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Going tp put this antenna up today
Hello All
Well after all the comments made and doing some research on the G5RV( I guess it has it place) I have decide to try something new and hopefully better.Here it is, the Mystery Antenna,I had all the parts at home, so it did not cost any money at this point. Built it last night took about 1.5 hrs to build,including making my own center insulators and end insulators.Here is the link to the site,I am sure allot of you in this group know about it and looking forward to your comments.Will let you all know how it works,and some of the readings I get from it. http://w5gi.com/mysteryantenna.htm thanx all Howard VE4ISP ps. thanx to all that commented on my G5RV post |
Going tp put this antenna up today
On 2008-09-29, Howard Kowall wrote:
Hello All Well after all the comments made and doing some research on the G5RV( I guess it has it place) I have decide to try something new and hopefully better.Here it is, the Mystery Antenna,I had all the parts at home, so it I built a Mystery Antenna a few years ago just to see how it performed. Since then, John has changed his explanation a bit, but it is supposed to have a better pattern for its designed frequency, ala 20m, than the g5rv. Of course you can design it for whatever frequency you wish. I use mine for an all band portable antenna and it seems to work well. I ended the parallel line in a pl-259. To that I attach, with a barrel connector, a ferrite bead balun (W2DU style) and then the coax to the tuner to prevent any common mode from coming back down the line. We use it at ham field day each year for the GOTA area. I will not claim any particular advantage over, say, a dipole fed with ladder line to a tuner and will not quote reports, which are at best nebulus anyway, but it was fun to build and does work fine. Incidentally, the cw operator at field day put together the D3 (the 1.5 wavelength ant) on John's W5GI site, tuned for the 40m cw freqs and it worked very well. Of course that requires a bit more real estate. 73 ...Edwin, KD5ZLB -- __________________________________________________ __________ "Once you have flown, you will walk the earth with your eyes turned skyward, for there you have been, there you long to return."-da Vinci http://bellsouthpwp2.net/e/d/edwinljohnson |
Going tp put this antenna up today
Howard Kowall wrote:
... comments.Will let you all know how it works,and some of the readings I get from it. http://w5gi.com/mysteryantenna.htm thanx all Howard VE4ISP ps. thanx to all that commented on my G5RV post Here is one guys thoughts on it: http://www.coolweb.ws/genesis/Downlo...%20Antenna.pdf Regards, JS |
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"Howard Kowall" wrote in
: Hello All .... I guess it has it place) I have decide to try something new and hopefully better.Here it is, the Mystery Antenna,I had all the parts .... Now, lets see... that the "multi-band wire antenna that performs exceptionally well even though it confounds antenna modeling software". Yet another 'magic' antenna who's 'magic' cannot be explained! Of course the claim is nonsense, the antenna can be modelled in NEC... just the complication is evaluating an equivalent load in NEC terms for the TL stubs. You can evaluate the equivalent impedance of a s/c stub of 16.5' of RG8X (or whatever you used) using the calculator at http://www.vk1od.net/tl/tllc.php. For example Z of s/c stub of 16.5' of RG8X at 7.1MHz is around 2.97 +j64.54, so it acts like a quite lossy inductor at that frequency. Once the feedpoint impedance is found, the transmission line losses and ATU losses can be evaluated for a system perspective. Ask yourself why they choose to not model the antenna. Often, when people claim that an antenna can't be modelled in reality just don't like the answers. Owen |
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Owen Duffy wrote:
... Ask yourself why they choose to not model the antenna. Often, when people claim that an antenna can't be modelled in reality just don't like the answers. Owen I think they did model it, and compared the results against actual hands-on observations, readings, contacts, etc. This is why they make the claim NEC is missing something ... they simply believe their eyes, ears and meter readings and signal reports ... Regards, JS |
Going tp put this antenna up today
Howard Kowall wrote:
Hello All Well after all the comments made and doing some research on the G5RV( I guess it has it place) I have decide to try something new and hopefully better.Here it is, the Mystery Antenna,I had all the parts at home, so it did not cost any money at this point. Built it last night took about 1.5 hrs to build,including making my own center insulators and end insulators.Here is the link to the site,I am sure allot of you in this group know about it and looking forward to your comments.Will let you all know how it works,and some of the readings I get from it. http://w5gi.com/mysteryantenna.htm thanx all Howard VE4ISP ps. thanx to all that commented on my G5RV post Dog bone, not wish bone. |
Going tp put this antenna up today
John Smith wrote:
