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From NEC2 to the real world with accuracy
Hi,
So I've spent months, rather years, carefully designing a new 5 band cubical quad for myself. I've always known that I would use #12 solid copper wire (not stranded), so that is what I used when running NEC2 to optimize this touchy antenna design (over millions of iterations). Well, real world things are starting to happen. For wire, I've decided what I would like to use is an enameled coated copperweld wire. I'll buy the plain copperweld wire and coat it myself. So my question is, what is the most accurate way to make sure when I build the quad that I account for the velocity factor (unknown) of the wire I use? Should I grid dip the elements and make sure they agree with my NEC2 model? Can I build a simple loop on a higher frequency with the wire and then somehow use that information to rescale my wire lengths? What is the "right" way to do this? I wonder how consistent velocity factor will be if I do my own coating on the wire....maybe hand coating is a bad idea for this reason.... I spent a lot of time designing a Yagi with NEC2. I used Leeson's correction for taper elements and to calculate the effect of the element to boom mounts. All that attention paid off - I thought my Yagi lived up to the NEC2 predictions very well. -Scott, WU2X |
From NEC2 to the real world with accuracy
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From NEC2 to the real world with accuracy
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From NEC2 to the real world with accuracy
wrote in message ... Hi, So I've spent months, rather years, carefully designing a new 5 band cubical quad for myself. I've always known that I would use #12 solid copper wire (not stranded), so that is what I used when running NEC2 to optimize this touchy antenna design (over millions of iterations). Well, real world things are starting to happen. For wire, I've decided what I would like to use is an enameled coated copperweld wire. I'll buy the plain copperweld wire and coat it myself. I think I would spend the extra money and buy copper wire. One nick in your coating and the copper surface and the wire will disappear. If you must use copperweld, try The Wireman or The RF Connection for copperweld that has a high density black polyethylene jacket. I would hate to spend all the time to design and build a quad and then have the wire be the weakest link. Dale W4OP |
From NEC2 to the real world with accuracy
If I might anticipate because the copper oxidizes and presents a high
resistance to current, I would point out that enamel is even more resistive and the current isn't going to travel through it either (the current will travel through the proverbial path of least resistance - under the coating of resistive layer covering no matter what it is composed of). (I am addressing more than one post with this response) Basically I want to use copperweld for durability. Its not a matter of saving money over hard drawn copper. I have no experience with (bare) copperweld, but I have read that if you get a nick in the copper layer, basically the steel will start rusting away at that point (very quickly). So I don't have problem using some enamel to protect it (and have done this in the past with hard drawn). I am using copperweld/ insulated wire for my low band antennas - but anyone who has actually built a quad knows insulated wire is much too heavy, esp. when you are dealing with 5 bands. I have used hard drawn copper with enamel on it on 10 meters and never had a problem with a wire breaking in 10 years. So, I am not really sure if I need copperweld, but I have no expierence with larger quad loops. The goal is to have this thing be reliable. Regardless still - whatever I wire I use - even if it was bare #12 solid copper wire, I still want to go through the exercise of getting the wire I actually used in sync with my model. So, I'd rather focus the conversation on that topic. With respect to manually tuning the quad (reflector) after its up, yes, that seems to be the conventional wisdom. How practical and possible is that for me? Mine will sit on 72' US Tower crankup/tilt over - short of renting some huge bucket/boom, I can't see myself manually tuning it when its up. The best I will do is take some type of field measurements and see if it displays anything close to the pattern NEC2 predicts - or better yet, just see if I am happy with it as is. I did not have to make any adjustments to the monoband yagi that went right from NEC2 to the tower, so, I am hoping for the same good fortune. -Scott, WU2X |
