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#1
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Frank wrote -
I have also used your software for modeling verticals, and it is in very close to the results produced by NEC. The one problem with NEC 2 (Though not with NEC 4) is that it cannot model buried radials, but can get very close to the ground. ================================ The only program I am reasonably familiar with is the several years old free EZNEC. I don't know whether it has been updated or not and I make very little use of it. Come to think of it, I don't make much use of my own programs either. Regarding shallow buried radials in conjunction with a vertical, have you tried my recent program RADIALS2 ? It is intended to demonstrate performance of the radials themselves in a given ground rather than antenna performance. Which I suspect is the reverse of NEC-4. As you probably know, the effects of above-ground radials change very rapidly as they get within a few inches of the ground surface. But once in the ground they tend to remain static. RADIALS2 uses an entirely different, unconventional form of performance analysis. If other programs don't take soil permittivity into account at HF, predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a form suitable for a direct comparison with my simple program? But in view of the large uncertainties involving ground conditions, accuracy is not worth making much of a song and dance about. ---- Reg, G4FGQ |
#2
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Unfortunate I have only a NEC 2 based program (Nittany Scientific's NEC-Win
Pro), but with the Sommerfeld/Norton ground model you can approach the ground to within 1/1000 of a wavelength closely approximating the results of buried wires, and in very close agreement with your "RADIALS2" program. NEC programs do require a value for ground permittivity. NEC 4 based software is more expensive, being in the $800.00 range, plus a $500.00 license from the Lawrence Livermore Laboratory. In any case the data are easily compared with your program results. I have measured my ground conductivity, but not yet attempted to measure the permittivity. If anybody wants to get serious with antenna modeling I recommend Ansoft's HFSS (Often known as "Highly Frustrating Structure Simulator"). It costs a mere $30,000, with a $10,000 per year support payment. Regards, Frank "Reg Edwards" wrote in message ... Frank wrote - I have also used your software for modeling verticals, and it is in very close to the results produced by NEC. The one problem with NEC 2 (Though not with NEC 4) is that it cannot model buried radials, but can get very close to the ground. ================================ The only program I am reasonably familiar with is the several years old free EZNEC. I don't know whether it has been updated or not and I make very little use of it. Come to think of it, I don't make much use of my own programs either. Regarding shallow buried radials in conjunction with a vertical, have you tried my recent program RADIALS2 ? It is intended to demonstrate performance of the radials themselves in a given ground rather than antenna performance. Which I suspect is the reverse of NEC-4. As you probably know, the effects of above-ground radials change very rapidly as they get within a few inches of the ground surface. But once in the ground they tend to remain static. RADIALS2 uses an entirely different, unconventional form of performance analysis. If other programs don't take soil permittivity into account at HF, predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a form suitable for a direct comparison with my simple program? But in view of the large uncertainties involving ground conditions, accuracy is not worth making much of a song and dance about. ---- Reg, G4FGQ |
#3
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Reg Edwards wrote:
The only program I am reasonably familiar with is the several years old free EZNEC. I don't know whether it has been updated or not and I make very little use of it. Come to think of it, I don't make much use of my own programs either. Regarding shallow buried radials in conjunction with a vertical, have you tried my recent program RADIALS2 ? It is intended to demonstrate performance of the radials themselves in a given ground rather than antenna performance. Which I suspect is the reverse of NEC-4. As you probably know, the effects of above-ground radials change very rapidly as they get within a few inches of the ground surface. But once in the ground they tend to remain static. RADIALS2 uses an entirely different, unconventional form of performance analysis. If other programs don't take soil permittivity into account at HF, predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a form suitable for a direct comparison with my simple program? Yes. I made a few comparisons long