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Elevated vs buried radials
I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? Owen |
Elevated vs buried radials
On Sep 29, 10:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? Owen because they are easier to install in most cases than raised radials, unless you get them far enough up to walk under them like i do. |
Elevated vs buried radials
On Sep 29, 3:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? Owen Because my vertical is located out in a pasture. Deer, horses, and the tractor will trip over the wires. Even buried, the tractor tires brought some up when I mowed the pasture late this summer, especially when I turned over the radial field. Paul |
Elevated vs buried radials
On Sep 29, 5:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? Owen I don't think the model is totally valid.. Partially though.. I agree that for a given number of radials used, slightly elevating off the ground is better than buried. But I don't agree that a small number of slightly elevated radials is equal to a large number of buried radials, and most certainly not equal to the same number of radials highly elevated. IE: vs 1/4 or 1/2 wave or more up.. Three slightly elevated radials are not sufficient to lower ground losses down to a low level over mediocre soil. I always think in terms of wavelength when calculating the approximate efficiency of an elevated radial set. For instance, three radials at 1/2 wave up will be pretty much equal to about 120 on the ground. Three at 1/4 wave will be equal to about 50-60 on the ground. Three at 1/8 wave might be equal to 15-20 on the ground. Three at cigarette pack height will be equal to about twice as many as actually used at best. "slightly guessing on that one, but my real world tests seem to pretty much agree". So being as the increase is fairly small at such low heights in wavelength, it is probably practical to just bury them so people won't trip over them.. :/ If tripping is no issue, then it might be worthwhile to get the slight edge in performance. But the increase over buried will be fairly small with them only 100mm up. |
Elevated vs buried radials
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Elevated vs buried radials
Owen writes:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. N6LF made extensive measurements and essenstially confirmed this. He wrote a 7 part series of articles for QEX. You can download them at his site: http://www.antennasbyn6lf.com/2009/1...periments.html This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Practicality. In most cases, you either want to be able to walk above the radials, (i.e. bury them or leave them on the ground) or below them. This means at least 2 - 2.5 m up, and there will be some sagging. Essentially, your vertical just got that much shorter. But if what remains is tall enough, it's a great choice. If a friendly farmer lets you borrow a field in wintertime, stringing four elevated radials is a lot less work than rolling out 32 on the ground. Jon LA4RT, Trondheim, Norway |
Elevated vs buried radials
On 30/09/10 17:49, LA4RT Jon wrote:
writes: I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. N6LF made extensive measurements and essenstially confirmed this. He wrote a 7 part series of articles for QEX. You can download them at his site: http://www.antennasbyn6lf.com/2009/1...periments.html This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Practicality. In most cases, you either want to be able to walk above the radials, (i.e. bury them or leave them on the ground) or below them. This means at least 2 - 2.5 m up, and there will be some sagging. Essentially, your vertical just got that much shorter. But if what remains is tall enough, it's a great choice. If a friendly farmer lets you borrow a field in wintertime, stringing four elevated radials is a lot less work than rolling out 32 on the ground. Hi Jon, Noted. One of the designs I am exploring is an eighth wave vertical over elevated quarter wave radials, three top guy ropes the the radial straining posts, and the top section of the guys are top hat wires. The whole thing is tuned low so that it is matched to 50 ohm line with a shunt inductor. Modelled performance is within 0.5dB of a ground mounted quarter wave with 32 buried radials. Elevating the radials with the shortened vertical is not a big structural challenge, even to 2.5+m to allow a bit of sag in the radial catenary. (I never said I wanted to rig the radials at 100mm, just that above that, the efficiency was relatively high and didn't vary much between 100mm and a few metres.) But, such a design does not conform to the Rules of Thumb commonly trotted out for low HF Marconis. I will look at Rudy's articles, always an interesting read. Thanks Owen |
Elevated vs buried radials
On 29/09/2010 23:44, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? People generally don't have the room to have several elevated radials 1/4 wave long for 20/30/40/80m whereas the buried ones can be laid in any shape required to fit. -- Conor I'm not prejudiced. I hate everyone equally. |
Elevated vs buried radials
On Sep 30, 1:24 am, Owen wrote:
On 30/09/10 12:05, wrote: ... So being as the increase is fairly small at such low heights in wavelength, it is probably practical to just bury them so people won't trip over them.. :/ If tripping is no issue, then it might be worthwhile to get the slight edge in performance. But the increase over buried will be fairly small with them only 100mm up. I am interested in the electrical performance rather than trip hazard. Once electrical performance is known, issues like trip hazard, mowing, rock etc can be dealt with for each application scenario. So, back to the electrical performance, do you have measurement data, or can you refer me to articles that contain sound objective measurement data that would suggest that my NEC4 model is not valid. I haven't done any careful measurements, but one of the books I have has an article and graph that pretty much matches the figures I gave as far as amount of elevated radials needed to match a certain amount on the ground. I'm not sure which book it is.. Maybe one of the Bill Orr radio or antenna handbooks.. I don't think it was the ARRL handbooks. But I have tried exactly what you are proposing. And it didn't really pan out too well. I could hardly tell the difference between having them on the ground, and slightly elevated. Sure, it worked OK, but it didn't mimic a large number of radials by any means. I also tried the elevated ground plane at various heights using the same four radials. You could tell an obvious difference between 1/4 wave up, and 1/8 wave up using the same four radials. The real world results I've seen seemed to fairly closely match the graph I have in that book. BTW, I know it was one of the Bill Orr handbooks which suggesting trying the elevated low radials.. I think the same one that had the graph. He seemed to suggest it was better than having them in the ground. But on the other hand his graph dealing