Owen Duffy wrote: ... Ask yourself why they choose to not model the antenna. Often, when people claim that an antenna can't be modelled in reality just don't like the answers. Owen I think they did model it, and compared the results against actual hands-on observations, readings, contacts, etc. This is why they make the claim NEC is missing something ... they simply believe their eyes, ears and meter readings and signal reports ... Let's not forget the possibility that they didn't know how to model the thing. I'm no great shakes at modeling, and that antenna is beyond my prowess at the present. Of course, I'm mpore likely to assume that discrepancies between computer world and reality are my fault. Some others might assume that the data they input was correct, so it must be the programs fault.... And some on the fringe might say the antenna CAN't work - the computer says it can't! - 73 de Mike N3LI - |
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Michael Coslo wrote:
Let's not forget the possibility that they didn't know how to model the thing. I'm no great shakes at modeling, and that antenna is beyond my prowess at the present. Of course, I'm mpore likely to assume that discrepancies between computer world and reality are my fault. Some others might assume that the data they input was correct, so it must be the programs fault.... And some on the fringe might say the antenna CAN't work - the computer says it can't! - 73 de Mike N3LI - Absolutely, I am NOT making any statement "they" are correct (I have never tried this antenna myself--either as a model or as an actual antenna in reality.) But, anyway you cut it, and on the ground floor, there ARE discrepancies in the basic equations, formulas and assumptions being put to use in the NEC, someday these will be fleshed out ... Regards, JS |
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On Sep 29, 11:04*pm, John Smith wrote:
Owen Duffy wrote: ... Ask yourself why they choose to not model the antenna. Often, when people claim that an antenna can't be modelled in reality just don't like the answers. Owen I think they did model it, and compared the results against actual hands-on observations, readings, contacts, etc. This is why they make the claim NEC is missing something ... they simply believe their eyes, ears and meter readings and signal reports ... Regards, JS The evaluation of this antenna should start with connecting a choke type BALUN at the input to isolate feedline radiation. A few feet of wire draped across some tree limbs can let you talk to the world. There have been an awful lot of hams who have thought they just invented the best antena out there while talking off their coax. Jimmie |
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JIMMIE wrote:
The evaluation of this antenna should start with connecting a choke type BALUN at the input to isolate feedline radiation. A few feet of wire draped across some tree limbs can let you talk to the world. There have been an awful lot of hams who have thought they just invented the best antena out there while talking off their coax. Jimmie Absolutely. The "truth" does have a way of outing itself ... However, not so very long ago, I did just that ... and being ignorant to just how much the coax (transmission) line was serving as an antenna--I lived in ignorant bliss ... I can't say I wasn't happy! LOL However, being the control freak I can be, I wanted that antenna "doing its' correct job"--when my ignorance had been dispelled (well, I am working on that one wink.) Man! I have to read that paper one more time! ... LOL Regards, JS |
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JIMMIE wrote in
: .... The evaluation of this antenna should start with connecting a choke type BALUN at the input to isolate feedline radiation. A few feet of Then it is not the same antenna as described. I agree with your implication that an antenna of this type warrants an attempt to minimise common mode current on the feedline... but that is not a part of the design as described. I have not said anywhere that this antenna doesn't 'work' or does 'work' whatever that means, just commented on their opinion that it defies conventional explanation. They haven't suggested that the problem is their own limitation in modelling or explaining, so it is reasonable to assume that their implication is that it just cannot be explained in conventional terms and using accepted tools. I have questioned that assertion, I think it is not all that difficult to model. It presents no more challenges than dealt with in my models of a Bazooka (http://www.vk1od.net/DoubleBazooka/index.htm)and G5RV (http://www.vk1od.net/G5RV/index.htm) assuming an ideal balun. I know the ideal balun condition appears inconsistent with my first par, the problem is that including the feedline common mode path in the model is complicated by the huge variability from one installation to another, both the length and route. (It is possible to model the antenna with a specific common mode configuration... it is just that is has limited applicability.) I am tempted to do it one day. Although it isn't the same antenna in that it has a balun, it is revealing. My gut feed is that the antenna with balun is probably not that band on at lease many of the HF bands... more if you ditch the coax section and use open wire feeder to a balanced ATU... but we are moving further from the original. Having said that, I do think the published VSWR figures at 145.3MHz are fanciful and highlight your average ham's obsession with VSWR, as if that single metric was a good indicator of system performance. Owen |