ago, shortly after you introduced your program, and found major disagreement. NEC-4 approximately agrees with the measurements made long ago by Brown, Lewis, and Epstein (whom I know you've never heard of), once you make reasonable assumptions of ground conductivity and dielectric constant. Your program gives very different answers. At the time, I concluded that there's considerable coupling between radials, which your program doesn't seem to account for. Interested readers should look in the google archives for postings in this group on the thread "Ground Radials" in July 1998 and "Evaluation of G4FGQ Freeware Antenna Software" in September 1998. But in view of the large uncertainties involving ground conditions, accuracy is not worth making much of a song and dance about. True, but in the past, you've used the results from your program to reach conclusions about radial systems that I didn't, and don't, believe to be valid. (See the threads mentioned above.) I don't think it's wise to draw conclusions from a program that gives results which are demonstrably very different from the only measurements regarded to be reasonably well made. Roy Lewallen, W7EL |
#4
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![]() "Roy Lewallen" wrote in message ... Reg Edwards wrote: The only program I am reasonably familiar with is the several years old free EZNEC. I don't know whether it has been updated or not and I make very little use of it. Come to think of it, I don't make much use of my own programs either. Regarding shallow buried radials in conjunction with a vertical, have you tried my recent program RADIALS2 ? It is intended to demonstrate performance of the radials themselves in a given ground rather than antenna performance. Which I suspect is the reverse of NEC-4. As you probably know, the effects of above-ground radials change very rapidly as they get within a few inches of the ground surface. But once in the ground they tend to remain static. RADIALS2 uses an entirely different, unconventional form of performance analysis. If other programs don't take soil permittivity into account at HF, predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a form suitable for a direct comparison with my simple program? Yes. I made a few comparisons long ago, shortly after you introduced your program, and found major disagreement. NEC-4 approximately agrees with the measurements made long ago by Brown, Lewis, and Epstein (whom I know you've never heard of), once you make reasonable assumptions of ground conductivity and dielectric constant. Your program gives very different answers. At the time, I concluded that there's considerable coupling between radials, which your program doesn't seem to account for. Interested readers should look in the google archives for postings in this group on the thread "Ground Radials" in July 1998 and "Evaluation of G4FGQ Freeware Antenna Software" in September 1998. But in view of the large uncertainties involving ground conditions, accuracy is not worth making much of a song and dance about. True, but in the past, you've used the results from your program to reach conclusions about radial systems that I didn't, and don't, believe to be valid. (See the threads mentioned above.) I don't think it's wise to draw conclusions from a program that gives results which are demonstrably very different from the only measurements regarded to be reasonably well made. Roy Lewallen, W7EL ============================== Roy, it's gratifying to see, once again, you take notice of my sayings. Such as, I repeat - "Fact 4. Computer programs do not tell gospel truths. They are at least as unreliable as their human programmers." ---- Reg. |
#5
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Modeled #14 AWG, copper conductor, 32ft monopole, 29 radials of 25ft, and
base 6" above (nominal lambda/1000) Sommerfeld/Norton ground of Er = 13, sigma = 0.013 S/m at 1.8 MHz. All segments 6". NEC2 computes: Zin = 2.87 - j1358 Efficiency 92% RADIALS2 computes (with radials 1mm below ground): Zin = 1.55 - j1310 Efficiency 23.5% Not a large amount of difference, but thought I had gotten closer results with a different monopole, but seem to have deleted the code (Not sure why such a large difference in efficiency). NEC2 is supposed to provide a reasonable approximation of a buried radial monopole when at about lambda/1000 above ground. Be interested in any comments, and what NEC4 provides if anybody has it. 73, Frank "Roy Lewallen" wrote in message ... Reg Edwards wrote: The only program I am reasonably familiar with is the several years old free EZNEC. I don't know whether it has been updated or not and I make very little use of it. Come to think of it, I don't make much use of my own programs either. Regarding shallow buried radials in conjunction with a vertical, have you tried my recent program RADIALS2 ? It is intended to demonstrate performance