with elevated radials suggested the results would be fairly lackluster. Which they were at this QTH. :( Now that I think about it, I think it was the Bill Orr antenna handbook that had that article and graph pertaining to the radials. Being as you are talking about radials which are very low, I assume the base of the radiator will be low also. Myself, I think a large part of the lack of success I saw was due to not enough radial density at the base of the low vertical. I think one would probably be better off taking the three or four 1/4 wave radials and chopping them into many short radials and laying them on the ground. And this is indeed a fairly common practice when laying them on the ground. I think having the higher density of wire at the base works better than have just a few radials elevated. Most of the ground loss seems to be in the area of the base. I know metal density under the radiator is quite critical as far as short mobile whips. I saw that when I tried mounting my mobile whip on a piece of angle iron that was running across the bed of my truck. It didn't work too well, and being I was used to using that antenna on various mounts, it was obvious something was wrong. And the angle iron was very well grounded as far as bonding to the truck. That wasn't the problem. I then moved it over to the top of the side mounted utility bed box, which is a part of the truck body and a good bit wider, and my usual performance was back. Just by adding more metal under the base of the whip did the trick. I think this is one reason why I always had so much better results with elevated vs ground mount verticals. At 1/4 wave up, the base of the antenna was much farther from ground, and the antenna started to perform more like a vertical dipole, than a grounded monopole. With the ground mount, I was up to 32 radials at one point. And the performance was still mediocre, and barely better than my dipole at 36 ft on longer paths. Only when I got it up at 1/4 wave did it really come alive. After that, it ate the dipole for lunch on longer paths, and with only four radials. All of my comparisons were done using a full 1/4 wave radiator. Anyway, maybe your model is more accurate than I think. But I've already tried doing that, and wasn't too impressed with the results. I've heard others that tried it also, with the same lackluster results. I think elevated or not, it's just too few a number of radials to really be effective at that low height in WL. Too much lossy dirt between the few radials is my theory. I just don't believe in too much of a free lunch when it comes to just a few radials at low heights in wavelength. My stance comes from actually trying it, vs modeling it. I don't totally trust the modeling programs in this area. I've seen too many differences comparing the models vs the actual vertical antennas. For instance, using "average" ground, most of the programs underestimated the performance of my 1/4 wave high ground plane. To get a model that more closely matched real life vs my dipole, I had to set the ground at a much higher conductivity. And in the case you are modeling, it seems to be over estimating the performance vs real life. Or at least what I have seen after trying it. I wished it would have panned out better. Would save a lot of wire. But for me, the improvement was minimal. But.. You are welcome to try it. Maybe you will have better luck than I did. |
Elevated vs buried radials
On Sep 30, 1:24*am, Owen wrote:
So, back to the electrical performance, do you have measurement data or can you refer me to articles that contain sound objective measurement data that would suggest that my NEC4 model is not valid. This topic was investigated experimentally quite some time ago by a broadcast consulting firm in the US, which generated measured data. Here is a clip from their paper describing the system tested, and the results (note that the convention used for "efficiency" here is that of the FCC practice based on the groundwave field intensity at 1 km with respect to the power applied to the antenna system): \\ In November of 1988, our firm supervised the construction of a temporary antenna system in Newburgh, New York under FCC Special Field Test Authority using call sign KPI-204. The antenna system consisted of a lightweight, 15 inch face tower, 120 feet in height, with a base insulator at the 15 foot elevation and six elevated radials, a quarter wave in length, spaced evenly around the tower and elevated 15 feet above the ground. The radials were fully insulated from ground and supported at the ends by wooden tripods. Approximately ten feet above ground, a T network for matching the antenna was mounted on a piece of marine plywood to isolate the components from contact with the lower section of the tower which was grounded. Power was fed to the system through a 200 foot length of coaxial cable with the cable shield connected to the shunt element of the T network and to the elevated radials. A balun or RF choke on the feedline was not employed and the feedline was isolated from the lower section of the tower. The system operated on 1580 kHz at a power of 750 watts. The efficiency of the antenna was determined by radial field intensity measurements along 12 radials extending out to a distance of up to 85 kilometers. The measured RMS efficiency was 287 mV/m for 1 kW, at one kilometer, which is the same measured value as would be expected for a 0.17 wave tower above 120 buried radials. The Newburgh tests gave empirical proof that the elevated system worked although, in an abundance of caution, we used six radials instead of four. For the limited time that the system was operational, the system was stable as determined by monitoring the field intensity at selected locations each day. The measured base impedance was in general agreement with a tower of this height above a standard, buried, ground system. Results of the KPI-204 tests were submitted to the FCC in January of 1989.// The complete paper is available at this URL: http://www.commtechrf.com/documents/nab1995.pdf RF |
Elevated vs buried radials
Owen wrote:
I have been exploring models of a quarter wave monopole over a set of radials on 80m using NEC4 models. If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil. This leaves me wondering why the popularity of extensive fields of buried radials for the lower bands. Comments? Owen I would think that the buried radials are more convenient (broad band, etc.) Look at the performance of your ankle biting radials when the dimensions are changed slightly.. For instance, if you shorten them by 5%, does it make a big difference? For the buried radials, the length is very, very non critical. Something else to look at is the sensitivity of "efficiency" (and your definition of radiated power in the hemisphere/power into antenna is fine) to soil properties.. if the soil conductivity or epsilon changes (as it will with changing water content) does the efficiency change rapidly? Also, what about the loss in any matching components needed (e.g. if you had a real efficient narrow band antenna, then operating off nominal means you'll need a different matching network, and the loss in it might be worth considering) |
