|
So, was that a yes or a no? I have trouble with your accent.
Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: Tell us, Cecil, at steady state at one frequency, can a lumped inductor (presumably like the experimenter's toroid) tell whether it's at the base of an antenna or simply in series between a generator and load impedance? This question proves you don't understand the problem. The inductor cannot tell if it is installed in an antenna or transmission line. So I will turn the question around: Does a standing wave antenna have standing waves? Reference _Antenna_Theory- by Balanis, page 17, section 1.4 Current Distrubution on a Thin Wire Antenna. Is Balanis correct when he says: "If the diameter of each wire is very small, the ideal standing wave pattern of the current along the arms of the (1/2WL) dipole is sinusoidal with a null at the end." This is after he takes an unterminated transmission line, discusses standing waves, and then slowly opens up the ends of the transmission line to create a 1/2WL dipole. I took Balanis' antenna course at ASU in 1995. I asked a lot of questions about inductively loaded antennas. The current and standing wave pattern on each side of a loading coil is NOT the same. |
No, that's not what I asked. Let me try again.
I have an antenna whose feedpoint impedance I measure as R + jX. I put a lumped (physically very small and short) coil in series with it and drive it with a generator. You and Yuri say that the current going into the coil is different from the current going out. Now, I replace the antenna with a series resistor and capacitor or inductor which also has a terminal impedance of R + jX ohms. My question is, does the inductor now have equal currents at its two terminals, and why or why not? Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: If you answered "yes", please explain how and why, and how we'd calculate the current through and voltage across the inductor. If we moved it an inch up the transmission line from the antenna base, can it still tell? Forget about an inductor becoming conscious. The impedance looking into a six foot whip is the same whether the coil is there or not. The impedance looking into the bottom of the coil is certainly not the same as looking into the six foot whip. I suspect this can be proven by modeling a mobile antenna and then moving the source point from just under the coil to just above the coil. If you answered "no", please write us the equations showing just how much the current should be expected to be different from one end of the inductor to the other. The current will be approximately the same as at the two points of wire it replaces in the antenna without the inductor. I earlier asked you a question that you seem to have missed. Do you agree or disagree with Fig 9-22 of ON4UN`s "Low-Band DXing", included on Yuri`s web pages.? And where those coulombs are going, that go into one end and don't come out the other. You can answer your own question. Where do the coulombs go that enter one end of a 1/4WL stub and don't exit the other end? Please stop using lumped circuit analysis on distributed network problems. You know and I know that it doesn't work. |
Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well. Look carefully at the currents along the stub and you'll find they're not equal and opposite on the two conductors. Such a radiating stub *is* very different from a coil. That shouldn't be surprising. I have a high level of confidence that if you built the antenna just like you modeled it, you would find the results to closely agree with the model. Roy Lewallen, W7EL Cecil Moore wrote: Wes Stewart wrote: On Mon, 03 Nov 2003 09:26:05 -0600, Cecil Moore wrote: |Roy Lewallen wrote: | If you could build an antenna from | straight conductors and lumped inductors, the result would be very close | to EZNEC's predictions. | |Hard to prove since lumped inductors are impossible in reality. Why |does EZNEC show so much difference between lumped inductors and stub |inductors? I see no such difference in my model. There shouldn't be a lot of difference. I have modeled two short dipoles, one loaded with a lumped inductive reactance and one modeled with the same reactance using an inductive stub. EZNEC reports the following: Inductance lumped j335 10'stub current in segment just before the coil .8374 amp .8384 amp current in segment just after the coil .7971 amp .5642 amp The relative difference just before the coil is quite small, 0.12%. The relative difference just after the coil is quite large, 41.28%. There just cannot be that amount of difference between a coil and a stub. |
Yes. If any real component has signficant length, it's best to model it