Going tp put this antenna up today
On Sep 30, 4:37*pm, Owen Duffy wrote:
JIMMIE wrote : ... The evaluation of this antenna should start with connecting a choke type BALUN at the input to isolate feedline radiation. A few feet of Then it is not the same antenna as described. I agree with your implication that an antenna of this type warrants an attempt to minimise common mode current on the feedline... but that is not a part of the design as described. I have not said anywhere that this antenna doesn't 'work' or does 'work' whatever that means, just commented on their opinion that it defies conventional explanation. They haven't suggested that the problem is their own limitation in modelling or explaining, so it is reasonable to assume that their implication is that it just cannot be explained in conventional terms and using accepted tools. I have questioned that assertion, I think it is not all that difficult to model. It presents no more challenges than dealt with in my models of a Bazooka (http://www.vk1od.net/DoubleBazooka/index.htm)andG5RV (http://www.vk1od.net/G5RV/index.htm) assuming an ideal balun. I know the ideal balun condition appears inconsistent with my first par, the problem is that including the feedline common mode path in the model is complicated by the huge variability from one installation to another, both the length and route. (It is possible to model the antenna with a specific common mode configuration... it is just that is has limited applicability.) I am tempted to do it one day. Although it isn't the same antenna in that it has a balun, it is revealing. My gut feed is that the antenna with balun is probably not that band on at lease many of the HF bands... more if you ditch the coax section and use open wire feeder to a balanced ATU... but we are moving further from the original. Having said that, I do think the published VSWR figures at 145.3MHz are fanciful and highlight your average ham's obsession with VSWR, as if that single metric was a good indicator of system performance. Owen With enough RG58 any antenna's VSWR looks pretty good. Jimmie |
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JIMMIE wrote:
With enough RG58 any antenna's VSWR looks pretty good. With 200' of RG58 at 460 MHz, one doesn't even need an antenna. :-) -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Going tp put this antenna up today
"JIMMIE" wrote in message ... snip A few feet of wire draped across some tree limbs can let you talk to the world. Yes, absolutely! I've told this story here before ... My first HF antenna was a 10m vertical dipole I made out of copper water pipes. Using a tuner, I could work 15m with it, but not 20 -- it needed more metal. So, I grabbed an alligator clip lead and electrically added an old 8-foot ladder that was literally laying on the ground next to the dipole. This enabled me to get a good SWR and I immediately worked Hawaii from here near San Diego. One wag asked if I was feeding it with ladder line. No. |
Going tp put this antenna up today
On Sep 30, 4:37*pm, Owen Duffy wrote:
JIMMIE wrote : ... The evaluation of this antenna should start with connecting a choke type BALUN at the input to isolate feedline radiation. A few feet of Then it is not the same antenna as described. I agree with your implication that an antenna of this type warrants an attempt to minimise common mode current on the feedline... but that is not a part of the design as described. Any additional feedline is also not part of the antenna as described but I am sure this attributes to many of the glowing reports that seems to refute antenna NEC analysis. Jimmie |
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JIMMIE wrote:
Any additional feedline is also not part of the antenna as described but I am sure this attributes to many of the glowing reports that seems to refute antenna NEC analysis. Jimmie Is that why NEC can't evaluate these antennas correctly?: http://assemblywizard.fr33webhost.co...ristics%20.pdf Regards, JS |
Going tp put this antenna up today
JIMMIE wrote in
: On Sep 30, 4:37*pm, Owen Duffy wrote: .... I agree with your implication that an antenna of this type warrants an attempt to minimise common mode current on the feedline... but that is not a part of the design as described. Any additional feedline is also not part of the antenna as described but I am sure this attributes to many of the glowing reports that seems to refute antenna NEC analysis. If that is to imply that common mode current only exists on the dipole side of the common mode choke, it is a mistaken analysis. A common mistake, but wrong nonetheless. Owen |
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John Smith wrote:
Michael Coslo wrote: Let's not forget the possibility that they didn't know how to model the thing. I'm no great shakes at modeling, and that antenna is beyond my prowess at the present. Of course, I'm mpore likely to assume that discrepancies between computer world and reality are my fault. Some others might assume that the data they input was correct, so it must be the programs fault.... And some on the fringe might say the antenna CAN't work - the computer says it can't! - 73 de Mike N3LI - Absolutely, I am NOT making any statement "they" are correct (I have never tried this antenna myself--either as a model or as an actual antenna in reality.) But, anyway you cut it, and on the ground floor, there ARE discrepancies in the basic equations, formulas and assumptions being put to use in the NEC, someday these will be fleshed out ... Regards, JS I doubt that any antenna an amateur is likely to build has some physics not adequately modeled by the math in NEC. Exactly what might be these discrepancies in equations, formulae and assumptions? Considering that NEC has had decades of validation against actual measurements and a lot of really, really smart people looking at how it works, I'd be kind of surprised. However, I can also easily believe that an amateur (or professional) could build an antenna that has measured performance different than expected from their NEC model of that antenna. The differences would lie, most likely, in these areas: 1) Inaccuracies in the model itself. Things like earth properties (NEC assumes uniform dielectric, it isn't) are an important source of error for antennas close to the ground. Most amateur models do not include a very good model of the surroundings (supports, trees, feedlines, etc.) 2) Inaccuracies in the measurements or not measuring the right things. A good example is using NEC to get feedpoint characteristics, then measuring at the rig, and not properly accounting for the transmission line, particularly if the feedpoint Z is reactive. In the professional antenna world, if someone models an antenna, then builds it and tests it on the range, and the measurements differ from what the model predicted, the usual assumption is that what was built differed from what was modeled, or the measurements were off. |
Going tp put this antenna up today
Jim Lux wrote:
John Smith wrote: [A bunch of chit Jim, obviously, will/and does differ with] Regards, JS I doubt that any antenna an amateur is likely to build has some physics not adequately modeled by the math in NEC. Exactly what might be these discrepancies in equations, formulae and assumptions? Considering that NEC has had decades of validation against actual measurements and a lot of really, really smart people looking at how it works, I'd be kind of surprised. I don't believe the above it correct. Indeed, if you will only review my past objections and reflections on how "the NEC engine" demonstrates "differences" you will be focused at the "focal point" of my "inquiries" ... However, I can also easily believe that an amateur (or professional) could build an antenna that has measured performance different than expected from their NEC model of that antenna. The differences would lie, most likely, in these areas: Look, the dipole, standard monopole (1/4, 1/2, longwire, etc.) is NOT in debate. Indeed, it is like NEC was designed to "explain/model" these, DUH! 1) Inaccuracies in the model itself. Things like earth properties (NEC assumes uniform dielectric, it isn't) are an important source of error for antennas close to the ground. Most amateur models do not include a very good model of the surroundings (supports, trees, feedlines, etc.) 2) Inaccuracies in the measurements or not measuring the right things. A good example is using NEC to get feedpoint characteristics, then measuring at the rig, and not properly accounting for the transmission line, particularly if the feedpoint Z is reactive. In the professional antenna world, if someone models an antenna, then builds it and tests it on the range, and the measurements differ from what the model predicted, the usual assumption is that what was built differed from what was modeled, or the measurements were off. I could pick apart the above, attempt to poke fun, etc. -- however, I would much rather join forces and attempt to focus on the points which would lead us to real answers -- i.e., the arrl and illiterates have already done enough damage, let us pursue a more productive path? Leave us leave our minds open, OK? Regards, JS |
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John Smith wrote:
... Leave us leave our minds open, OK? Regards, JS And damn, and DAMN, and well DAMN ... If you want to build something from a 19?? to 1950 (or beyond?) publications, do I stop you? Is this what "you have you back up your ego about?" It this what threatens you? If so, go ahead, go to your grave with your pursuits, without my critique! ... I am here about what "I AM", about "WHAT I THINK", about "WHAT I SEE", about "WHAT I SUSPECT", about "WHAT I WONDER", about "MY QUESTIONS TO OTHER MEN/WOMEN", about what I simply want to think about and want answers to ... yanno, I think you are really endangerd by those "others" here, I find ... Cecil, where are you? evil grin If you can't participate, if you think I am am an idiot, if you think I am a moron ... could you do it politely until I give you reason to do differently ... indeed, I may feel threaten my "moronic brains" and respond ... please don't take insult, just reassure me I am not wrong ... Regards, JS |
