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HF-Ground
Question:
Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John |
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To cut down on copper corrosion, I use enameled #14 magnet wire.
"Bob Miller" wrote in message ... On 19 Jan 2006 06:04:29 -0800, wrote: Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John In one of my antenna books, by W6SAI, he recommended uninsulated wire for buried radials. bob k5qwg |
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On 19 Jan 2006 06:04:29 -0800, John wrote:
Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? It makes no real noticeable difference in operation, however, insulated wire will not deteriorate as fast in the ground. Danny, K6MHE email: k6mheatarrldotnet http://www.k6mhe.com/ |
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If you are also going to use the radials for a lightning protection
ground then bare wire is much better. 73 Gary K4FMX On 19 Jan 2006 06:04:29 -0800, wrote: Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John |
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On 19 Jan 2006 16:28:53 GMT, "Bill Turner" wrote:
wrote: Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Insulated will reduce corrosion. Don't bury them any deeper than necessary. Lying on top of the ground is better. Dirt is not a good antenna element. Bill, I often see the assertion that it is better to not bury radials. Can you point me to any reputable texts or experimental evidence that shows the difference between shallow buried radials and radials lying "on top of the ground"? Corrosion is often cited as a reason to use insulated buried radials, but is corrosion a significant risk in most locations. We widely use buried copper water pipes here, and copper clad earthing electrodes for the MEN power supply earthing, yet they don't seem to suffer significant corrosion in most places. It seems to me that insulated buried radials are likely to be less effective in a lightning protection role. Owen -- |
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"Bill Turner" wrote in news:xn0ehew7u3qjhj000
@cnews.newsguy.com: wrote: Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Insulated will reduce corrosion. Don't bury them any deeper than necessary. Lying on top of the ground is better. Dirt is not a good antenna element. Electric fence wire is adequate and cheap. I use it right on the surface, as you say. It's a bit springy so you may have to work it to straighten it, though. -- Dave Oldridge+ ICQ 1800667 |
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Ok, but do you use steel or aluminum fence wire? Galvanized steel fence wire
will rust in a few years. "Dave Oldridge" wrote in message 9... Electric fence wire is adequate and cheap. I use it right on the surface, as you say. It's a bit springy so you may have to work it to straighten it, though. -- Dave Oldridge+ ICQ 1800667 |
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Owen Duffy wrote:
Bill, I often see the assertion that it is better to not bury radials. Can you point me to any reputable texts or experimental evidence that shows the difference between shallow buried radials and radials lying "on top of the ground"? Corrosion is often cited as a reason to use insulated buried radials, but is corrosion a significant risk in most locations. We widely use buried copper water pipes here, and copper clad earthing electrodes for the MEN power supply earthing, yet they don't seem to suffer significant corrosion in most places. It seems to me that insulated buried radials are likely to be less effective in a lightning protection role. Owen ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Your mind is already made up. Do as you like. Bill, W6WRT |
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On 20 Jan 2006 02:08:57 GMT, "Bill Turner" wrote:
Owen Duffy wrote: Bill, I often see the assertion that it is better to not bury radials. Can you point me to any reputable texts or experimental evidence that shows the difference between shallow buried radials and radials lying "on top of the ground"? Corrosion is often cited as a reason to use insulated buried radials, but is corrosion a significant risk in most locations. We widely use buried copper water pipes here, and copper clad earthing electrodes for the MEN power supply earthing, yet they don't seem to suffer significant corrosion in most places. It seems to me that insulated buried radials are likely to be less effective in a lightning protection role. Owen ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Your mind is already made up. Do as you like. I note that you decline to subtantiate the reasons underlying your advice. Has anyone references to sound evidence that supports Bill's advice that radials "Lying on top of the ground is better." than buried. Owen -- |
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Owen Duffy wrote:
