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#1
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Why do you feel modeling is "so unreliable"?
========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. |
#2
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On Tue, 18 May 2004 20:05:37 +0000 (UTC), "Reg Edwards"
wrote: Why do you feel modeling is "so unreliable"? ========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. I see........ Then I assume then that your diagnosis of computer modeling applies to your antenna computer programs as well? Danny |
#3
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"Dan Richardson wrote Why do you feel modeling is "so unreliable"? ========================= Because 99.99 percent of models are never properly tested in practice and in an anateur's back yard, if they WERE tested, the radiation patterns would only bear a vague resemblance to predictions. Predictions are mostly wishful thinking. Nevertheless, they can be a satisfying intellectual activity. ---- Reg. I see........ Then I assume then that your diagnosis of computer modeling applies to your antenna computer programs as well? Danny ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
#4
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On Tue, 18 May 2004 23:50:17 +0000 (UTC), "Reg Edwards"
wrote: But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. If you will review my posting of the results i reported you will see that I was comparing 1/4, 1/2 and 5/8-wave whips under the same conditions for each antenna. I was not and did not report the reported gain figures, rather, I posted the difference or lack there of for the comparisons. Under those conditions I feel comfortable stating that difference between using a 1/4, 1/2 or 5/8-wave whips on a vehicle (mounted top-dead-center on the roof) the overall gain differences would be one dB or less. I have observed, operating mobile using the above antennas that I really couldn't notice any real difference. Bad areas remained bad areas and I would loose the repeater in about the same location regardless which antenna I used. In other words I was comparing apples to apples and not looking for an absolute gain value, rather, the difference and I am very comfortable with the findings - both via computer modeling and actual use. very 73 Danny, K6MHE |
#5
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You are only sort of correct.
At HF where the local world is as large as your antenna, that statement is true, at 6m it starts to become less of an issue as the outside world becomes big and somewhat ignorable. At 2m it is quite ignorable, and at 70cm it pretty much isn't there. Yagis I have designed for 432 perform exactly as predicted by the software, with the exception of the ones too long for the antenna range, where the only discrepency was the gain tests a bit lower than the model predicts. And then it was .3 dB on an 18.4 dBd prediction. It is possible to predict antenna gain and pattern very accurately if you do it at a frequency that does not interact with the local environment to a any large degree. It is also possible to build those antennas and have them perform the same time and time again. I've built dozens of long boom yagis for 2, 222, and 432, and after the first one, you don't even have to measure the SWR before setting the T match shorts unless you are anal or an EME're. Oops, same thing, I guess. And every antenna I've ever range tested (thank you Central States VHF) has been within .1 dB of predicted except one. The designs I have built over the last 15 years didn't need to be adjusted at all from the model plus predicted boom corrections. The only variable was the matching system on the driven element, which none of the modeling programs handles very well. And for you picky folks, yes, the losses from foliage, etc. aren't ignorable, but the antenna system still performs as predicted unless you mount it in the center of a tree. And I do realize I'm preaching to the non-choir here, since this group is (apparently) not very interested in anything above 10m. tom K0TAR Reg Edwards wrote: ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
#6
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Oh, I forgot the important bit - I did it in my back yard.
tom K0TAR Tom Ring wrote: You are only sort of correct. At HF where the local world is as large as your antenna, that statement is true, at 6m it starts to become less of an issue as the outside world becomes big and somewhat ignorable. At 2m it is quite ignorable, and at 70cm it pretty much isn't there. Yagis I have designed for 432 perform exactly as predicted by the software, with the exception of the ones too long for the antenna range, where the only discrepency was the gain tests a bit lower than the model predicts. And then it was .3 dB on an 18.4 dBd prediction. It is possible to predict antenna gain and pattern very accurately if you do it at a frequency that does not interact with the local environment to a any large degree. It is also possible to build those antennas and have them perform the same time and time again. I've built dozens of long boom yagis for 2, 222, and 432, and after the first one, you don't even have to measure the SWR before setting the T match shorts unless you are anal or an EME're. Oops, same thing, I guess. And every antenna I've ever range tested (thank you Central States VHF) has been within .1 dB of predicted except one. The designs I have built over the last 15 years didn't need to be adjusted at all from the model plus predicted boom corrections. The only variable was the matching system on the driven element, which none of the modeling programs handles very well. And for you picky folks, yes, the losses from foliage, etc. aren't ignorable, but the antenna system still performs as predicted unless you mount it in the center of a tree. And I do realize I'm preaching to the non-choir here, since this group is (apparently) not very interested in anything above 10m. tom K0TAR Reg Edwards wrote: ==================================== Danny, there's nothing wrong with antenna pattern-prediction progams. There are good nunber-crunched side effects. It's only their practical application by people in their own back yards which is unreliable. It cannot be helped. It's a fact of life. The environment and performance of an antenna cannot be accurately predicted unless it is at a height of several wavelengths above and away from obstructions. Not a single one of my programs predicts a radiation pattern. Only an exceedingly few professional antenna design engineers would ever make real use of such facilities even if I could write them. I don't like wasting time. I havn't all that amount of time anyway. Nobody has produced a new type of antenna for many decades. (Not even Fractal). It's all been done before. But, as I say, radiation prediction programs are entertaining, educational and satisfying. Enjoy them while you can. Nevertheless, unreliable is a fair practical description. ---- Reg, G4FGQ |
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