as wires if possible. If it's not possible or practical to model it as wires, about the best you can do is as I've suggested befo model it as a wire of the length and diameter of the component, and insert one or more loads to represent the low frequency impedance of the component. That's about as good as you can do with the limited set of modeling objects you have. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: I use lumped circuit analysis when dealing with lumped circuits, and distributed circuit analysis when dealing with distributed circuits. EZNEC's loads are lumped elements, so when you're talking about EZNEC loads, you're talking about lumped elements. Roy Lewallen, W7EL Hi Roy, Wouldn't it be better not to lump any portions of an antenna that are a part of its electrical length? 73, Jim AC6XG |
Roy Lewallen wrote:
I don't have Balanis. Can you provide a short quote where he states that the current at the terminals of a two-terminal lumped component are unequal? He doesn't use lumped components and probably for good reason. But here's the quote that allows my analysis. "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents 'If' and 'Ib' in Figure 10.1(a)." This means that net total current equals If+Ib. The fact that the feedpoint current occurs at a current maximum point ties both ends down. 'If' must traverse 90 degrees and 'Ib' must traverse 90 degrees in addition to the 180 degree phase shift due to reflection from the open end. Besides the coil, an 8' whip gives about 22 degrees phase shift in a round trip. Adding the 180 degree phase shift due to the open end reflection gives 202 degrees. But we know the phase shift is actually 360 degrees. Where can the additional 158 degrees of phase shift come from except from the coil? Center-loaded mobile antennas are still an electrical 1/4 wavelength. If there's no phase shift through the coil, where's the missing 158 degrees of phase shift taking place? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Roy Lewallen wrote:
A radiating stub does act differently than a lumped inductor, in both modeling and reality. EZNEC should reflect this difference accurately. If you're aware of a situation where you think it doesn't, please email me the models illustrating the difficulty. I have already done that but I just sent them to you again. If you model a stub using a transmission line model, it should behave exactly the same as a lossless lumped inductor at a given frequency. However, it's an accurate model of reality only if the real stub has exactly equal and opposite currents on the two conductors. That is, it's an entirely non-radiating stub. The difference in current between the two configurations that I sent to you means the vertical stubs are radiating better than the horizontal antenna which is unlikely since EZNEC doesn't show any vertical radiation. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Roy Lewallen wrote:
So, was that a yes or a no? I have trouble with your accent. It's a no. Lumped inductors are not conscious of anything including their locations. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Roy Lewallen wrote:
No, that's not what I asked. Let me try again. I have an antenna whose feedpoint impedance I measure as R + jX. I put a lumped (physically very small and short) coil in series with it and drive it with a generator. You and Yuri say that the current going into the coil is different from the current going out. Yes, just as the current going into a 1/4WL stub is different from the current going out. If you use an inductive stub, is the current the same going in as going out? If so, you have invented faster than light transfer of current. Now, I replace the antenna with a series resistor and capacitor or inductor which also has a terminal impedance of R + jX ohms. My question is, does the inductor now have equal currents at its two terminals, and why or why not? You have replaced a distributed network with a lumped circuit. If the lumped circuit model worked on distributed networks, you would be right and there would be no need for a distributed network model (but there is). -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Roy Lewallen wrote:
Because you're seeing different currents at the two stub terminals, you must be modeling it with wires, which should reflect reality quite well. Look carefully at the currents along the stub and you'll find they're not equal and opposite on the two conductors. Such a radiating stub *is* very different from a coil. Instead of a knee-jerk defense of your ideas, why don't you actually take a look at the problem? Those stubs are vertical. EZNEC shows virtually zero vertically polarized radiation. According to EZNEC, those stubs are radiating a negligible amount, just like the lumped inductance. Why the 40% difference in current between the two configurations? Is this a characteristic of NEC? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Mon, 03 Nov 2003 15:19:44 -0600, Cecil Moore