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John Smith wrote:
... If you can't participate, if you think I am am an idiot, if you think I am a moron ... could you do it politely until I give you reason to do differently ... indeed, I may feel threaten my "moronic brains" and respond ... please don't take insult, just reassure me I am not wrong ... Regards, JS Look, that got "corrupted" in my frustration, I am not a writer, I just attempt to bring my skills up to speed to participate here ... Let me change all that: If I can't think here ... If I can use this as a "note pad" ... If I can speculate here ... If I can't search for others here, if I can't use other as a "backboard" here ... .... then let it all be damned ... I am an idiot ... and let's look over your past, present and future questions, speculations, advances, etc. ... Regards, JS |
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John Smith wrote:
... Regards, JS Geesh, can = can't in a lot of that. Those with a brain will know ... those without will point it out ... don't think it necessary ... PLEASE! I am taking a break from all this ... I need to ... :-) Regards, JS |
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On Thu, 02 Oct 2008 15:10:05 -0700, John Smith
wrote: Geesh, can = can't in a lot of that. Those with a brain will know ... those without will point it out It took 3 posts to point it out, but you managed.... ... don't think it necessary ... PLEASE! Not in the least necessary from any perspective; but I can see why some modeling would be impossible to confirm against the data offered. Afterall, if the prognosis of, say, netzheimers were based on a ±0.1dB DNA error, then all bets are off in proving sanity. Knowledge may give weight, but accomplishments give lustre, and many more people see than weigh. Lord Chesterfield 73's Richard Clark, KB7QHC |
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John Smith wrote:
I don't believe the above is correct. Proof that NEC cannot model everything is at: http://www.w5dxp.com/SUPRGAIN.EZ Would you believe a vertical with 24 dBi omnidirectional gain? -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
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Richard Clark wrote:
... Not in the least necessary from any perspective; but I can see why some modeling would be impossible to confirm against the data offered. Afterall, if the prognosis of, say, netzheimers were based on a ±0.1dB DNA error, then all bets are off in proving sanity. Knowledge may give weight, but accomplishments give lustre, and many more people see than weigh. Lord Chesterfield 73's Richard Clark, KB7QHC LOL! Thanks Richard! I will place a grain (or two, or more) of salt upon the joint of the thumb with wrist (and, thinking fondly of you, as reason enough), swag down the shot of Tequila and touch tongue to the joint ... but for now, more important things beckon me ... Ahh yes, if only for a night ... I suppose you will be here tomorrow ... may your dreams be filled with the such of mine ... here is too our further "exploits" :-) Regards, JS |
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Cecil Moore wrote:
... Would you believe a vertical with 24 dBi omnidirectional gain? Cecil: With the "truth" we have maintained before us, and especially here, another shot of Ta-Kill-Ya (or, Tequila), I'd believe anything you would state!--await my returned "brain" tomorrow--please? ROFLOL Regards my friend, JS |
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John Smith wrote:
--await my returned "brain" tomorrow--please? ROFLOL Good luck on that one. :-) -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
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Cecil Moore wrote:
John Smith wrote: I don't believe the above is correct. Proof that NEC cannot model everything is at: http://www.w5dxp.com/SUPRGAIN.EZ Would you believe a vertical with 24 dBi omnidirectional gain? Got a regular NEC deck or text description (can't read the EZ-NEC file..) |
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Jim Lux wrote:
Cecil Moore wrote: John Smith wrote: I don't believe the above is correct. Proof that NEC cannot model everything is at: http://www.w5dxp.com/SUPRGAIN.EZ Would you believe a vertical with 24 dBi omnidirectional gain? Got a regular NEC deck or text description (can't read the EZ-NEC file..) Although NEC can't model everything (for example, it can't model a patch antenna containing dielectric), this model is not at all proof of that. All it shows is that models can be carelessly or intentionally constructed in such a way as to cause NEC to malfunction. An experienced EZNEC or NEC modeler would immediately run an Average Gain test and seen that the reported gain is about 16.7 dB too high due to numerical problems. If desired, this can be subtracted from the reported gain to give the gain that's much closer to reality. A resistance, the jagged schematic symbol that we use in basic circuit analysis, can be and often is used as a simple model of a resistor. But anyone who has ever done any electronic design or analysis quickly discovers that this model is adequately accurate only under some limited group of conditions -- for example, it's a lousy model of a leaded resistor at 