On 20 Jan 2006 02:08:57 GMT, "Bill Turner" wrote: Owen Duffy wrote: Bill, I often see the assertion that it is better to not bury radials. Can you point me to any reputable texts or experimental evidence that shows the difference between shallow buried radials and radials lying "on top of the ground"? Corrosion is often cited as a reason to use insulated buried radials, but is corrosion a significant risk in most locations. We widely use buried copper water pipes here, and copper clad earthing electrodes for the MEN power supply earthing, yet they don't seem to suffer significant corrosion in most places. It seems to me that insulated buried radials are likely to be less effective in a lightning protection role. Owen ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Your mind is already made up. Do as you like. I note that you decline to subtantiate the reasons underlying your advice. Has anyone references to sound evidence that supports Bill's advice that radials "Lying on top of the ground is better." than buried. Owen -- Owen; Given the expertise that Bill has accumulated over the years and the good advise he has given to anyone who asks I think that your attitude needs modification. When you were in school did you challenge your teachers this way? I think not. If you diagreed you kept it to your self or checked it out on your own. For what it's worth everything that I have read tends towards placing the radials on the open ground, usually staked down so as to prevent tripping or getting caught in a lawn mower. Dave WD9BDZ Note to Bill: I know you don't need anyone to defend you but this guy isn't going to take any answer from anyone. I suggest that we drop him as a thread. I just know his response is going to be at me demanding positive confirmation. ;^).. |
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On Thu, 19 Jan 2006 21:22:25 -0600, "David G. Nagel"
wrote: Owen; Given the expertise that Bill has accumulated over the years and the good advise he has given to anyone who asks I think that your attitude needs modification. When you were in school did you challenge your teachers this way? I think not. If you diagreed you kept it to your self or checked it out on your own. Dave, I don't agree, I have never had a teacher worth his salt who responded to polite questions as Bill did. No, I don't believe something just because I read it on the 'net, I would like to know why. For what it's worth everything that I have read tends towards placing the radials on the open ground, usually staked down so as to prevent tripping or getting caught in a lawn mower. Yes, I see lots of web articles describing that in ham stations, but it is not the only approach that I see documented and talked about. In my limited experience, I have not seen commercial HF installations with radial / ground wires laid above ground in preference to being buried. The only cases I can recall were because of rock. Whilst there are articles around about the performance of shallow buried radials, I have not seen any that deal quantitatively with radials laid on the ground, or pinned to the ground as you describe, and the effects of those different installations on antenna efficiency. That is what I was asking about. Equally, there a plenty of articles where the author insists that radials cannot work near the ground and they need to be some distance above, some stating a quarter wave above. They can't all have "better" efficiency, the only way to know is to seek reasons why a configuration is better. Bill proposed a "better" configuration and declined to explain why / how it is better. Owen -- |
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I used about 1300 ft of insulated copper wire for my 72 radials. I chose to
pin mine to the ground witrh fenc e staples. I did this work in the dead of winter. I then also covered them with a thin layer of soil. Come spring the grass grows and the radials are hidden and beneath the applied soil layer so as to not interfere with the lawn mower. Pictures of my installation for my Hustler 5BTV at this web page 1. http://www.ad5th.com/5-BTV.html -- Charlie-AD5TH www.deepsouthnet.net "Gary Schafer" wrote in message ... If you are also going to use the radials for a lightning protection ground then bare wire is much better. 73 Gary K4FMX On 19 Jan 2006 06:04:29 -0800, wrote: Question: Is you are making a HF-ground (radials just below the surface) Should these radials be insulated or not? 73 John |
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David G. Nagel wrote:
Dave WD9BDZ Note to Bill: I know you don't need anyone to defend you but this guy isn't going to take any answer from anyone. I suggest that we drop him as a thread. I just know his response is going to be at me demanding positive confirmation. ;^).. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Thanks, Dave. He's history. (I wonder if he'd like positive confirmation?) 73, Bill W6WRT |
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On Fri, 20 Jan 2006 04:16:39 GMT, Owen Duffy wrote:
In my limited experience, I have not seen commercial HF installations with radial / ground wires laid above ground in preference to being buried. The only cases I can recall were because of rock. Whilst there are articles around about the performance of shallow buried radials, I have not seen any that deal quantitatively with radials laid on the ground, or pinned to the ground as you describe, and the effects of those different installations on antenna efficiency. That is what I was asking about. Equally, there a plenty of articles where the author insists that radials cannot work near the ground and they need to be some distance above, some stating a quarter wave above. Hi Owen, These three paragraphs reveal arguments that vary by application, rather than by degree. It seems to me that most AM stations' ground fields are shallow buried in gravel simply to permit foot traffic. The HAARP site uses a grid that is elevated sufficiently to allow vehicular traffic. Neither really attend lightning as they are more ground screens and principally constructed for RF. I found a much more compelling report in: UNITED STATES DEPARTMENT OF AGRICULTURE Rural Electrification Administration REA BULLETIN 1751F-802 SUBJECT: Electrical Protection Grounding Fundamentals Which is vastly more comprehensive and directly answers these questions when viewed in the terms of the resistivity of the earth connection. 73's Richard Clark, KB7QHC |