Elevated vs buried radials
On Thu, 30 Sep 2010 03:46:40 -0700 (PDT), Richard Fry
wrote: On Sep 30, 1:24*am, Owen wrote: So, back to the electrical performance, do you have measurement data or can you refer me to articles that contain sound objective measurement data that would suggest that my NEC4 model is not valid. This topic was investigated experimentally quite some time ago by a broadcast consulting firm in the US, which generated measured data. Here is a clip from their paper describing the system tested, and the results (note that the convention used for "efficiency" here is that of the FCC practice based on the groundwave field intensity at 1 km with respect to the power applied to the antenna system): \\ In November of 1988, our firm supervised the construction of a temporary antenna system in Newburgh, New York under FCC Special Field Test Authority using call sign KPI-204. The antenna system consisted of a lightweight, 15 inch face tower, 120 feet in height, with a base insulator at the 15 foot elevation and six elevated radials, a quarter wave in length, spaced evenly around the tower and elevated 15 feet above the ground. The radials were fully insulated from ground and supported at the ends by wooden tripods. Approximately ten feet above ground, a T network for matching the antenna was mounted on a piece of marine plywood to isolate the components from contact with the lower section of the tower which was grounded. Power was fed to the system through a 200 foot length of coaxial cable with the cable shield connected to the shunt element of the T network and to the elevated radials. A balun or RF choke on the feedline was not employed and the feedline was isolated from the lower section of the tower. The system operated on 1580 kHz at a power of 750 watts. The efficiency of the antenna was determined by radial field intensity measurements along 12 radials extending out to a distance of up to 85 kilometers. The measured RMS efficiency was 287 mV/m for 1 kW, at one kilometer, which is the same measured value as would be expected for a 0.17 wave tower above 120 buried radials. The Newburgh tests gave empirical proof that the elevated system worked although, in an abundance of caution, we used six radials instead of four. For the limited time that the system was operational, the system was stable as determined by monitoring the field intensity at selected locations each day. The measured base impedance was in general agreement with a tower of this height above a standard, buried, ground system. Results of the KPI-204 tests were submitted to the FCC in January of 1989.// The complete paper is available at this URL: http://www.commtechrf.com/documents/nab1995.pdf RF Hi Richard, Solid piece of information - thanx. Odd to notice none have acknowledged field data. 73's Richard Clark, KB7QHC |
Elevated vs buried radials
On 01/10/10 03:51, Jim Lux wrote:
Thanks Jim. I would think that the buried radials are more convenient (broad band, etc.) Yes, I understand that there are advantages to buried radials, but I don't understand the preponderance of cases where I see 120 radials pinned on the top of infertile dirt. They still present a trip hazard, and less money spent on just a few elevated radials may perform just as well. Look at the performance of your ankle biting radials when the dimensions are changed slightly.. For instance, if you shorten them by 5%, does it make a big difference? For the buried radials, the length is very, very non critical. Yes, of course the feedpoint impedance is more sensitive to change in length or conversely change in frequency. Something else to look at is the sensitivity of "efficiency" (and your definition of radiated power in the hemisphere/power into antenna is fine) to soil properties.. if the soil conductivity or epsilon changes (as it will with changing water content) does the efficiency change rapidly? Yes, efficiency is sensitive to soil parameters... for both types, but not very sensitive. Because of the impedance change mentioned above, the impedance transformation needs adjustment for wide range frequency operation. Not such an issue in the intended application, the DX window on 80m here is just 50kHz. Also, what about the loss in any matching components needed (e.g. if you had a real efficient narrow band antenna, then operating off nominal means you'll need a different matching network, and the loss in it might be worth considering) Matching network loss was not included in my analysis because both quarter wave options present fairly similar load impedances that need transformation to 50 ohms. The shortened verticle is slightly lower R (23 vs 38 IIRC), and slightly more loss can be expected, but it is practical to match with a shunt coil of copper tube and matching loss should be real low in the system context, and in comparison of elevated vs buried radials. If I haven't got something quite wrong in the modelling, it would seem worthwhile to prototype the shortened version with a view to extending the system to a four-square if suitable. I have still to read Rudy's papers... I am away from home (less bandwidth) and I will download them later today when I get home. I suppose that the proposed design challenges the norm of a very large number of buried radials. In our case, part of the property is quite rocky, and a configuration with just a few elevated radials offers deployment opportunities that aren't suited to buried radials. So, my original question is no so much suggesting everyone else got it wrong, but why don't I seem more people doing it this way. Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? Owen |
Elevated vs buried radials
On Sep 30, 3:13*pm, Owen wrote:
Could I be forgive(n) in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? The BL&E experiments were conducted using 3 MHz signals, so their applicability to the 80m band is not a large uplift. |
Elevated vs buried radials
On 01/10/10 06:39, Richard Fry wrote:
On Sep 30, 3:13 pm, wrote: Could I be forgive(n) in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? The BL&E experiments were conducted using 3 MHz signals, so their applicability to the 80m band is not a large uplift. I stand corrected. I have read the paper many times, and my recollection was that it was below 2MHz. Must be time to read it again! Owen |
Elevated vs buried radials
Owen wrote:
On 01/10/10 03:51, Jim Lux wrote: Thanks Jim. I would think that the buried radials are more convenient (broad band, etc.) Yes, I understand that there are advantages to buried radials, but I don't understand the preponderance of cases where I see 120 radials pinned on the top of infertile dirt. They still present a trip hazard, and less money spent on just a few elevated radials may perform just as well. never underestimate the power of tradition. It was written by BL&E that 120 radials work, and the FCC accepts that for broadcast, so by golly, that's what we do. Why 120? it was at the point of diminishing returns or practicality back when the study was done (e.g. there was no detectable change from going to more) As for laying on ground.. I think that's more the laying on grass, and eventually, the wire sinks into the grass/turf. There's also the whole "the radials must be resonant" misconception.. Look at the performance of your ankle biting radials when the dimensions are changed slightly.. For instance, if you shorten them by 5%, does it make a big difference? For the buried radials, the length is very, very non critical. Yes, of course the feedpoint impedance is more sensitive to change in length or conversely change in frequency. While for a buried radial system (probably because of the losses) it's going to be less frequency sensitive. Something else to look at is the sensitivity of "efficiency" (and your definition of radiated power in the hemisphere/power into antenna is fine) to soil properties.. if the soil conductivity or epsilon changes (as it will with changing water content) does the efficiency change rapidly? Yes, efficiency is sensitive to soil parameters... for both types, but not very sensitive. Maybe less sensitive for the buried radials? Or, it was "good enough" for BL&E, so being so written, so shall it be done. Because of the impedance change mentioned above, the impedance transformation needs adjustment for wide range frequency operation. Not such an issue in the intended application, the DX window on 80m here is just 50kHz. If I haven't got something quite wrong in the modelling, it would seem worthwhile to prototype the shortened version with a view to extending the system to a four-square if suitable. The shortened version will, of course, aggravate the tuning sensitivity. I have still to read Rudy's papers... I am away from home (less bandwidth) and I will download them later today when I get home. I suppose that the proposed design challenges the norm of a very large number of buried radials. In our case, part of the property is quite rocky, and a configuration with just a few elevated radials offers deployment opportunities that aren't suited to buried radials. So, my original question is no so much suggesting everyone else got it wrong, but why don't I seem more people doing it this way. Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? Tradition is a powerful force. Look how many years it took for someone (e.g. Rudy) to put the substantial work into doing a real quantitative experiment. For most hams, they're only going to do something once, and if works ok, that's how it stays. Almost none are going to do a well controlled A/B study, especially if there's a (not necessarily valid) tradition that says A works better (where better is ill defined and probably a combination of radiation efficiency and installation convenience) Until recently, modeling tools available to most amateurs were not suitable for making the call, although there have been some people who did models and published it, but, in the face of decades of "lay down 120 radials" it was a tough sell. The other thing is whether the difference is big enough to "make a difference" in observed system performance. For a lot of operators, a 1 dB change in performance might not be noticeable. If you're in a "either propagation is there, or it isn't" situation the difference between good and bad is 10s of dB. There are relatively few people who work at 0dB SNR (where tenths count) on a regular and continuing basis, and they're not necessarily the ones who are interested in doing experiments on antennas on the scale needed. Owen |
Elevated vs buried radials
Referring to my earlier post in this thread with a link to the
measured field intensity data of a MW antenna system using elevated, 1/4-wave radials taken by a consulting engineering firm ... On Sep 30, 1:53 pm, Richard Clark wrote: Odd to notice none have acknowledged field data. A non sequitur, possibly? |
Elevated vs buried radials
On Fri, 01 Oct 2010 06:13:56 +1000, Owen wrote:
Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? The original field research was done at 3 MHz - very much closer to 80M than to 300M. 73's Richard Clark, KB7QHC |
Elevated vs buried radials
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote: It was written by BL&E that 120 radials work, 2, 15, 30, 60, and 113. No discussion whatever of 120. 73's Richard Clark, KB7QHC |
Elevated vs buried radials
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote: For a lot of operators, a 1 dB change in performance might not be noticeable. That 1dB is at the periphery of a radius where surface area (customers) mounts up by the square. 73's Richard Clark, KB7QHC |
Elevated vs buried radials
On 01/10/10 07:44, Jim Lux wrote:
Owen wrote: On 01/10/10 03:51, Jim Lux wrote: Thanks Jim. I would think that the buried radials are more convenient (broad band, etc.) Yes, I understand that there are advantages to buried radials, but I don't understand the preponderance of cases where I see 120 radials pinned on the top of infertile dirt. They still present a trip hazard, and less money spent on just a few elevated radials may perform just as well. never underestimate the power of tradition. It was written by BL&E that 120 radials work, and the FCC accepts that for broadcast, so by golly, that's what we do. Why 120? it was at the point of diminishing returns or practicality back when the study was done (e.g. there was no detectable change from going to more) As for laying on ground.. I think that's more the laying on grass, and eventually, the wire sinks into the grass/turf. There's also the whole "the radials must be resonant" misconception.. Look at the performance of your ankle biting radials when the dimensions are changed slightly.. For instance, if you shorten them by 5%, does it make a big difference? For the buried radials, the length is very, very non critical. Yes, of course the feedpoint impedance is more sensitive to change in length or conversely change in frequency. While for a buried radial system (probably because of the losses) it's going to be less frequency sensitive. I expect so. Something else to look at is the sensitivity of "efficiency" (and your definition of radiated power in the hemisphere/power into antenna is fine) to soil properties.. if the soil conductivity or epsilon changes (as it will with changing water content) does the efficiency change rapidly? Yes, efficiency is sensitive to soil parameters... for both types, but not very sensitive. Maybe less sensitive for the buried radials? Or, it was "good enough" for BL&E, so being so written, so shall it be done. BL&E were measuring ground wave, I think solely. My efficiency measure is the hemisphere, so ground losses play a different role. Because of the impedance change mentioned above, the impedance transformation needs adjustment for wide range frequency operation. Not such an issue in the intended application, the DX window on 80m here is just 50kHz. If I haven't got something quite wrong in the modelling, it would seem worthwhile to prototype the shortened version with a view to extending the system to a four-square if suitable. The shortened version will, of course, aggravate the tuning sensitivity. Yes, but the model suggests that the variation in R is very small, and variation in VSWR (with shunt coil match) is small... in that band segment. I have still to read Rudy's papers... I am away from home (less bandwidth) and I will download them later today when I get home. I suppose that the proposed design challenges the norm of a very large number of buried radials. In our case, part of the property is quite rocky, and a configuration with just a few elevated radials offers deployment opportunities that aren't suited to buried radials. So, my original question is no so much suggesting everyone else got it wrong, but why don't I seem more people doing it this way. Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? Tradition is a powerful force. Look how many years it took for someone (e.g. Rudy) to put the substantial work into doing a real quantitative experiment. For most hams, they're only going to do something once, and if works ok, that's how it stays. Almost none are going to do a well controlled A/B study, especially if there's a (not necessarily valid) tradition that says A works better (where better is ill defined and probably a combination of radiation efficiency and installation convenience) Until recently, modeling tools available to most amateurs were not suitable for making the call, although there have been some people who did models and published it, but, in the face of decades of "lay down 120 radials" it was a tough sell. The other thing is whether the difference is big enough to "make a difference" in observed system performance. For a lot of operators, a 1 dB change in performance might not be noticeable. If you're in a "either propagation is there, or it isn't" situation the difference between good and bad is 10s of dB. There are relatively few people who work at 0dB SNR (where tenths count) on a regular and continuing basis, and they're not necessarily the ones who are interested in doing experiments on antennas on the scale needed. Yes, there will be differing view on what is significant difference. I am not in the school of declaring less than one or two S points is insignificant in general. In the case of a four square in the DX segment, users are looking for performance... and it seems to me that the elevated three radials, eight wave vertical with capacity had is very close to quarter wave over buried radials... depending of course on the soil type. You mention the modelling tools, I am not so much concerned as to whether the elevated radials model is good, but whether the NEC4 buried radials model is good, and likewise for radials on and just above the ground because those models are setting the benchmark for the performance of the alternative. Owen Owen |
Elevated vs buried radials
On 01/10/10 08:17, Richard Clark wrote:
On Fri, 01 Oct 2010 06:13:56 +1000, wrote: Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? The original field research was done at 3 MHz - very much closer to 80M than to 300M. 73's Richard Clark, KB7QHC Yes, I accepted that advice from Richard Fry an hour or so earlier... Owen |
Elevated vs buried radials
On Sep 30, 4:44*pm, Jim Lux wrote:
never underestimate the power of tradition. *It was written by BL&E that 120 radials work, and the FCC accepts that for broadcast, so by golly, that's what we do. A minor point, but in the interest of accuracy - the greatest number of buried radials used in the BL&E experimental work was 113. There's also the whole "the radials must be resonant" misconception.. While the physical lengths of the buried radials in the BL&E experiments were stated in free space wavelengths, that does not mean that those physical lengths will behave the same when buried as they will when not buried. As shown in the link I posted earlier in this thread (and by NEC), a few elevated wires used as a counterpoise in place of the BL&E buried wires need to have an electrical wavelength of 1/4-lambda for best antenna system radiation efficiency, even with "short" vertical monopoles. And even when those elevated counterpoise wires are close to the earth in terms of a free space wavelength, their electrical length is not much different than their physical dimension in terms of a free space wavelength -- as is the case when they are buried. Theory and practice both show that such wires perform differently when they are buried than when they are elevated above the surface of the earth. RF |
Elevated vs buried radials
LA4RT Jon wrote in :
.... N6LF made extensive measurements and essenstially confirmed this. He wrote a 7 part series of articles for QEX. You can download them at his site: http://www.antennasbyn6lf.com/2009/1...les-on-ground- system-experiments.html Hi Jon, All of the articles are interesting. I have previously read the last, but wasn't aware it was only one of a series. Looking at Article 3, Fig 1 suggests that efficiency improves very slowly beyond about 32 radials lying on the surface, and 4 such radials are about 5.4dB below 32 radials. In Fig 2, he shows 4 radials just 6" (150mm) above ground as about 5dB better than 4 radials on the ground. My interpretation of Fig 1 and Fig 2 then is that 4 radials at 6" are about 0.5dB behind 32 radials lying on the ground. That is the type of effect I was referring to when I said "If my models are valid, and they use 'average ground', the indication is that while it may require a large number of buried radials (16) before efficiency levels off a bit, similar efficiency can be obtained with just three radials elevated more than 100mm above the soil" in my first post. Rudy reports some further small improvement (1dB) in raising the radials to 4'(1.2m). These are very similar effect to those predicted by my NEC4 model. In the case of my model of radials from 100mm depth to some distance above ground, the improvement was mostly in the range of heights from 0mm to about 20mm. Obviously, the model is sensitive to soil type, and different soil types can be expected to yield different response... but it would seem that just 3 radials at 1 to 3m height give similar system efficiency to 16+ radials shallow buried for a range of common soil types. I know my interpretation of Rudy's measurements and my NEC4 model don't fall in line with some traditional thinking, and will not appeal to many. I think it is time to build a prototype. Thanks for the links. Owen |
Elevated vs buried radials
Owen Duffy wrote in
: .... I should have noted that the efficiency figures I spoke of from NEC models are technically not directly comparable with Rudy's |S21|, the error in interpeting |S21| as system gain is likely to be small for the kind of load impedances encountered. Owen |
Elevated vs buried radials
On Oct 1, 12:45 am, Owen Duffy wrote:
Obviously, the model is sensitive to soil type, and different soil types can be expected to yield different response... but it would seem that just 3 radials at 1 to 3m height give similar system efficiency to 16+ radials shallow buried for a range of common soil types. The two links below may be of interest in comparing NEC modeling with empirical results. The first link is a clip from the "benchmark" 1937 I.R.E paper of BL&E, showing that the radiated fields measured 3/10 of a mile from monopoles ranging from about 45 to past 90 degrees in height, and using 113 each 0.412-lambda buried radials is within several percent of the theoretical maximum for a perfect monopole of those heights when driven against a zero-ohm connection to a perfect ground plane. The BL&E tests were conducted in the sandy soil of New Jersey, where earth conductivity was/is 4 mS/m or less. Those measured results indicate those systems were radiating 90% or more of the applied power, and that the conductivity of the earth in which those radials were buried is relatively unimportant. The second link is a NEC model of a 1/4-lambda monopole driven against four, elevated counterpoise wires with no antenna system connection to a perfect ground plane, showing that its peak gain is 5.15 dBi -- which is the theoretical maximum for a perfect 1/4-lambda monopole driven against a zero-ohm connection to a perfect ground plane. Adding an ohm or two in the connection from the source to the four elevated radials reduces the gain/field of the NEC model such that it is approximately what was shown in the BL&E study, indicating that a similar value must have been present in their buried radial ground system consisting of 113 each 0.412-lambda wires. Using NEC-4 to incorporate buried (or elevated) radials into the model should show groundwave fields within 1 km of the monopole that are very close to the theoretical maximum for the applied power when radiated along a perfect ground plane, if the model is optimal, and accurate. The theoretical maximum inverse distance voltage field intensity at 1 km for 1 kW of radiated power from a perfect 1/4-lambda monopole system is about 313 mV/m. http://i62.photobucket.com/albums/h85/rfry-100/G.gif http://i62.photobucket.com/albums/h8...tedRadials.jpg RF |
Elevated vs buried radials
Owen wrote:
So, my original question is no so much suggesting everyone else got it wrong, but why don't I seem more people doing it this way. Could I be forgive in thinking that the popular, nearly universal, way is to uplift the BL&E research at MF and apply it to 80m? There are lots of reasons that people don't use elevated radials versus buried ones. But before that, keep in mind that comparing elevated to buried is not really comparing 4 elevated to 120 buried. 120 is overkill, and not many of us who have verticals have that many. I found that 32 was getting into diminishing returns for me, so stopped. Also buried radials are more forgiving of length variations. My case was that the antenna had to be located a good bit closer to one end of the yard than the other. So the radials on one side were from 10 to 25 feet shorter. Also, many of us are married, and the spouse doesn't like all that many wires running around. My wife also mows the yard, something that would be relegated to me if I had a lot of wires running around the yard. And I've used and been around an elevated radial system. It was a royal pain. You have to declare a rather large area off limits, we added little ties to it to warn people, and it didn't really help at night. That experience told me that elevated HF radials was not the way I wanted to go - ever. Even if you are way out in the middle of nowhere, an elevated radial vertical is a liability unless you put a wire link fence around it - check with your insurance company. Just my opinion of course, but it seems to be shared by many. So with the buried radials not being all that much more work,(unless you insist on 120 of them) the greater flexibility of buried radials when dealing with real estate limitations, the appearance and liability issues, just makes a buried radial system a more attractive and practical option to many of us. - 73 de Mike N3LI - |
Elevated vs buried radials
Richard Fry wrote in
: On Oct 1, 12:45 am, Owen Duffy wrote: .... Using NEC-4 to incorporate buried (or elevated) radials into the model should show groundwave fields within 1 km of the monopole that are very close to the theoretical maximum for the applied power when radiated along a perfect ground plane, if the model is optimal, and accurate. It may do, I can not comment. My interest is for an antenna for sky wave path, and I have not explored ground wave performance. In the cases of 32 buried radials and three elevated radials, the patterns are similar, efficiencies are similar, and maximum gain is similar. Reducing the number of buried radials degrades its performance significantly. The elevated radials configuration allows a shortened radiator with capacity hat with negligible degradation in performance. I haven't modelled the same thing over buried radials, but I expect performance degradation would be significantly worse. Owen |
Elevated vs buried radials
Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux wrote: It was written by BL&E that 120 radials work, 2, 15, 30, 60, and 113. No discussion whatever of 120. I stand corrected.. thanks.. So they extrapolated to 120 as a "nice round number" for the future purposes of the FCC. |
Elevated vs buried radials
Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux wrote: For a lot of operators, a 1 dB change in performance might not be noticeable. That 1dB is at the periphery of a radius where surface area (customers) mounts up by the square. For broadcasters, sure. But the discussion is in reference to the potential performance difference for ham use, and I would think that there is more than 1 dB variation in the "other end" of the link. The hard core DXer or QRPer digging the signal out of the noise will care, but that's a small fraction of the overall ham population. |
Elevated vs buried radials
Owen wrote:
On 01/10/10 07:44, Jim Lux wrote: The other thing is whether the difference is big enough to "make a difference" in observed system performance. For a lot of operators, a 1 dB change in performance might not be noticeable. If you're in a "either propagation is there, or it isn't" situation the difference between good and bad is 10s of dB. There are relatively few people who work at 0dB SNR (where tenths count) on a regular and continuing basis, and they're not necessarily the ones who are interested in doing experiments on antennas on the scale needed. Yes, there will be differing view on what is significant difference. I am not in the school of declaring less than one or two S points is insignificant in general. yeah, but there's a big difference between 6-12 dB and 1dB.. I think most users would care about 6 dB. Many fewer about 1 dB. And even fewer care about 1 dB AND have the desire and means to perform the experiment in a controlled way. (well, this latter category probably has less than 10 people in it, and only 1 has published in the last 50 years) In the case of a four square in the DX segment, users