10 GHz --, and any calculations made using it will be far from measurement of a real resistor (assuming the measurements are capably made). Like the resistance and all other models, NEC also has limitations and boundaries over which it's valid. And it can be misapplied to produce just as egregiously bad results as using a resistance to model a leaded resistor at 10 GHz. In this case, the model violates at least one NEC rule by spacing two 40-foot #14 wires 1/2 wire diameter (0.032") apart. (NEC guidelines specify that parallel wires should be at least several wire diameters apart.) It would be a great challenge to actually construct this antenna. But, even at that, the NEC results are probably not bad once the average gain correction is made. Here's the model in .NEC format for those who don't have EZNEC. I've changed the current source to a voltage source which simplifies the model without making any difference in results. CM 40m triangular loop CE GW 1,31,.003048,0.,3.048,.003048,0.,12.192,8.138E-4 GW 2,53,0.,0.,12.192,0.,16.4592,12.192,8.138E-4 GW 3,61,0.,16.4592,12.192,0.,0.,3.048,8.138E-4 GW 4,31,0.,0.,3.048,0.,0.,12.192,8.138E-4 GE 1 LD 5,1,0,0,5.7471E+7,1. LD 5,2,0,0,5.7471E+7,1. LD 5,3,0,0,5.7471E+7,1. LD 5,4,0,0,5.7471E+7,1. FR 0,1,0,0,7. GN 2,0,0,0,13.,.005 EX 0,1,16,0,1.414214,0. RP 0,181,1,1000,90.,0.,-1.,0.,0. EN Roy Lewallen, W7EL |
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Jim Lux wrote:
I doubt that any antenna an amateur is likely to build has some physics not adequately modeled by the math in NEC. Exactly what might be these discrepancies in equations, formulae and assumptions? Considering that NEC has had decades of validation against actual measurements and a lot of really, really smart people looking at how it works, I'd be kind of surprised. However, I can also easily believe that an amateur (or professional) could build an antenna that has measured performance different than expected from their NEC model of that antenna. The differences would lie, most likely, in these areas: 1) Inaccuracies in the model itself. Things like earth properties (NEC assumes uniform dielectric, it isn't) are an important source of error for antennas close to the ground. Most amateur models do not include a very good model of the surroundings (supports, trees, feedlines, etc.) 2) Inaccuracies in the measurements or not measuring the right things. A good example is using NEC to get feedpoint characteristics, then measuring at the rig, and not properly accounting for the transmission line, particularly if the feedpoint Z is reactive. In the professional antenna world, if someone models an antenna, then builds it and tests it on the range, and the measurements differ from what the model predicted, the usual assumption is that what was built differed from what was modeled, or the measurements were off. A lot of people are aware that some knowledge and skill is necessary in order to construct a good model which will give accurate results. What's not so widely appreciated is how difficult it is to make decent measurements of even such seemingly simple things as impedance, let alone gain and pattern. Some years ago I was at what was then NOSC (Naval Ocean Systems Center) in San Diego, where they had the very best equipment to do full 3D pattern measurements of antennas mounted on carefully constructed and accurate scale models of U.S. Navy ships. They were also modeling the antennas and ship structures with NEC-4. One of the engineers confided to me that they'd learned that when the measurement disagreed with the model result, the model result was probably better. Of course, these people were very highly skilled in using the modeling software and how to avoid and detect its limitations. But they were also very highly skilled in making the best possible measurements. A claim by an antenna manufacturer or creator that an antenna "can't be modeled by NEC" very often means that NEC's accurate results don't back up the manufacturer's or creator's inflated claims. If the claim includes a statement that some alleged physical rule or phenomenon ("critical coupling" comes to mind) isn't "accounted for" by NEC, you can be certain that the disparity is due to inflated claims rather than a shortcoming of NEC. There are, of course, some antennas that truly can't be modeled with NEC, for example a patch antenna with dielectric between the patch and ground plane. But those of NEC's limitations that aren't apparent from the basic program structure have been pretty well discovered and documented in the 30 years or so it's been in use. Antennas are being designed every day with NEC and EZNEC which provide critical communications functions for military and government agencies, aerospace companies, space agencies, domestic and international broadcasters, scientific researchers, and many others. Many of those antennas have been carefully measured and verified before being put into service, and countless of them are in current use. I personally have designed a number of antennas with EZNEC, tested physical prototypes at a professional range, and seen them put into daily service performing critical functions as designed. Roy Lewallen, W7EL |
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Roy Lewallen wrote:
Jim Lux wrote: I doubt that any antenna an amateur is likely to build has some physics not adequately modeled by the math in NEC. Exactly what might be these discrepancies in equations, formulae and assumptions? Considering that NEC has had decades of validation against actual measurements and a lot of really, really smart people looking at how it works, I'd be kind of surprised. However, I can also easily believe that an amateur (or professional) could build an antenna that has measured performance different than expected from their NEC model of that antenna. The differences would lie, most likely, in these areas: 1) Inaccuracies in the model itself. Things like earth properties (NEC assumes uniform dielectric, it isn't) are an important source of error for antennas close to the ground. Most amateur models do not include a very good model of the surroundings (supports, trees, feedlines, etc.) 2) Inaccuracies in the measurements or not measuring the right things. A good example is using NEC to get feedpoint characteristics, then measuring at the rig, and not properly accounting for the transmission line, particularly if the feedpoint Z is reactive. In the professional antenna world, if someone models an antenna, then builds it and tests it on the range, and the measurements differ from what the model predicted, the usual assumption is that what was built differed from what was modeled, or the measurements were off. A lot of people are aware that some knowledge and skill is necessary in order to construct a good model which will give accurate results. What's not so widely appreciated is how difficult it is to make decent measurements of even such seemingly simple things as impedance, let alone gain and pattern. Some years ago I was at what was then NOSC (Naval Ocean Systems Center) in San Diego, where they had the very best equipment to do full 3D pattern measurements of antennas mounted on carefully constructed and accurate scale models of U.S. Navy ships. They were also modeling the antennas and ship structures with NEC-4. One of the engineers confided to me that they'd learned that when the measurement disagreed with the model result, the model result was probably better. Of course, these people were very highly skilled in using the modeling software and how to avoid and detect its limitations. But they were also very highly skilled in making the best possible measurements. A claim by an antenna manufacturer or creator that an antenna "can't be modeled by NEC" very often means that NEC's accurate results don't back up the manufacturer's or creator's inflated claims. If the claim includes a statement that some alleged physical rule or phenomenon ("critical coupling" comes to mind) isn't "accounted for" by NEC, you can be certain that the disparity is due to inflated claims rather than a shortcoming of NEC. There are, of course, some antennas that truly can't be modeled with NEC, for example a patch antenna with dielectric between the patch and ground plane. But those of NEC's limitations that aren't apparent from the basic program structure have been pretty well discovered and documented in the 30 years or so it's been in use. Antennas are being designed every day with NEC and EZNEC which provide critical communications functions for military and government agencies, aerospace companies, space agencies, domestic and international broadcasters, scientific researchers, and many others. Many of those antennas have been carefully measured and verified before being put into service, and countless of them are in current use. I personally have designed a number of antennas with EZNEC, tested physical prototypes at a professional range, and seen them put into daily service performing critical functions as designed. Roy Lewallen, W7EL Oh, I C ... You will excuse me then, I have had a bit too much to drink, to be coherent ... but then, from your response, you are in the same shape, right? :-) Regards, JS |
Going tp put this antenna up today
Roy Lewallen wrote:
... Roy Lewallen, W7EL Gawd, that is a lot Roy. Like I have said before, I have read your articles, since I was a boy ... And, I shouldn't even be here, from the party ... but then, ya' already know that ... wink Did I get that right? I mean, I like what I'm drinkin', what are you drinin', can I get you one? goofy-smile Regards, JS |
Going tp put this antenna up today
Correction:
Roy Lewallen wrote: . . . An experienced EZNEC or NEC modeler would immediately run an Average Gain test and seen that the reported gain is about 16.7 dB too high due to numerical problems. If desired, this can be subtracted from the reported gain to give the gain that's much closer to reality. I should have said "experienced *and not careless*". The average gain is about 30.4 dB, not 16.7. . . . In this case, the model violates at least one NEC rule by spacing two 40-foot #14 wires 1/2 wire diameter (0.032") apart. (NEC guidelines specify that parallel wires should be at least several wire diameters apart.) It would be a great challenge to actually construct this antenna. But, even at that, the NEC results are probably not bad once the average gain correction is made. The wires are just about one wire diameter, not 1/2, apart. This antenna is a very difficult one for NEC. In a few minutes of fiddling, I couldn't get a decent average gain at much less than about 0.3 foot wire spacing, and then it required specific segmentation. I'll modify my earlier statement to say that this is an antenna that is indeed pretty difficult for NEC to handle. But it's easily recognized as such by looking at the average gain result. If you don't know about the average gain test, look in the EZNEC manual -- included also with the demo program -- index under "Average Gain". It's an important and valuable tool. Roy Lewallen, W7EL |