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On Thu, 19 Jan 2006 22:19:04 -0800, Richard Clark
wrote: These three paragraphs reveal arguments that vary by application, rather than by degree. It seems to me that most AM stations' ground fields are shallow buried in gravel simply to permit foot traffic. The HAARP site uses a grid that is elevated sufficiently to allow vehicular traffic. Neither really attend lightning as they are more ground screens and principally constructed for RF. Yes, I understand that a ground system may be called upon to perform a role as the other terminal of a Marconi for instance, and as the drain for lightning or other EMP. Hence my earlier comment that a rule of thumb that buried radials should be insulated seems to deny fullest lightning protection to mitigate a small risk of corrosion. I found a much more compelling report in: UNITED STATES DEPARTMENT OF AGRICULTURE Rural Electrification Administration REA BULLETIN 1751F-802 SUBJECT: Electrical Protection Grounding Fundamentals Which is vastly more comprehensive and directly answers these questions when viewed in the terms of the resistivity of the earth connection. OK, I found it and it is substantial. It will be an interesting read, thank you for the pointer. It appears to be focused more on power / lighting protection that an antenna ground system. For others, the URL is http://www.usda.gov/rus/telecom/publ...s/1751f802.pdf .. One of the things that intrigues me is the common "expert" advice to cut radials for 7MHz to 33' long and bury them. It seems to me that when buried and considering the wire as a transmission line, the velocity factor will be somewhere between 0.3 and 0.8 depending on the soil type, so that 33' is likely to be closer to a half wave electrically, and present a relatively high and reactive impedance at the antenna base if it were not for the attenuation of the wave on the radial. It would seem a length more like 17' to 20' would be a better estimate by the SWAG method (Scientific Wild Arsed Guess), although if ground attenuation is high enough, it could be cut shorter and the extra wire used for another radial for a more effective solution. More when I digest some of the article. Thanks again... Owen -- |
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On Fri, 20 Jan 2006 06:56:42 GMT, Owen Duffy wrote:
One of the things that intrigues me is the common "expert" advice to cut radials for 7MHz to 33' long and bury them. It seems to me that when buried and considering the wire as a transmission line, the velocity factor will be somewhere between 0.3 and 0.8 depending on the soil type, so that 33' is likely to be closer to a half wave electrically, and present a relatively high and reactive impedance at the antenna base if it were not for the attenuation of the wave on the radial. Hi Owen, Calling them "tuned" radials is an artifact of their length being described in free space wavelength. The proximity of earth negates such illusions. The association with the necessity of being a quarter wave long comes by the field data obtained by Brown, Lewis, and Epstein. This was simply an arbitrary selection born more of the available wire being portioned out in binary increasing counts (2,4,8,16....) such that 119 radials depleted their stock (short of that magic 128). Their work has been offered on the web through the interests of our discussions here, and by one or several correspondent's scanning and posting their report. Google this newsgroup for that link using the authors as a keyword search. This was offered last summer. 73's Richard Clark, KB7QHC |
HF-Ground
Owen Duffy wrote:
Bill, I often see the assertion that it is better to not bury radials. Can you point me to any reputable texts or experimental evidence that shows the difference between shallow buried radials and radials lying "on top of the ground"? Corrosion is often cited as a reason to use insulated buried radials, but is corrosion a significant risk in most locations. We widely use buried copper water pipes here, and copper clad earthing electrodes for the MEN power supply earthing, yet they don't seem to suffer significant corrosion in most places. It seems to me that insulated buried radials are likely to be less effective in a lightning protection role. Owen You're asking some good questions. Many years ago, I was doing some ground system experiments with a vertical antenna. I had strung a bunch of radials, consisting of small insulated hookup wire, on the surface of the ground and I was measuring antenna input impedance. I was surprised to discover distinct resonance effects as the radial lengths were changed, something which I hadn't expected -- they were acting more like elevated than buried radials. It was summertime and the clay ground was pretty dry. Then I took some pieces of mild steel wire and "stapled" the wires down to conform with the ground and bring the wires into close contact with it. The resonant effects disappeared and the radials acted more like buried ones. I concluded that even a very small air gap between the radials and the ground provided some independence from the ground. More recently I've done some modeling to try and understand the phenomenon a bit better. I'll give those results in my response to another of your recent postings. Oh, as for corrosion -- I'm sure it depends on the soil. But there's no harm in using insulated wire, as far as RF ground effectiveness goes. I suppose it would limit the lighning protection voltage to the insulation voltage, however. Roy Lewallen, W7EL |
