually 113) radials of 0,4 w/l form the ground system for a 1/4 wl radiator, the efficiency is 98.7% efficient, REGARDLESS OF THE SOIL RESISTIVITY UNDER THE RADIALS. This is shown by obtaining the field strength of 192 mv/meter at 1 mile for 1000 watts delivered to the antenna under the conditions described above, compared to 194.5 mv/meter with a perfect ground having an efficiency of 100% It is only the soil resistivity of the ground external to the radial system that determines the field stength external to the radial system. Consequently, the soil resistivity (or conductivity, if you like) is significant only in the areas external to the radial system. Walt, W2DU ======================================= Walt, just what is it you cannot agree with? You appear to be making an argument where none exists. It is obvious there must be a distant point beyond which a large number of radials will approach 100% efficiency regardless of ground resistivity. B.L & E and the FCC arbitraliry decided on 1/2-wavelength and 120. Both nice round figures. I'm sorrry to say you appear unable to agree that for the remaining 99.9% of all possible cases, ie., for cases less than 1/2-wavelength and fewer than 120 radials, that GROUND RESISTIVITY in the immediate vicinity of the antenna DOES HAVE A SIGNIFICANT EFFECT ON EFFICIENCY and it cannot be disregarded. My only criticism of B.L & E is that they forgot to measure soil resistivity before leaving the site. And apparently, nobody has ever bothered to go back and do it for them. The only mention of their work occurs on this newsgroup. When laying radials, 99% of amateurs forget B.L & E (if they have ever heard of them) and the magic number of 120. Hasan Schiers has recently given a blow-by-blow account of a sensible way to lay a set of radials with the reasoning behind it. ---- Reg. |
#24
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Reg, G4FGQ wrote:
"The broadcaster takes into account the revenue accruing from selling the service." Yes, and he takes into account the ease of compliance with FCC Riles in the USA. Although 90 radials is a minimum requirement for the U.S. AM broadcaster, here is the rule quoted by John Edward Cunningham in his 1977 "The Complete Broadcast Antenna Handbook", from page 311: "The current FCC Rules specify that the radials should be at least 1/4 wavelength long and that there should be as many as practicable, but in no case less than 90. The Rules add that a system of 120 radials spaced every 3-degrees and extending 0.35-0.4 wavelength from the tower is considered an excellent ground system. In addition, a square ground screen 24 or 48 on a side is often provided at the base of the tower, particularly when the tower height is such as to cause a high voltage. Whenever a less than optimum ground system is used, the FCC requires a complete field-intensity survey to establish that the effective field at one mile meets minimum requirements." Best regards, Richard Harrison, KB5WZI |
#25
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Reg Edwards wrote:
. . . My only criticism of B.L & E is that they forgot to measure soil resistivity before leaving the site. And apparently, nobody has ever bothered to go back and do it for them. What would you suggest as a method of measuring the RF ground resistivity to a depth of 17 - 100 feet (3 skin depths at 3 MHz for the range of likely resistivities)? Assuming it's very likely that the resistivity would be quite different at different depths within that range, how should they have used that information? How would you use that information if someone "bothered to go back and do it for them"? Reg, all signs point to your being seriously in need of a holiday. How about a little trip to New Jersey to show us Yanks how it should have been done? For less than the price of a couple of bottles of decent wine, you can buy everything you'll need -- bucket, pocket DVM, trowel -- right there, so you won't even have to carry any equipment with you. Roy Lewallen, W7EL |
#26
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Dear Roy,
I'm sorry to say your worthless comments constitute an abject admission of defeat in an argument which exists only in your imagination. By the way, are you still using your S-meter as the North American Standard of signal strength? ---- Reg. |
#27
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Dan Richardson wrote - Had BL&E been Englishmen I sure things would be different.G ==================================== They sure would! They would have been instructed to go back and finish the job. ---- Reg. |
#28
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Dan Richardson wrote -
Had BL&E been Englishmen I sure things would be different.G ==================================== They sure would! They would have been instructed (by their employers) to go back and finish the job. ---- Reg. ===================================== I am reminded of the military engineer who was dispatched by Napolion, an engineer himself, in connection with standardisation of the Metre, to measure the distance between the Earth's Equator and the North Pole. Measurements began, but the further the engineer departed from his beautiful lady friend in Paris the more difficult it became to make progress along the route. Eventually, he couldn't withstand the mental and physical stress. He returned to her Parisian boudoir and resorted to cooking the books in what time he had to spare. So, the International Standard of Length, The Metre, held in Paris, France, carefully guarded by the German occupying forces during WW2, may or may not be equal to 39.37 English inches. Actually, the most fundamental physical measurement standard is the Mass of the Standard Kilogram on which everything else depends. But it is quite an arbitrary quantity. I have just finished a bottle of Blossom Hill, Californian, white wine. Makes a pleasant change to arguing about what 'amateurs' BL&E might, or might not have done before leaving the site. ---- Reg. |
#29
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Reg Edwards wrote:
Dan Richardson wrote - Had BL&E been Englishmen I sure things would be different.G ==================================== They sure would! They would have been instructed (by their employers) to go back and finish the job. ---- Reg. ===================================== I am reminded of the military engineer who was dispatched by Napolion, an engineer himself, in connection with standardisation of the Metre, to measure the distance between the Earth's Equator and the North Pole. Measurements began, but the further the engineer departed from his beautiful lady friend in Paris the more difficult it became to make progress along the route. Eventually, he couldn't withstand the mental and physical stress. He returned to her Parisian boudoir and resorted to cooking the books in what time he had to spare. So, the International Standard of Length, The Metre, held in Paris, France, carefully guarded by the German occupying forces during WW2, may or may not be equal to 39.37 English inches. Actually, the most fundamental physical measurement standard is the Mass of the Standard Kilogram on which everything else depends. But it is quite an arbitrary quantity. I have just finished a bottle of Blossom Hill, Californian, white wine. Makes a pleasant change to arguing about what 'amateurs' BL&E might, or might not have done before leaving the site. ---- Reg. All the above is/maybe true but remember that all the formule work using the values represented in the METRE and the KILOGRAM. Something must be correct here.... Dave WD9BDZ |
#30
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On Thu, 8 Sep 2005 16:09:23 +0000 (UTC), "Reg Edwards"
wrote: I have just finished a bottle of Blossom Hill, Californian, white wine. Makes a pleasant change to arguing about what 'amateurs' BL&E might, or might not have done before leaving the site. ---- Reg. I hate to say this, Reg, but calling BL&E 'amateurs' is insulting to me. I knew them well, and worked with B and E in the RCA Laboratories antenna lab, and they are anything but 'amateurs'. I think you finished off the Blossom Hill to rapidly. Perhaps you should take a look at the long list of IRE articles published by Dr. G.H. Brown, reporting his work that shaped the present design of all AM BC antennas. Did you ever wonder what happened to the diamond-shaped AM towers? And why the AM towers constructed since 1940 have a uniform cross section? And did you know that Dr. Brown gave John Kraus, W8JK, the idea of close spaced elements that culminated in the 'W8JK Beam' antenna? L, (Bob Lewis) is also a fine engineer, although he is also a ham, W2EBS. Walt |
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