are looking for performance... and it seems to me that the elevated three radials, eight wave vertical with capacity had is very close to quarter wave over buried radials... depending of course on the soil type. Hmm.. and there the real question is what kind of performance are we talking about: the power radiated in a desired direction (Tx) or the ability to null unwanted signals (Rx). Given the generally high noise levels on low bands for Rx, a 1 dB change in efficiency of the antenna might not make any difference for the latter. A bigger effect on a phased array is the relative phasing. For a 4 element array, you can have pretty big errors in phase on transmit without changing the forward gain much (30 degree phase error on one element might give you a 1dB change). But a 30 degree phase error on receive could turn a -30dB null into a -7dB one.. And for that, the lower loss of your elevated radials might make things "pickier".. that is, as frequency or surroundings change, the reactive term for each element changes, which could change the power distribution and phasing among the elements (depending on the feed system used). (obviously, one of the "current forcing" drive schemes would be less sensitive to this) You mention the modelling tools, I am not so much concerned as to whether the elevated radials model is good, but whether the NEC4 buried radials model is good, and likewise for radials on and just above the ground because those models are setting the benchmark for the performance of the alternative. The modeling performance of NEC4 for buried wires and wires just above the surface is quite good. Where I would be suspicious is for a wire ON the surface or partly embedded in the surface. Look for that paper by Burke and Poggio on validating NEC3 and NEC4 (it was published at some conference in Ankara Turkey) |
Elevated vs buried radials
Owen Duffy wrote:
LA4RT Jon wrote in : I know my interpretation of Rudy's measurements and my NEC4 model don't fall in line with some traditional thinking, and will not appeal to many. I think you can take the fact that Rudy's measurements match the model pretty well as experimental validation of the model. indeed, NEC4 works. Traditional thinking (or more accurately, mindless repetition of tradition) could well be wrong, eh? I think it is time to build a prototype. Thanks for the links. Owen |
Elevated vs buried radials
On Oct 1, 4:00*pm, Jim Lux wrote:
Richard Clark wrote: On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux wrote: It was written by BL&E that 120 radials work, 2, 15, 30, 60, and 113. No discussion whatever of 120. I stand corrected.. thanks.. So they extrapolated to 120 as a "nice round number" for the future purposes of the FCC. The only reason the FCC used 120, is overkill for the stations to be able to avoid a costly survey. If they didn't use 120, they had to do tests to prove that the system was efficient enough. So most used 120 to avoid all that. For most cases, 120 is almost twice overkill.. For ham use 60 is usually plenty to get well into the near optimum range. Any more than that is a small increase, and usually not worth the cost of the wire. Since Owen posted this question, I did a lot more checking around, and I had already seen the MW BC examples. Seems I'm not the only one that doubts that a small number of barely elevated radials will give a large increase over ones on the ground. One that is in my camp is... Yuri will love this.. W8JI.. Tom seems to agree with my stance from what I can tell. He has done tests in this regard and his results did not show much of an increase over the ground installed radials. In fact, he gave one example where they changed a MW station from four elevated radials to the usual buried radials.. I assume 120 of them.. They then had to explain to the FCC why the buried radials suddenly gave 5 db+ gain over the supposedly "near perfect" elevated set.. :/ He also did tests on 80m comparing this same thing. The results did not pan out and pretty much were in the same ballpark as the results I saw when I tried it. IE: the elevated radials are slightly better than the same number on the ground, but only by a small amount. His tests showed that the usual buried radials using 60 or more greatly outperformed the three or four elevated radials. By 5 db+.. Myself, I think for four elevated radials to equal even sixteen on the ground would require them to be almost 1/8 wave off the ground. So it seems I'm not alone in my doubt of this supposed free lunch program. W8JI seems to be in my doubtful camp. A few others too actually. |
Elevated vs buried radials
Jim Lux wrote in news:i85i4p$enq$1
@news.jpl.nasa.gov: Richard Clark wrote: On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux wrote: It was written by BL&E that 120 radials work, 2, 15, 30, 60, and 113. No discussion whatever of 120. There might not have been much "discussion" though it is mentioned, but the summary does contain the following: "It is also found that a ground system consisting of 120 buried radial wires, each on half wave long, is desirable". I stand corrected.. thanks.. So they extrapolated to 120 as a "nice round number" for the future purposes of the FCC. It does appear that if someone blessed the number 120, it was probably BL&E who did it, even if they were talking about half wave radials. Owen |
Elevated vs buried radials
Thanks Jim, all fair comment and noted.
The end application is a four square phased array for the 80m DX window. The location is at another ham's property, a rural location with ground ranging from dryish clay to rock. I do expect noise to be lowish compared to residential precincts. The excercise is really about a design for a monopole that gives reasonably good performance if extended to the four square configuration. Yes, I note your points about the phase sensitivity to components. That would be a challenge even with buried radials as although we have been in drought for a long time with 'controlled' low moisture content of the soil, rain changes that and the soil is no longer as homogenous. Nothing is as perfect as a modeller's world, but the discussion and some of the links offered give confidence that a shortened vertical with capacity hat and three radials, and shunt coil matched should give similar performance to full quarter wave verticals with 32 buried radials. I have just reread Cebik's article on buried radials, and my own models seem fairly consistent. As you say, Rudy's work is further confirmation allowing for the difference in configuration and the |S21| use. Owen |
Elevated vs buried radials
On Oct 1, 3:02*pm, Owen Duffy wrote:
Richard Fry wrote: Using NEC-4 to incorporate buried (or elevated) radials into the model should show groundwave fields within 1 km of the monopole that are very close to the theoretical maximum for the applied power when radiated along a perfect ground plane, if the model is optimal, and accurate. It may do, I can not comment. My interest is for an antenna for sky wave path, and I have not explored ground wave performance. Just to point out that for vertical monopole heights of 5/8-lambda and less, the peak elevation plane relative field (E / E max) _always_ occurs in the horizontal plane, regardless of the r-f losses in the buried radial system or counterpoise wires they are driven against, and the conductivity of the earth in which those radial wires are buried, or above which they are elevated. IOW, the relative field actually "launched" at all angles above the horizontal plane from such antenna systems _always_ is LESS than that in the horizontal plane. The reason for this is related to the r-f current distribution, and its relative phase along the lengths of those monopoles. NEC analyses showing low to zero relative field in the horizontal plane being launched by a monopole of 5/8-lambda height and less and regardless of the r-f ground they are driven against need to be understood in due context. The link next below leads to further development of this ... http://i62.photobucket.com/albums/h8...at_Compare.gif The longest, great-circle, single-hop, skywave paths are related to the relative fields launched by a monopole system at elevation angles of less than ten degrees (see Figure 55 in the link below) -- where a NEC analysis may show very low relative field. But if such low relative fields really were true for the fields actually launched by such monopoles, then the nighttime skywave coverage of MW AM broadcast stations would be much different than is shown by real-world experience (and applicable theory). http://i62.photobucket.com/albums/h8...Comparison.gif RF |
Elevated vs buried radials
On 10/01/2010 06:13 PM, Jim Lux wrote:
Owen wrote: On 01/10/10 07:44, Jim Lux wrote: A bigger effect on a phased array is the relative phasing. For a 4 element array, you can have pretty big errors in phase on transmit without changing the forward gain much (30 degree phase error on one element might give you a 1dB change). But a 30 degree phase error on receive could turn a -30dB null into a -7dB one.. How come ? Can you elaborate how can these differences happen ? Thanks -- Ing. Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map foF2: http://1fcr.com.ar |
Elevated vs buried radials
On Oct 2, 4:28*am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote: On 10/01/2010 06:13 PM, Jim Lux wrote: Owen wrote: On 01/10/10 07:44, Jim Lux wrote: A bigger effect on a phased array is the relative phasing. For a 4 element array, you can have pretty big errors in phase on transmit without changing the forward gain much (30 degree phase error on one element might give you a 1dB change). But a 30 degree phase error on receive could turn a -30dB null into a -7dB one.. How come ? Can you elaborate how can these differences happen ? it's the difference between the effect on a peak vs effect on a null. consider a simple 2 element array.. for sake of argument, say it's 1/4 wavelength apart and phased 90 degrees, so it has a cardioid pattern.... a gain of 2 in one direction (where the signals from the two antennas align), and a gain of zero in the opposite direction. The gain is 1+cos(phi - spacing*cos(theta)) where phi is the feed phasing, and theta is the direction.. in the preferred direction 1+cos(90 - 90*cos(0)) = 1+cos(0) = 2 in the 45 degree direction: 1+cos(90-90*cos(45)) = 1+cos(90-90*.707) = 1.895 in the 90 degree direction: 1+cos(90-90*cos(90)) = 1+cos(90) = 1 in the 180 degree direction: 1+cos(90-90*cos(180)) = 1+cos(90-90*-1) = 1+cos(180) = 0 Now spoil the feed phase (phi) by 10 degrees... (80 on boresight: 1+cos(80-90*cos(0)) = 1+cos(-10) = 1.984 on 45: 1+cos(80-90*cos(45)) = 1.959 on 90: 1+cos(80-90*cos(90)) = 1.174 at 180: 1+cos(80-90*cos(180)) = 1+cos(80+90) = 1.52E-2 The gain on boresight didn't change much... from 2 to 1.984 (0.03dB) But the null in the back came up from zero to 1.5E-2.. (instead of - infinity, it's now -18dB) Change the phase error to 45 degrees...) @theta=0: 1+cos(45-90*cos(0)) = 1.707 @theta=180: 1+cos(45-90*cos(180)) = .292 So, from the 10 degree error case, the forward gain went from 1.984 to 1.707, about 0.6dB... but the null went from 1.52E-2 to .292 (from -17dB to -5 dB).. The thing to remember on any gain antenna is that it takes very little power to disrupt a null (after all, a -30dB null means that if you're radiating 1kW in the forward direction, you're radiating 1 W in the null.. so just another watt will double the energy in the null, turning it from -30dB to -27dB...) (And, you can see why making antennas with sidelobes -60dB is VERY challenging... ) Now, change the phasing to, say, 80 degrees.. in the preferred direction, the gain is now 1+cos(10degrees) |
Elevated vs buried radials
Mark wrote:
I always think in terms of wavelength when calculating the approximate efficiency of an elevated radial set. For instance, three radials at 1/2 wave up will be pretty much equal to about 120 on the ground. Three at 1/4 wave will be equal to about 50-60 on the ground. Three at 1/8 wave might be equal to 15-20 on the ground. Three at cigarette pack height will be equal to about twice as many as actually used at best. "slightly guessing on that one, but my real world tests seem to pretty much agree". So being as the increase is fairly small at such low heights in wavelength, it is probably practical to just bury them so people won't trip over them.. :/ I have explored what you have said in an NEC4 model of a quarter wave monopole with three quarter wave radials at varying heights over 'average ground'. The results are summarised at http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is the efficiency of the same antenna with 120 buried radials in the same soil type. If the models are correct, laying just a few radials on or very close to the ground (eg the popular method of pinned into the turf) would appear to be a very poor option. The model indicates efficiency improves with a very small increase in height above the dirt, just 30mm is a 6dB improvement of lying on the dirt, just half a metre achieves 90% of the available efficiency. Owen |
Elevated vs buried radials
On Sun, 03 Oct 2010 22:31:07 +0000, Owen Duffy wrote:
I have explored what you have said in an NEC4 model of a quarter wave monopole with three quarter wave radials at varying heights over 'average ground'. The results are summarised at http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is the efficiency of the same antenna with 120 buried radials in the same soil type. If the models are correct, laying just a few radials on or very close to the ground (eg the popular method of pinned into the turf) would appear to be a very poor option. The model indicates efficiency improves with a very small increase in height above the dirt, just 30mm is a 6dB improvement of lying on the dirt, just half a metre achieves 90% of the available efficiency. Owen Owen, Based upon your findings above, have you thought of increasing the height of your model to determine at what height would be necessary to equal the same efficiency as your 120 radial reference? Danny, K6MHE |
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