From NEC2 to the real world with accuracy
On Mar 12, 9:50 am, wrote:
If I might anticipate because the copper oxidizes and presents a high resistance to current, I would point out that enamel is even more resistive and the current isn't going to travel through it either (the current will travel through the proverbial path of least resistance - under the coating of resistive layer covering no matter what it is composed of). (I am addressing more than one post with this response) Basically I want to use copperweld for durability. Its not a matter of saving money over hard drawn copper. I have no experience with (bare) copperweld, but I have read that if you get a nick in the copper layer, basically the steel will start rusting away at that point (very quickly). So I don't have problem using some enamel to protect it (and have done this in the past with hard drawn). I am using copperweld/ insulated wire for my low band antennas - but anyone who has actually built a quad knows insulated wire is much too heavy, esp. when you are dealing with 5 bands. I have used hard drawn copper with enamel on it on 10 meters and never had a problem with a wire breaking in 10 years. So, I am not really sure if I need copperweld, but I have no expierence with larger quad loops. The goal is to have this thing be reliable. Regardless still - whatever I wire I use - even if it was bare #12 solid copper wire, I still want to go through the exercise of getting the wire I actually used in sync with my model. So, I'd rather focus the conversation on that topic. With respect to manually tuning the quad (reflector) after its up, yes, that seems to be the conventional wisdom. How practical and possible is that for me? Mine will sit on 72' US Tower crankup/tilt over - short of renting some huge bucket/boom, I can't see myself manually tuning it when its up. The best I will do is take some type of field measurements and see if it displays anything close to the pattern NEC2 predicts - or better yet, just see if I am happy with it as is. I did not have to make any adjustments to the monoband yagi that went right from NEC2 to the tower, so, I am hoping for the same good fortune. -Scott, WU2X Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Art |
From NEC2 to the real world with accuracy
On Mar 12, 11:01 am, Art Unwin wrote:
Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Is NEC4 better in this regard? Here is the NEC2 reported performance of my current design. Its a 24 foot boom, with 5 bands. 4 elements on 15-10 and 3 elements on 20-17. Optimization is for F/R around mid-band and bandwidth (under 2.5:1 at edges). http://remote.wu2x.com:8888/WU2X/ Click each band.html file to see the graphs on each band. The non- active driven elements are shorted. Here is the stock Cubex 4 element quad, which is 4 elements on all bands (on a 24' boom). http://remote.wu2x.com:8888/Cubex/ -Scott, WU2X |
From NEC2 to the real world with accuracy
On Mar 12, 10:22 am, wrote:
On Mar 12, 11:01 am, Art Unwin wrote: Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Is NEC4 better in this regard? Here is the NEC2 reported performance of my current design. Its a 24 foot boom, with 5 bands. 4 elements on 15-10 and 3 elements on 20-17. Optimization is for F/R around mid-band and bandwidth (under 2.5:1 at edges). http://remote.wu2x.com:8888/WU2X/ Click each band.html file to see the graphs on each band. The non- active driven elements are shorted. Here is the stock Cubex 4 element quad, which is 4 elements on all bands (on a 24' boom). http://remote.wu2x.com:8888/Cubex/ -Scott, WU2X I would suggest that you check with what Cebik says about it. At the same time you are also now dealing with vertcal polarisation which also has a lot of characteristics that are different from planar yagis where the elements are not in equilibrrium as compared to a quad design. One can discharge corona where the other doesn't soto use the same calculations for designs that behave differently raises a lot of questions besides the sharp corners. Niether of the programs are perfect but better than shooting in the dark. Art |
From NEC2 to the real world with accuracy
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From NEC2 to the real world with accuracy
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From NEC2 to the real world with accuracy
On Mar 12, 2:27 pm, Roy Lewallen wrote:
wrote: On Mar 12, 11:01 am, Art Unwin wrote: Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Is NEC4 better in this regard? Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement is incorrect as quoted. Roy Lewallen, W7EL That really is good news. Capacitive coupling between close spaced wires, progressive change in capacitive coupling to ground from a verticle ,or wires at different spacings and closeness must be a devil to calculate tho I suppose you can also insert fudge factors for alignment from impirical results which I presume is the reason for program modifications despite the inflexibility to change of Maxwells laws. Well you highlight one of the answers the gentleman was asking for so a forthcoming complete answer from you should give him closure on this subject Art. |
From NEC2 to the real world with accuracy
Art Unwin wrote:
On Mar 12, 2:27 pm, Roy Lewallen wrote: wrote: On Mar 12, 11:01 am, Art Unwin wrote: Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Is NEC4 better in this regard? Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement is incorrect as quoted. Roy Lewallen, W7EL That really is good news. Capacitive coupling between close spaced wires, progressive change in capacitive coupling to ground from a verticle ,or wires at different spacings and closeness must be a devil to calculate tedious, but not complex. Like all Finite Element approaches, you basically break the problem up into little tiny chunks and rigorously apply basic laws of physics. Charge is charge. The challenge is not in the theory of operation, but in the practical implementation. There's also the practical implementation detail of taking a high level description of something and turning it into many, many smaller pieces. (For instance, turning an arbitrary 3-d shape into lots of little plane triangles or quadrilaterals). You want a small number of pieces so the computational work is less (many of these techniques have "work" that goes as the cube of the number of pieces, so going from 10 segments to 100 segments takes 1000 times as much computation), but also you want the pieces small enough that they approximate the original continuous curve to an adequate level of accuracy (the calculus problem) As you make the chunks smaller, round off errors and numerical precision become a bigger issue (e.g. on a computer with finite precision, summing a million millionths might not equal one). So even if you have a 1000 processor Beowulf cluster, it might not help. tho I suppose you can also insert fudge factors for alignment from impirical results which I presume is the reason for program modifications despite the inflexibility to change of Maxwells laws. No fudge factors in NEC. What changes there are between versions do things like: handle insulated wires, or wires embedded in a medium other than free space. If you accept the constraint of uninsulated wires in a vacuum, you can simplify the equations, which takes less computation (i.e. you don't have to take epsilonr or sigma into account). Since air is pretty close to a vacuum, and most people build antennas out of uninsulated wires, the first version of NEC did the easy case. better approximations of the charge distribution on the segment from a numerical analysis standpoint. i.e. rather than using sin(x) for values of x near pi/2, where small changes in x result in very small changes in sin(x), you use 1-cos(x). You could have also just used a zillion digit sin calculation, but that gets back to the computational efficiency thing. More accurate calculations of the interaction between chunks. NEC essentially calculates the coupling between every possible pair of segments in your model. Calculating coupling between two segments some distance apart assuming the segment is very much smaller in diameter than the spacing and where they are parallel is fairly straightforward. Calculating coupling between two conductors of diameter d, separated by a distance close to d, with them at an angle, is a bit tougher. (a lot of it is back to the issue of precision of trig functions) Easier ways to define a model. NEC4 includes a function to enter a wire with a catenary curve. For NEC2, you'd have to do that outside, and then enter the wire as a series of smaller wires. There's a readily available paper out there that explains all the improvements from NEC2 to the later versions, and how they were experimentally validated. {https://e-reports-ext.llnl.gov/pdf/210389.pdf} Well you highlight one of the answers the gentleman was asking for so a forthcoming complete answer from you should give him closure on this subject Art. Here's the reference to the paper by Burke: Title: Recent advances to NEC (Numerical Electromagnetics Code): Applications and validation Authors: Burke, G. J. Affiliation: Lawrence Livermore National Lab., CA. Journal: Presented at the Conference on Modern Antenna Design Using Computers and Measurement Application To Antenna Problems of Military Interest, Ankara, Turkey, 19-20 Oct. 1989 Publication Date: 03/1989 Category: Communications and Radar Origin: STI NASA/STI Keywords: ANTENNA DESIGN, COMPUTER AIDED DESIGN, COMPUTER PROGRAMS, ELECTROMAGNETISM, MATHEMATICAL MODELS, ELECTRICAL INSULATION, INTEGRAL EQUATIONS, WIRING Bibliographic Code: 1989STIN...9011917B Abstract Capabilities of the antenna modeling code NEC are reviewed and results are presented to illustrate typical applications. Recent developments are discussed that will improve accuracy in modeling electrically small antennas, stepped-radius wires and junctions of tightly coupled wires, and also a new capability for modeling insulated wires in air or earth is described. These advances will be included in a future release of NEC, while for now the results serve to illustrate limitations of the present code. NEC results are compared with independent analytical and numerical solutions and measurements to validate the model for wires near ground and for insulated wires. |
From NEC2 to the real world with accuracy
Dale Parfitt wrote:
wrote in message ... Hi, So I've spent months, rather years, carefully designing a new 5 band cubical quad for myself. I've always known that I would use #12 solid copper wire (not stranded), so that is what I used when running NEC2 to optimize this touchy antenna design (over millions of iterations). Well, real world things are starting to happen. For wire, I've decided what I would like to use is an enameled coated copperweld wire. I'll buy the plain copperweld wire and coat it myself. I think I would spend the extra money and buy copper wire. One nick in your coating and the copper surface and the wire will disappear. If you must use copperweld, try The Wireman or The RF Connection for copperweld that has a high density black polyethylene jacket. I would hate to spend all the time to design and build a quad and then have the wire be the weakest link. Dale W4OP Dale He already said he didn't want to use insulated wire because of the weight of the insulation. -- Tom Horne "This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison |
From NEC2 to the real world with accuracy
On Mar 12, 6:13 pm, Jim Lux wrote:
Art Unwin wrote: On Mar 12, 2:27 pm, Roy Lewallen wrote: wrote: On Mar 12, 11:01 am, Art Unwin wrote: Scott, you are introducing sharp corners to your new antenna so NEC will stray from accuracy. Is NEC4 better in this regard? Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement is incorrect as quoted. Roy Lewallen, W7EL That really is good news. Capacitive coupling between close spaced wires, progressive change in capacitive coupling to ground from a verticle ,or wires at different spacings and closeness must be a devil to calculate tedious, but not complex. Like all Finite Element approaches, you basically break the problem up into little tiny chunks and rigorously apply basic laws of physics. Charge is charge. The challenge is not in the theory of operation, but in the practical implementation. There's also the practical implementation detail of taking a high level description of something and turning it into many, many smaller pieces. (For instance, turning an arbitrary 3-d shape into lots of little plane triangles or quadrilaterals). You want a small number of pieces so the computational work is less (many of these techniques have "work" that goes as the cube of the number of pieces, so going from 10 segments to 100 segments takes 1000 times as much computation), but also you want the pieces small enough that they approximate the original continuous curve to an adequate level of accuracy (the calculus problem) As you make the chunks smaller, round off errors and numerical precision become a bigger issue (e.g. on a computer with finite precision, summing a million millionths might not equal one). So even if you have a 1000 processor Beowulf cluster, it might not help. tho I suppose you can also insert fudge factors for alignment from impirical results which I presume is the reason for program modifications despite the inflexibility to change of Maxwells laws. No fudge factors in NEC. What changes there are between versions do things like: handle insulated wires, or wires embedded in a medium other than free space. If you accept the constraint of uninsulated wires in a vacuum, you can simplify the equations, which takes less computation (i.e. you don't have to take epsilonr or sigma into