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Owen Duffy wrote:
. . . One of the things that intrigues me is the common "expert" advice to cut radials for 7MHz to 33' long and bury them. It seems to me that when buried and considering the wire as a transmission line, the velocity factor will be somewhere between 0.3 and 0.8 depending on the soil type, so that 33' is likely to be closer to a half wave electrically, and present a relatively high and reactive impedance at the antenna base if it were not for the attenuation of the wave on the radial. It would seem a length more like 17' to 20' would be a better estimate by the SWAG method (Scientific Wild Arsed Guess), although if ground attenuation is high enough, it could be cut shorter and the extra wire used for another radial for a more effective solution. Modeling clearly shows that using elevated radials which are too long degrades the efficiency. A quarter wavelength can be determined by constructing a dipole at the height of the proposed radial system and adjusting it to resonance. The maximum length of the radials is half the length of that dipole. If you make them longer, efficiency drops. Very close to the ground, the length of a quarter wavelength decreases substantially, so a free-space quarter wavelength can easily be too long. Modeling presents a very idealized situation, overly so when dealing with ground. But I believe the general trends and conclusions are instructive. I modeled a 40 meter vertical over average ground. It had four 34 foot radials, which were quarter wave resonant when elevated very high. As I lowered the radials from one foot high to 0.1 inch high, the gain dropped 4 dB. The main cause of the drop was that the radials were becoming too long at the low height above ground. Shortening them to 19.6 feet, the resonant length at that height, increased the field strength by 2.45 dB. Burying them lowered the field strength to 1.7 dB below the field strength when 0.1 inch above the ground and of proper length. There wasn't any substantial change in field strength as the length was increased beyond about 30 feet, or when the depth was varied from 0.1 to 6 inches. These changes in field strength are solely due to changes in efficiency; the pattern shape stays the same. When modeled at 0.1 inch above the ground, the radial current distribution is approximately sinusoidal, as in elevated radials. When buried, even an inch, the current decays in an approximately exponential fashion away from the center. In the case of the modeled antenna system, the current was substantially zero beyond about 40 feet. Conclusions a 1. A small number of radials just above the ground are theoretically a bit more efficient than the same number of buried radials, providing that they're not longer than a resonant quarter wavelength at that height. I say theoretically, though, because I believe it would be impossible to maintain current balance in the radials at a low height. So one or two radials would likely hog all the current, resulting in a less efficient system. 2. Making elevated radials too long, even if the elevation consists of being just above the ground, can seriously reduce the antenna efficiency. Buried radials, on the other hand, are insensitive to length provided they're sufficiently long. This latter fact is well known. I've found in other modeling I've done that making elevated radials shorter than a resonant quarter wavelength doesn't negatively impact the efficiency. So if you have to guess, guess on the short side. The amount of differences you'll see in real antenna systems will vary quite a bit depending on ground characteristics, frequency, and number of radials. And it would be impossible to suspend radials precisely over a perfectly flat and homogeneous ground as I've done with the models. But I believe the conclusions are valid. Roy Lewallen, W7EL |
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Owen Duffy wrote:
Whilst there are articles around about the performance of shallow buried radials, I have not seen any that deal quantitatively with radials laid on the ground, or pinned to the ground as you describe, and the effects of those different installations on antenna efficiency. Will NEC-4 accurately model radials at different depths? -- 73, Cecil http://www.qsl.net/w5dxp |
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Owen Duffy wrote:
One of the things that intrigues me is the common "expert" advice to cut radials for 7MHz to 33' long and bury them. It seems to me that when buried and considering the wire as a transmission line, the velocity factor will be somewhere between 0.3 and 0.8 depending on the soil type, so that 33' is likely to be closer to a half wave electrically, and present a relatively high and reactive impedance at the antenna base if it were not for the attenuation of the wave on the radial. You seem to be referring to the feedpoint impedance of a single radial the virtual impedance of which would depend upon the magnitude and phase of the forward and reflected wave on the radial wire. The single-wire transmission line formula gives a Z0 for each radial as less than 100 ohms. Given the probability of a high degree of attenuation and the number of radials in parallel, the impedance presented at the base is likely to be relatively low no matter what the length of the radials assuming an electrical length of longer than 1/4WL. -- 73, Cecil http://www.qsl.net/w5dxp |
HF-Ground
On Thu, 19 Jan 2006 23:31:32 -0600, "Charlie" wrote:
I used about 1300 ft of insulated copper wire for my 72 radials. So they're about 18 feet long, giving you a short, dense pattern? bob k5qwg I chose to pin mine to the ground witrh fenc e staples. I did this work in the dead of winter. I then also covered them with a thin layer of soil. Come spring the grass grows and the radials are hidden and beneath the applied soil layer so as to not interfere with the lawn mower. Pictures of my installation for my Hustler 5BTV at this web page 1. http://www.ad5th.com/5-BTV.html |
HF-Ground
"Roy Lewallen" wrote
2. Making elevated radials too long, even if the elevation consists of being just above the ground, can seriously reduce the antenna efficiency. Buried radials, on the other hand, are insensitive to length provided they're sufficiently long. This latter fact is well known. I've found in other modeling I've done that making elevated radials shorter than a resonant quarter wavelength doesn't negatively impact the efficiency. So if you have to guess, guess on the short side. _____________ NEC studies of a 1/4-wave vertical radiator working against three 1/4-wave horizontal radials at 120 degrees, when all elements are elevated 12 feet above a perfect ground plane show virtually identical peak gain as when the same radiator minus the radials is mounted with its base at the perfect ground plane, and connects to it though two ohms (about the same ground loss as produced by 120 buried radials, each 1/4-wave long). As few as four elevated radials have been used at AM broadcast sites where a typical system of 120 buried radials was impossible due to rocky terrain. The FCC "efficiency" of these radiator systems meets/exceed FCC requirements for radiation at 1 km. Conclusion: a few elevated radials can be the electrical equivalent of a classic "Brown, Lewis & Epstein" system of 113 (or 120)buried radials. RF |
HF-Ground
"Owen Duffy" wrote Whilst there are articles around about the performance of shallow buried radials, I have not seen any that deal quantitatively with radials laid on the ground, or pinned to the ground as you describe, and the effects of those different installations on antenna efficiency. ======================================== Owen, you may be interested in Program RADIALS3. Once radials are buried in the ground, just below soil surface, I don't think depth of burial matters very much. But things happen very fast when resting on the soil surface and conductor height is raised only slightly. Consider a single counterpoise wire. When resting on the soil surface propagation velocity along the wire is about half of the free space velocity. Propagation velocity increases fast as height increases. When height is greater than length the VF is very nearly 1. When resting on the ground there is an equivalent loss resistance distributed along the wire due to the mutual impedance with the ground. The input resistance at resonance is high. Q is very small. But a lot depends on soil resistivity. With increasing height the coupling with ground decreases and so does the input resistance. When height is greater than length the input resistance is due only to conductor loss resistance and resonant Q is high. Resonant frequency increases with height due to the increase in VF. The increase in VF is due to decrease in capacitance to ground when considered as a transmission line. Changes in capacitance, VF, resonant frequency, and induced loss in the ground, mostly occur in first few inches of height above ground but cannot be neglected until height is roughly greater than length. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
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"Richard Fry"
NEC studies of a 1/4-wave vertical radiator working against three 1/4-wave horizontal radials at 120 degrees, when all elements are elevated 12 feet above a perfect ground plane... etc ____________ I neglected to include that the frequency in these studies was 1 MHz, so elevation of the system was about 0.012 wavelengths (Reg). RF |
HF-Ground
"John, N9JG" wrote in
news:OVTzf.501279$084.368852@attbi_s22: Ok, but do you use steel or aluminum fence wire? Galvanized steel fence wire will rust in a few years. Copperclad steel, usually. -- Dave Oldridge+ ICQ 1800667 |
HF-Ground
On Fri, 20 Jan 2006 13:56:28 +0000 (UTC), "Reg Edwards"
wrote: Owen, you may be interested in Program RADIALS3. For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp Reg, did I miss something. RADIALS3 doesn't seem to be in the index at http://www.btinternet.com/~g4fgq.reg...3.html#S301%22 Owen -- |