of the radials themselves in a given ground rather than antenna performance. Which I suspect is the reverse of NEC-4. As you probably know, the effects of above-ground radials change very rapidly as they get within a few inches of the ground surface. But once in the ground they tend to remain static. RADIALS2 uses an entirely different, unconventional form of performance analysis. If other programs don't take soil permittivity into account at HF, predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a form suitable for a direct comparison with my simple program? Yes. I made a few comparisons long ago, shortly after you introduced your program, and found major disagreement. NEC-4 approximately agrees with the measurements made long ago by Brown, Lewis, and Epstein (whom I know you've never heard of), once you make reasonable assumptions of ground conductivity and dielectric constant. Your program gives very different answers. At the time, I concluded that there's considerable coupling between radials, which your program doesn't seem to account for. Interested readers should look in the google archives for postings in this group on the thread "Ground Radials" in July 1998 and "Evaluation of G4FGQ Freeware Antenna Software" in September 1998. But in view of the large uncertainties involving ground conditions, accuracy is not worth making much of a song and dance about. True, but in the past, you've used the results from your program to reach conclusions about radial systems that I didn't, and don't, believe to be valid. (See the threads mentioned above.) I don't think it's wise to draw conclusions from a program that gives results which are demonstrably very different from the only measurements regarded to be reasonably well made. Roy Lewallen, W7EL |
#6
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![]() "Frank" wrote in message news:H4hkd.141267$9b.112169@edtnps84... Modeled #14 AWG, copper conductor, 32ft monopole, 29 radials of 25ft, and base 6" above (nominal lambda/1000) Sommerfeld/Norton ground of Er = 13, sigma = 0.013 S/m at 1.8 MHz. All segments 6". NEC2 computes: Zin = 2.87 - j1358 Efficiency 92% RADIALS2 computes (with radials 1mm below ground): Zin = 1.55 - j1310 Efficiency 23.5% Not a large amount of difference, but thought I had gotten closer results with a different monopole, but seem to have deleted the code (Not sure why such a large difference in efficiency). NEC2 is supposed to provide a reasonable approximation of a buried radial monopole when at about lambda/1000 above ground. Be interested in any comments, and what NEC4 provides if anybody has it. 73, Of course the higher efficiency is due to NEC calculating only the I^2R losses, and not the TRP. TRP should be fairly easy to calculate since the pattern is "phi" independent. Have not checked to see if there is a TRP card. Note that a 32 ft monopole mounted on a perfect ground has an input impedance of 1.58 - j1311 Ohms. The efficiency is reduced to 86% due to increased I^2R losses. Frank |
#7
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![]() "Frank" wrote in message news:t4okd.90482$VA5.33610@clgrps13... "Frank" wrote in message news:H4hkd.141267$9b.112169@edtnps84... Modeled #14 AWG, copper conductor, 32ft monopole, 29 radials of 25ft, and base 6" above (nominal lambda/1000) Sommerfeld/Norton ground of Er = 13, sigma = 0.013 S/m at 1.8 MHz. All segments 6". NEC2 computes: Zin = 2.87 - j1358 Efficiency 92% RADIALS2 computes (with radials 1mm below ground): Zin = 1.55 - j1310 Efficiency 23.5% Not a large amount of difference, but thought I had gotten closer results with a different monopole, but seem to have deleted the code (Not sure why such a large difference in efficiency). NEC2 is supposed to provide a reasonable approximation of a buried radial monopole when at about lambda/1000 above ground. Be interested in any comments, and what NEC4 provides if anybody has it. 73, Of course the higher efficiency is due to NEC calculating only the I^2R losses, and not the TRP. TRP should be fairly easy to calculate since the pattern is "phi" independent. Have not checked to see if there is a TRP card. Note that a 32 ft monopole mounted on a perfect ground has an input impedance of 1.58 - j1311 Ohms. The efficiency is reduced to 86% due to increased I^2R losses. Frank From the calculated field strength (as a function of Theta) the TRP for 100 W input, which includes copper and ground losses, shows 27.4 W, or 27.4% efficient. In very close agreement with the RADIALS2 program. The only noticeable discrepancy appears to be in the real part of Zin. Frank |
#8
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Frank, as you say, the height of the radials in the NEC2 model is only 6