account). Since air is pretty close to a vacuum, and most people build antennas out of uninsulated wires, the first version of NEC did the easy case. better approximations of the charge distribution on the segment from a numerical analysis standpoint. i.e. rather than using sin(x) for values of x near pi/2, where small changes in x result in very small changes in sin(x), you use 1-cos(x). You could have also just used a zillion digit sin calculation, but that gets back to the computational efficiency thing. More accurate calculations of the interaction between chunks. NEC essentially calculates the coupling between every possible pair of segments in your model. Calculating coupling between two segments some distance apart assuming the segment is very much smaller in diameter than the spacing and where they are parallel is fairly straightforward. Calculating coupling between two conductors of diameter d, separated by a distance close to d, with them at an angle, is a bit tougher. (a lot of it is back to the issue of precision of trig functions) Easier ways to define a model. NEC4 includes a function to enter a wire with a catenary curve. For NEC2, you'd have to do that outside, and then enter the wire as a series of smaller wires. There's a readily available paper out there that explains all the improvements from NEC2 to the later versions, and how they were experimentally validated. {https://e-reports-ext.llnl.gov/pdf/210389.pdf} Well you highlight one of the answers the gentleman was asking for so a forthcoming complete answer from you should give him closure on this subject Art. Here's the reference to the paper by Burke: Title: Recent advances to NEC (Numerical Electromagnetics Code): Applications and validation Authors: Burke, G. J. Affiliation: Lawrence Livermore National Lab., CA. Journal: Presented at the Conference on Modern Antenna Design Using Computers and Measurement Application To Antenna Problems of Military Interest, Ankara, Turkey, 19-20 Oct. 1989 Publication Date: 03/1989 Category: Communications and Radar Origin: STI NASA/STI Keywords: ANTENNA DESIGN, COMPUTER AIDED DESIGN, COMPUTER PROGRAMS, ELECTROMAGNETISM, MATHEMATICAL MODELS, ELECTRICAL INSULATION, INTEGRAL EQUATIONS, WIRING Bibliographic Code: 1989STIN...9011917B Abstract Capabilities of the antenna modeling code NEC are reviewed and results are presented to illustrate typical applications. Recent developments are discussed that will improve accuracy in modeling electrically small antennas, stepped-radius wires and junctions of tightly coupled wires, and also a new capability for modeling insulated wires in air or earth is described. These advances will be included in a future release of NEC, while for now the results serve to illustrate limitations of the present code. NEC results are compared with independent analytical and numerical solutions and measurements to validate the model for wires near ground and for insulated wires. Jim, that was a comprehensive answer for the gen tleman. Probably more information that he can deal with.Seems like it is quite easy to have a yagi behave when using NEC2 but I still hear stories of the diffuculty of tuning them after following the nec2 instructions. Never had a quad before so I am really out in left field on that one Regards Art |
From NEC2 to the real world with accuracy
Jim Lux wrote:
tedious, but not complex. Like all Finite Element approaches, you basically break the problem up into little tiny chunks and rigorously apply basic laws of physics. Charge is charge. The challenge is not in the theory of operation, but in the practical implementation. . . . The last time I read any of Art's postings, he was using AO, which is MININEC-based. MININEC has a number of peculiarities due to choices made in its implementation, including problems with close spaced parallel wires, wires connected at an angle, and very simplified ground model. (See "MININEC: The Other Edge of the Sword", _QST_, Feb. 1991 for more detail.) NEC-2 and NEC-4 also have peculiarities due to their implementations, but ones which are different from MININEC and in some cases from each other. Roy Lewallen, W7EL |
From NEC2 to the real world with accuracy
Scott, WU2X wrote:
"I`ve always known that I would use #12 solid copper wire (not stranded) so that is what I used when running NEC2 to optimize this touchy antenna design (over millions of iterations). Bill Orr, W6SAI devotes Chapter 9 in "All About Cubical Quad Antennas" to tuning and adjustment. John Devoldere, ON4UN wrote on page 13-52 of the 2nd edition of "Low-Band DXing: "I designed the quad (for 80-m) with two quad loops of identical length (for a 2 mm OD conductor or no.12 wire). William I.(Bill)Orr, W6SAI wrote on page 77 of the 2nd edition of "All About Cubical Quad Antennas"; 'the individual gamma devices are made of #12 solid copper wire and a small variable capacitor." Ed Laport pictures a Wind Turbine Company insulator used for two wire balanced lines on page 485 of "Radio Antenna engineering". These were used in WW-2 Signal Corps fhombic antenna kits to support the 600-ohm feedline. The wire was a cable made from (3) #12 twisted Copperweld wires. This same cable also was used to make the rhombic curtain which contained (3) of these Copperweld cables. I used many miles of this cable and never saw a breakage in normal use despite years of aging in all weather and the fact that we were using 100 KW in an antenna kit designed for 5 KW. We did redesign the stainless steel dissipation lines to withstand the high power. With new dissipation lines installed we got many letters of complaint from South America from listeners who had benefited from our bidirectional antennas which were intended to cover central Europe only. Too bad, but other broadcasters claimed the target area we were temporarily occupying in South America. Best regards, Richard Harrison, KB5WZI |
From NEC2 to the real world with accuracy
Jim Lux writes:
Here's the reference to the paper by Burke: Title: Recent advances to NEC (Numerical Electromagnetics Code): Applications and validation Authors: Burke, G. J. Affiliation: Lawrence Livermore National Lab., CA. Journal: Presented at the Conference on Modern Antenna Design Using Computers and Measurement Application To Antenna Problems of Military Interest, Ankara, Turkey, 19-20 Oct. 1989 Publication Date: 03/1989 Thanks. Recent advances, dated 1989. What's been going on during the 18 years since then? Polishing NEC-4, and developing other codes for specialized purposes, presumably. I know it's a big question, but maybe somebody here can tell us something. 73 LA4RT Jon, Trondheim, Norway |
From NEC2 to the real world with accuracy
Ok, so I guess my subject line might have open the door to this
discussion. So NEC4 can model insulated wire, which seems like it might be helpful for me. I see the price is down to 300$ now for US non-commercial. Now if I can just get them to answer my email. So what is included in the package - its not listed on the LLNL website. Does NEC4 use the same input deck as NEC2? I assume there are maybe some more input cards? What about the output? I wrote my own cubical quad optimizing software that reads and writes to the NEC2 input deck and reads the output directly - hopefully it wouldn't require to much change to adapt it to use a NEC4 engine instead. -Scott, WU2X |
From NEC2 to the real world with accuracy
Jon Kåre Hellan wrote:
Jim Lux writes: Here's the reference to the paper by Burke: Title: Recent advances to NEC (Numerical Electromagnetics Code): Applications and validation Authors: Burke, G. J. Affiliation: Lawrence Livermore National Lab., CA. Journal: Presented at the Conference on Modern Antenna Design Using Computers and Measurement Application To Antenna Problems of Military Interest, Ankara, Turkey, 19-20 Oct. 1989 Publication Date: 03/1989 Thanks. Recent advances, dated 1989. What's been going on during the 18 years since then? Polishing NEC-4, and developing other codes for specialized purposes, presumably. I know it's a big question, but maybe somebody here can tell us something. That's about it. NEC-4 is sort of about as good as you can get for a "wires" based MoM code. The math isn't changing, the implementation does the math, etc. These days, it's other modeling techniques that are being developed: various FDTD schemes various voxel based schemes all manner of harmonic balance lots of work on automatic meshing (i.e., take the mechanical solid model from Unigraphics or Pro/E or Solidworks or whatever and turn it into something that a EM code can process) 73 LA4RT Jon, Trondheim, Norway |
From NEC2 to the real world with accuracy
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From NEC2 to the real world with accuracy
AI4QJ wrote:
"Art Unwin" wrote in message ... but I still hear stories of the diffuculty of tuning them after following the nec2 instructions. You are violating the 11 commandment: Thou shalt NEVER criticize NEC in rraa lest the fleas of 1000 camels infest thy armpits. Been plenty of NEC2 commentary in here. NEC2 has it's limitations. That's why NEC4 came along 8^) - 73 de Mike N3LI - |
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