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Owen Duffy wrote:
I often see the assertion that it is better to not bury radials. Here's a data point for all. I'm taking down my 102 ft dipole and putting back up my 130 ft dipole. I measured the resonant frequency and feedpoint impedance of the 102 ft dipole both while hanging in the air as a V and laying on the ground. Hanging in the air: Resonant at 4.52 MHz with a feedpoint impedance of 22 ohms. Laying on the ground: Resonant at 2.17 MHz with a feedpoint impedance of 108 ohms. Laying the insulated wires on the ground resulted in a reduction of VF of about 50%. The marked increase in feedpoint impedance was due to the attenuation of the reflected waves arriving back at the feedpoint and agrees closely with my calculations of such. -- 73, Cecil http://www.qsl.net/w5dxp |
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Cecil Moore wrote:
Laying the insulated wires on the ground resulted in a reduction of VF of about 50%. The marked increase in feedpoint impedance was due to the attenuation of the reflected waves arriving back at the feedpoint and agrees closely with my calculations of such. I forgot to say: Note that the dipole laying on the ground was close to one wavelength, yet the feedpoint impedance is not all that high. It would appear that 1/2 wavelength buried radials do NOT present a high impedance. -- 73, Cecil http://www.qsl.net/w5dxp |
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"Owen Duffy" wrote For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp Reg, did I miss something. RADIALS3 doesn't seem to be in the index at http://www.btinternet.com/~g4fgq.reg...3.html#S301%22 Owen ---------------------------------------------------------------------- ---- Very sorry Owen. My mistake. The name of the program is RADIALS2. It deals only with buried radials. I am at present trying to write a program about coil-loaded counterpoises and artificial grounds at low heights. But it's a beast to model mathematically. ---- Reg. |
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"Cecil Moore" wrote
It would appear that 1/2 wavelength buried radials do NOT present a high impedance. =============================== Cec, .. . . . as demonstrated by program RADIALS2 which treats radials as transmission lines. As they truly are. The permittivity of soil surrounding buried radials is high. It is due to the moisture content of the soil. Water has a high permittivity K = about 80. If the moisture content is 20 percent then the permittivity is roughly K = 16 plus a little bit for the dry content. The poor, low conductance of the dielectric material, in conjunction with wire inductance, also has an effect. The soil is mainly, minute rock particles and a little air. Rock has K = 4 or 5. Air = 1. Velocity factor of any transmission line = 1 / Sqrt( Permittivity ). In some circumstances, there may be no point in having radial lengths longer than 1/10th or 1/12th of the free-space wavelength. If the soil has any magnetic material in it then the velocity is even lower. But it's a waste of time trying to tune buried radials by sprinkling iron filings around your garden. The attenuation along buried radials is usually so high that even 1/4-wave resonance doesn't show up. Impedance versus length at low HF is a smooth curve approximately equal to Zo. But input impedance of a set of radials is NOT equal to the impedance of the individual wires all in parallel. They interact with each other. The Law of Diminishing Returns applies. In perfectly dry desert sand with a resistivity of 5,000 or 10,000 ohm-metres and K = 3, the 1/2-wave resonance may appear on an impedance vs frequency curve. Program RADIALS2 shows this effect as evidence of reasonable modelling accuracy. This is a case of ground loss decreasing as soil resistivity increases further. It appears attenuation is a maximum when soil resistivity is around several hundred ohm-metres (377?). Which is quite a poor soil. (I once had a garden of sandy soil. Resistivity was 400 ohm-metres even in wet weather. Eventually I moved house. SR fell to 70. On the 160m band 7 radials, each about 10 feet long, plus the cold water pipe, were good enough with a 3/8 wave inverted -L. I never tried B,L,E's 118 radials, 1 wavelength long.) But in bone dry sand-desert soil, just rock mixed with air, at low HF one would not use a system of radials under a 1/4-wave vertical. The antenna could be a horizontal dipole lying on the ground. ;o) ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
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Reg Edwards wrote:
. . . . as demonstrated by program RADIALS2 which treats radials as transmission lines. As they truly are. . . . I disagree. Transmission lines have two conductors. Radials don't. The fields from transmission lines are confined to the region between the conductors. The fields from radials can couple quite strongly, altering their performance a great deal. It might be possible to model a radial system as a system of coupled transmission lines. Is that what your program does? Roy Lewallen, W7EL |
HF-Ground
On Fri, 20 Jan 2006 01:14:18 -0800, Richard Clark