wrote: Roy Lewallen wrote: Because you're seeing different currents at the two stub terminals, you must be modeling it with wires, which should reflect reality quite well. Look carefully at the currents along the stub and you'll find they're not equal and opposite on the two conductors. Such a radiating stub *is* very different from a coil. Instead of a knee-jerk defense of your ideas, why don't you actually take a look at the problem? Ah the quality of sneer review. Those stubs are vertical. EZNEC shows virtually zero vertically polarized radiation. According to EZNEC, those stubs are radiating a negligible amount, just like the lumped inductance. Why the 40% difference in current between the two configurations? Is this a characteristic of NEC? Cecil, you have two stubs and they are driven antiphase (typical of a doublet) and through symmetry would have equal antiphase currents when compared to their opposites, but not necessarily equal currents within their twin-pair of lines. The sum of ALL currents (and not just the myopic view of one of two stubs) would suggest exactly what Roy has offered. This, of course, returns us to the question of what part of the radiator radiates. Sadly, the convention of the current pulse (or maxima, or other equivalent term) trips up discussion just in these matters. ALL elements radiate, it is only in the far field where their contributions negate, not literally within the structure. 73's Richard Clark, KB7QHC |
Richard KB7QHC writes:
I've visited your page, and in an effort to wean out the details (you don't go to much effort to offer an unadorned, plain, vanilla specification for the radiator). I had to guess about the coil length (judging from its only metric being 2.5" diameter) and inductance (you dismiss the importance of its Q and wholly leave this metric blank). Our major disagreement with W8JI was is the current accros antenna loading the same or is it appreciably different. I was just trying to argue that it is signifficantly different, W9UCW has done multitude of tests and measurements and supported my position. I had practical experience of knowing that bottom of uniform coils get hotter than middle or top, I fried heatshrink tubing at the bottom of the coils and this was good enough for my practical knowledge that current must by higher at the bottom. ON4UN book also supports that. The second step in our quest of properly mastering the phenomena is to investigate the relationships, magnitudes and have it properly accounted for in the modeling software. W9UCW provided some info, data and pictures, he has lot more and is willing to cooperate further, be it writing concise article (he is preparing material for the book also) or answer questions or provide more info. I told him about this NG, so he might show up. So if we are ready to bridge the idea that current is different and implement that in the software, lets roll. So far it appears that Roy still has a problem with understanding what is going on. Yuri |
|
Richard Clark wrote:
Cecil, you have two stubs and they are driven antiphase (typical of a doublet) and through symmetry would have equal antiphase currents when compared to their opposites, but not necessarily equal currents within their twin-pair of lines. The sum of ALL currents (and not just the myopic view of one of two stubs) would suggest exactly what Roy has offered. Sorry, Richard, 1/2 of the loaded dipole turned vertical doesn't show a trace of horizontal radiation. Sorry about that. Roy suggested the stubs might be radiating. EZNEC says they are not. You can add two inches of vertical wire to the ends of the antenna and see the red vertical radiation. The stubs are 0.04WL, #14 wire, and 6 inch spacing. How much could they radiate on 75m? The difference in current is not due to radiation. It is due to the phase shift between Ifwd and Iref through the stub. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Richard Clark wrote:
That, in fact, is not so. The problem as expressed by you was acknowledged and a solution offered BEFORE you started this thread. For those of us who missed the solution, what was it? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
So, in short, I don't disagree with Balanis.
Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: I don't have Balanis. Can you provide a short quote where he states that the current at the terminals of a two-terminal lumped component are unequal? He doesn't use lumped components and probably for good reason. But here's the quote that allows my analysis. "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents 'If' and 'Ib' in Figure 10.1(a)." This means that net total current equals If+Ib. The fact that the feedpoint current occurs at a current maximum point ties both ends down. 'If' must traverse 90 degrees and 'Ib' must traverse 90 degrees in addition to the 180 degree phase shift due to reflection from the open end. Besides the coil, an 8' whip gives about 22 degrees phase shift in a round trip. Adding the 180 degree phase shift due to the open end reflection gives 202 degrees. But we know the phase shift is actually 360 degrees. Where can the additional 158 degrees of phase shift come from except from the coil? Center-loaded mobile antennas are still an electrical 1/4 wavelength. If there's no phase shift through the coil, where's the missing 158 degrees of phase shift taking place? |
Good. And I see from your other response that you understand the question.