inches above ground. The radials are shallow-buried in the RADIALS2 model. It can't do elevated radials. The ground loss resistance as height decreases, as seen by the antenna, increases very fast percentage-wise as the radials get within a few inches of the ground. It is due to very close magnetic and electric coupling to ground. Radials are transmission lines, insulated from but running very close to a resistive slab of soil. This would account for the computed higher input resistance of the radials ( 3.5 - j*3.3 ohms ) ( for 29, 25-feet long radials. Rg=77, K=13 ) in program RADIALS2. The calculated antenna input impedance in RADIALS2 is that of the antenna alone. For feedpoint impedance add the input impedance of the radial system. Presumably, NEC2 does not compute the input reactance of radials. Efficiency is calculated in the usual way from the sum of antenna input resistance and radials' input resistance. If you contrive to change the radials input reactance without changing frequency or the antenna, you will notice the loading coil tunes it out along with antenna reactance. Incidentally, when elevated radials are near the ground their velocity factor decreases fast which makes a mess of the usual recommendation to prune them to 1/4-wave free-space length. When radials are actually lying on the ground surface the velocity factor decreases to roughly 0.5 of the velocity of light. When buried the underground VF can fall to as small as 0.15 depending on soil permittivity. (or moisture content.) ---- Reg, G4FGQ |
#9
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Reg, thanks for the info. I see I was making an error with RADIALS2. Did
not realize that you had to add the radial impedance to the antenna impedance; I thought it was computed in the final result. I figured something was weird since the input impedance was similar to the antenna modeled over a perfect ground. NEC2 does compute the input impedance of the complete structure, but as mentioned before it is limited, in that all wires must be = lambda/1000 above the ground (at 1.8 MHz about 6"). I am just starting to delve into computational electromagnetics, so do not know that much about NEC. It uses the "Method of moments" in its computations. The theory of operation manual is available for download at several web sites. Anyway, with RADIALS2, I now get an input impedance of 5.1 - j1303, and with NEC2 2.9 - j1358. I did try entering a negative number for the depth of the radials, but RADIALS2 did not like it. Your comments about the effect on the radials of being buried are also very interesting, and obviously indicate the reason for our slightly different results. The very low VF of buried radials indicates that the length is less important. As for efficiency, NEC2 computes a normalized far E-field at 1 meter. For phi independent structures it becomes trivial to integrate the power density over a hemispherical region to arrive at the true total radiated power. 100 Watts into the antenna radiates 27 Watts, again, very close to RADIALS2' computed efficiency of 23.5%. Come to think of it, I guess I could have estimated the losses -- as you do -- by comparing the input impedance of an antenna over a perfect ground with the same antenna over a lossy ground. Still I think it was more fun playing with Excel spread sheets and coming up with a similar answer. Regard, Frank "Reg Edwards" wrote in message ... Frank, as you say, the height of the radials in the NEC2 model is only 6 inches above ground. The radials are shallow-buried in the RADIALS2 model. It can't do elevated radials. The ground loss resistance as height decreases, as seen by the antenna, increases very fast percentage-wise as the radials get within a few inches of the ground. It is due to very close magnetic and electric coupling to ground. Radials are transmission lines, insulated from but running very close to a resistive slab of soil. This would account for the computed higher input resistance of the radials ( 3.5 - j*3.3 ohms ) ( for 29, 25-feet long radials. Rg=77, K=13 ) in program RADIALS2. The calculated antenna input impedance in RADIALS2 is that of the antenna alone. For feedpoint impedance add the input impedance of the radial system. Presumably, NEC2 does not compute the input reactance of radials. Efficiency is calculated in the usual way from the sum of antenna input resistance and radials' input resistance. If you contrive to change the radials input reactance without changing frequency or the antenna, you will notice the loading coil tunes it out along with antenna reactance. Incidentally, when elevated radials are near the ground their velocity factor decreases fast which makes a mess of the usual recommendation to prune them to 1/4-wave free-space length. When radials are actually lying on the ground surface the velocity factor decreases to roughly 0.5 of the velocity of light. When buried the underground VF can fall to as small as 0.15 depending on soil permittivity. (or moisture content.) ---- Reg, G4FGQ |
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