wrote: such illusions. The association with the necessity of being a quarter wave long comes by the field data obtained by Brown, Lewis, and Epstein. This was simply an arbitrary selection born more of the available wire being portioned out in binary increasing counts (2,4,8,16....) such that 119 radials depleted their stock (short of that magic 128). Their work has been offered on the web through the interests of our discussions here, and by one or several correspondent's scanning and posting their report. Google this newsgroup for that link using the authors as a keyword search. This was offered last summer. Thanks, yes I have read the BLE paper. Owen -- |
HF-Ground
On Sat, 21 Jan 2006 00:55:13 +0000 (UTC), "Reg Edwards"
wrote: Very sorry Owen. My mistake. The name of the program is RADIALS2. It deals only with buried radials. Ok, I have had a play with it (again). I am at present trying to write a program about coil-loaded counterpoises and artificial grounds at low heights. But it's a beast to model mathematically. I wish you would explain the models a bit more in these tools, so that the user can have a better understanding of the approach and its applicability to the problem. Owen -- |
HF-Ground
On Thu, 19 Jan 2006 22:19:04 -0800, Richard Clark
wrote: I found a much more compelling report in: UNITED STATES DEPARTMENT OF AGRICULTURE Rural Electrification Administration REA BULLETIN 1751F-802 SUBJECT: Electrical Protection Grounding Fundamentals Which is vastly more comprehensive and directly answers these questions when viewed in the terms of the resistivity of the earth connection. Thanks Richard, I have read the document quickly overall, and a bit more detailed in some key areas. It covers similar material to documents I collected when studying power earthing and lighting protections in years past, but it is a bit more comprehensive... so a good read and a good reference document. One good pickup was the functions for predicting the low frequency resistance of shallow buried radials (which is relevant when radial wires are required to provide a level of power / lightning protection. I created a graph to add to an existing web page from the functions for 3mm (bare) wires buried 0.1m, the graph is at http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt, this graph does not predicting the RF characteristics of the radials.) Tks... Owen PS apologies for the delay in responding, I have been up to the big smoke (the city... Sydney) over the weekend... reminds me of why I left there thirty something years ago. -- |
HF-Ground
Your mind is already made up. Do as you like.
I note that you decline to substantiate the reasons underlying your advice. Hello sir, maybe YOU should nip in the yard with a shovel and some wire (don't forget your strippers) and do some tests yourself! old man |
HF-Ground
"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency resistance of shallow buried radials (which is relevant when radial wires are required to provide a level of power / lightning protection. __________________ For those using buried radials as one terminal of a vertical monopole... The only path consisting of physical conductors that can exist between a series-fed vertical monopole and buried, uninsulated radials is through the PA output, and the antenna matching network at the tower base. This is not adequate to control/prevent system damage from lightning. Three added techniques are used in most MW broadcast applications: 1. A "static drain choke" is installed between the tower base and the junction of the radials. 2. An arc gap is installed across the base insulator and set to flash over at some margin above normal peak voltage 3. The tx contains r-f phase sensors that kill tx output for a few milliseconds after an arc is sensed, so as not to sustain it. RF |
HF-Ground
"Owen Duffy" wrote One good pickup was the functions for predicting the low frequency resistance of shallow buried radials (which is relevant when radial wires are required to provide a level of power / lightning protection. I created a graph to add to an existing web page from the functions for 3mm (bare) wires buried 0.1m, the graph is at http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt, this graph does not predict the RF characteristics of the radials.) ========================================= Owen, I assume the curves in your graphs have been obtained by treating the conductors as transmission lines. As far as I am aware there's no other way of doing it. Except perhaps EZNEC number-crunching mathematical modelling methods. At VLF the inductance of the conductors and the capacitance due to relatively high permittivity of the dielectic material (soil) can be neglected. This leaves only conductor resistance and conductance (or resistivity) of the soil. It is then quite simple for single wires. To predict performance at RF it is necessary to take inductance and capacitance into account. What is unknown is the way in which soil permittivity and resistivity change with frequency. But this hardly matters as the uncertainty at 60 Hz is sufficient to swamp it. I won't ask you what you did about calculating the effects of multiple radial wires, and the interaction between individual wires, which causes "The Law of Diminishing Returns" to be followed. There is sufficient information in your graph to demonstrate that Magician Marzipan's magic high number of 120 is never necessary for amateur purposes. ---- Reg. |
HF-Ground
"Roy Lewallen" wrote - I disagree. Transmission lines have two conductors. Radials don't. ======================================= Roy, try using your imagination! ---- Reg. |
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