Going tp put this antenna up today
Roy Lewallen wrote:
All it shows is that models can be carelessly or intentionally constructed in such a way as to cause NEC to malfunction. SUPRGAIN.EZ was a complete accident from many years ago. I noticed that the closer together the wires were, the higher the reported gain. When it got to about 5 dBi omni, I knew I had done something wrong. All I was trying to do is show there are certain things that NEC cannot do. I suspect that modeling Art's antenna is one of those things. -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Going tp put this antenna up today
Cecil Moore wrote:
Roy Lewallen wrote: All it shows is that models can be carelessly or intentionally constructed in such a way as to cause NEC to malfunction. SUPRGAIN.EZ was a complete accident from many years ago. I noticed that the closer together the wires were, the higher the reported gain. When it got to about 5 dBi omni, I knew I had done something wrong. All I was trying to do is show there are certain things that NEC cannot do. I suspect that modeling Art's antenna is one of those things. Let me see, is there a way to "carelessly or intentionally" construct data into my formula to compute the volume of a sphere to make the formula "malfunction?" Nope! Guess that formula is returning valid results! Nice being able to have real confidence in it! ;-) Regards, JS |
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On Fri, 03 Oct 2008 10:20:12 -0700, John Smith
wrote: Let me see, is there a way to "carelessly or intentionally" construct data into my formula to compute the volume of a sphere to make the formula "malfunction?" Nope! Guess that formula is returning valid results! Nice being able to have real confidence in it! ;-) What is the volume of a sphere with the measured dimension of a chord = 1 meter? That was too easy! 73's Richard Clark, KB7QHC |
Going tp put this antenna up today
Richard Clark wrote:
What is the volume of a sphere with the measured dimension of a chord = 1 meter? That was too easy! 73's Richard Clark, KB7QHC Ahhh, once again that magnificent intellect of yours is displayed ... However, somehow, it has lost its' shock value ... :-| Regards, JS |
Going tp put this antenna up today
On Fri, 03 Oct 2008 12:31:34 -0700, John Smith
wrote: On Fri, 03 Oct 2008 10:20:12 -0700, John Smith wrote: Let me see, is there a way to "carelessly or intentionally" construct data into my formula to compute the volume of a sphere to make the formula "malfunction?" Nope! Guess that formula is returning valid results! Nice being able to have real confidence in it! ;-) What is the volume of a sphere with the measured dimension of a chord = 1 meter? That was too easy! Ahhh, once again that magnificent intellect of yours is displayed ... However, somehow, it has lost its' shock value ... :-| Which is to say even with your confidence and your formula that you don't know. It demonstrates you can neither construct careless nor careful data. This was anticipated and all would note that Sarah Lee could answer her own questions even if she couldn't answer others'. (Did they draft Cecil to Sedona last weekend for her rehearsals?) Brett, let's just cut to the chase and point out all of your "objections" to modeling are offered with rather pale logic and even less demonstrables. Simpler yet, you are just trolling - but that isn't groundbreaking news here.... 73's Richard Clark, KB7QHC |
Going tp put this antenna up today
Richard Clark wrote:
Brett, let's just cut to the chase and point out all of your "objections" to modeling are offered with rather pale logic and even less demonstrables. Simpler yet, you are just trolling - but that isn't groundbreaking news here.... 73's Richard Clark, KB7QHC Hey, I do a lot of what some would consider "illogical", like a belief in a superior being/race! However, the failing of NEC need not be taken as a leap of faith, they can be demonstrated ... But, ya'all wouldn't know a real troll if it bit ya' on the arse ... Regards, JS |
Going tp put this antenna up today
John Smith wrote:
[chit] Richard Clark wrote: ... Simpler yet, you are just trolling - but that isn't groundbreaking news here.... 73's Richard Clark, KB7QHC Or, let me summarize, by stating an example: Take two boxes, mark one "TROLLS", the other "NON-TROLLS." Now, put members of this group (or from your life experience--that'll work also) into the two boxes, sorting as you see fit. Now, if you notice that one box is full of all those who agree with you and who you worship, and the other is full of all those not agreeing with you--you don't have a "troll problem" at all! Regards, JS |
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