So now we have a simple series circuit consisting of a generator, the loading inductor, and the R + L or C we used to substitute for the antenna. And your "no" response indicates you've agreed that the voltage across and current through the inductor are the same as when it was connected to the antenna. Now, choose any values you'd like for the generator voltage or current and the component values, and write the equations showing that the current into the inductor is different in any way (amplitude or phase) from the current going out. Or, if that's too taxing, I'll choose some values for you. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: So, was that a yes or a no? I have trouble with your accent. It's a no. Lumped inductors are not conscious of anything including their locations. |
Cecil Moore wrote:
Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: |
Are you referring to something you emailed me? If so, I haven't received
it. I'll be glad to look at it when I do. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: Because you're seeing different currents at the two stub terminals, you must be modeling it with wires, which should reflect reality quite well. Look carefully at the currents along the stub and you'll find they're not equal and opposite on the two conductors. Such a radiating stub *is* very different from a coil. Instead of a knee-jerk defense of your ideas, why don't you actually take a look at the problem? Those stubs are vertical. EZNEC shows virtually zero vertically polarized radiation. According to EZNEC, those stubs are radiating a negligible amount, just like the lumped inductance. Why the 40% difference in current between the two configurations? Is this a characteristic of NEC? |
Roy Lewallen wrote:
So, in short, I don't disagree with Balanis. Yes, you have disagreed with an analysis based on Balanis's 'If' and 'Ib'. You even quoted some author saying that an antenna could not be analyzed in the manner that Balanis proposes. Cecil Moore wrote: He doesn't use lumped components and probably for good reason. But here's the quote that allows my analysis. "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents 'If' and 'Ib' in Figure 10.1(a)." This means that net total current equals If+Ib. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
To assume a short-circuited, sideways protuding stub line, intended to
replace a loading coil, will give the same results as the coil it replaces is unbelievable STUPIDITY. EZNEC will quite correctly treat the stub line as an essential part of the radiating system, itself much longer than the overall height of the short vertical antenna it is supposed to be loading. In fact, the length of wire in the stub line needed to resonate the antenna, plus the height of the antenna, will be of the same order as a full size quarter-wave vertical. Roy, you are wasting your time trying to educate such baffle-gabbing old wives. I make this comment because you are a much too polite gentleman (approaching the English variety) to send them packing yourself with tails between their legs. And I've just finished a glass of South African red. Yelp, yelp! Can you hear them fading into the distance? ;o) ;o) ;o) --- Reg. |
Roy Lewallen wrote:
So now we have a simple series circuit consisting of a generator, the loading inductor, and the R + L or C we used to substitute for the antenna. No we don't, Roy. You cannot be allowed, once again, to get away with turning a distributed network problem into a lumped circuit problem. If you can prove me wrong with a distributed network analysis, I will be the first to admit my mistake. If you cannot prove me wrong with a distributed circuit analysis, that's a big clue. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
David Robbins wrote:
Cecil Moore wrote: Roy Lewallen wrote: So what's the answer, Dave? Is there a current phase shift through a mobile antenna loading coil or not? Seems to me, if a coil can propagate the current phase in zero time, that is faster than light operation. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
I disagree with what you think Balanis says, not what he says. There's
quite a difference. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: So, in short, I don't disagree with Balanis. Yes, you have disagreed with an analysis based on Balanis's 'If' and 'Ib'. You even quoted some author saying that an antenna could not be analyzed in the manner that Balanis proposes. Cecil Moore wrote: He doesn't use lumped components and probably for good reason. But here's the quote that allows my analysis. "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents 'If' and 'Ib' in Figure 10.1(a)." This means that net total current equals If+Ib. |
Roy Lewallen wrote:
Are you referring to something you emailed me? If so, I haven't received it. I'll be glad to look at it when I do. Yep, I've emailed it twice to you. I had to email them three times to Yuri before he received it. Apparently, some of the new internet filters are deleting emails. Would you agree to cease the ad hominem attacks until you have actually viewed the problem? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
I'm sorry. If I'm bothering the readers, I'll be glad to bow out.
My postings aren't really directed to Cecil -- I know much better than to imagine that I'll ever change his mind, and I'm a firm believer in not wasting time on things I can't change. No, you and the other readers are really the audience, and the whole reason for the postings. If you and the other readers would rather I shut up, I'll be more than happy to spend my time at more productive pursuits. Just let me know. Roy Lewallen, W7EL David Robbins wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: Cecil Moore wrote: Roy Lewallen wrote: |
Woops, slipped through my fingers once again. My hat's off to the master.
Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: So now we have a simple series circuit consisting of a generator, the loading inductor, and the R + L or C we used to substitute for the antenna. No we don't, Roy. You cannot be allowed, once again, to get away with turning a distributed network problem into a lumped circuit problem. If you can prove me wrong with a distributed network analysis, I will be the first to admit my mistake. If you cannot prove me wrong with a distributed circuit analysis, that's a big clue. |
I'm sorry, I haven't received anything from you. If you'll send them
again, I'll be glad to take a look and to comment. Roy Lewallen, W7EL Cecil Moore wrote: The difference in current between the two configurations that I sent to you means the vertical stubs are radiating better than the horizontal antenna which is unlikely since EZNEC doesn't show any vertical radiation. |
Reg Edwards wrote:
In fact, the length of wire in the stub line needed to resonate the antenna, plus the height of the antenna, will be of the same order as a full size quarter-wave vertical. BS, Reg. I have a 102 foot tall, center-loaded vertical, with a ten foot long shorted horizontal stub in the middle to resonate it on 75m. Do you have EZNEC? I can send you a copy of the antenna file. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Roy Lewallen wrote:
I disagree with what you think Balanis says, not what he says. There's quite a difference. Balanis says one can analyze an antenna based on forward and reflected currents. You say that's not valid. Yes, you do disagree with Balanis. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
David Robbins wrote:
Cecil Moore wrote: Roy Lewallen wrote: Aww Dave! Cecil, Dave and a few others here serve as both education and somethimes entertainment. Go with the flow, learn something and when you get tired stop reading the thread. It's all good, man! |
It doesn't sound like a complicated model. If that's the case, it
wouldn't take long to enter it manually. So in EZNEC, click Outputs, then Show Description. Copy the description and paste it into a posting. After I make an EZNEC file out of it, I'll upload it to my web site so everyone with EZNEC can see what we're talking about. In general, I strongly prefer that people send me an .EZ file instead of a text description, both to save time and to reduce the probability of an error. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: Are you referring to something you emailed me? If so, I haven't received it. I'll be glad to look at it when I do. Yep, I've emailed it twice to you. I had to email them three times to Yuri before he received it. Apparently, some of the new internet filters are deleting emails. Would you agree to cease the ad hominem attacks until you have actually viewed the problem? |
Cecil and his confederates will soon be taking into account the difference
in current, and the forward and backward reflections, between the ends of the coil in a Pi-match network between the plate and 50-ohm output socket of the common or garden 807 linear power amplifier. The Pi-match network behaves as a 1/4-wave transmission line impedance matching transformer. Keep going Cec! And it has not yet been sorted out whether or not a congugate match exists. There's a long way still to go. ;o) ;o) --- Reg. |
On Mon, 03 Nov 2003 12:20:02 -0600, Cecil Moore
wrote: |Wes Stewart wrote: | On Mon, 03 Nov 2003 09:26:05 -0600, Cecil Moore | wrote: | | |Roy Lewallen wrote: | | If you could build an antenna from | | straight conductors and lumped inductors, the result would be very close | | to EZNEC's predictions. | | | |Hard to prove since lumped inductors are impossible in reality. Why | |does EZNEC show so much difference between lumped inductors and stub | |inductors? | | I see no such difference in my model. | |There shouldn't be a lot of difference. I have modeled two short dipoles, |one loaded with a lumped inductive reactance and one modeled with the |same reactance using an inductive stub. EZNEC reports the following: | | Inductance lumped j335 10'stub | |current in segment just before the coil .8374 amp .8384 amp | |current in segment just after the coil .7971 amp .5642 amp | |The relative difference just before the coil is quite small, 0.12%. | |The relative difference just after the coil is quite large, 41.28%. | |There just cannot be that amount of difference between a coil and a |stub. If you use the ideal transmission line model, there is *zero* difference between an ideal inductor and a transmission line stub. You are comparing a mess of wire with a ideal lumped inductor. Apples and oranges. If you really want to model this stuff accurately take a few hundred $K out of your next retirement check and buy a high frequency structure simulator. www.hfss.com |
Roy Lewallen wrote:
It doesn't sound like a complicated model. If that's the case, it wouldn't take long to enter it manually. There may be a bug in Netscape 7.1. The "reply all" function may not work right. I'll send you an email direct. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote:
Cecil and his confederates will soon be taking into account the difference in current, and the forward and backward reflections, between the ends of the coil in a Pi-match network between the plate and 50-ohm output socket of the common or garden 807 linear power amplifier. The Pi-match network behaves as a 1/4-wave transmission line impedance matching transformer. Keep going Cec! Reg, I cannot believe you would side with the people who claim there is zero delay through a p-net coil. Do you also believe in faster than light propagation of signals? If there is a delay in one foot of copper wire, don't you think it is reasonable to expect a delay in 20 feet of copper wire formed into a coil? Is the number of degrees on a Smith Chart between 0+j0 ohms and 0+j50 ohms just a figment of my imagination? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Wes Stewart wrote:
You are comparing a mess of wire with a ideal lumped inductor. Apples and oranges. Not my fault. Why does EZNEC treat these two inductances so differently? If you really want to model this stuff accurately take a few hundred $K out of your next retirement check and buy a high frequency structure simulator. No thanks, Wes. I can live without that. But let me ask you a question. If there is a one degree delay through one foot of copper wire, how can there be a zero delay through 20 feet of wire coiled into a one foot coil? Given the pressure on the electrons, one would expect 20 times the delay through the coil as through the one foot wire. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Mon, 03 Nov 2003 16:36:59 -0800, Roy Lewallen
wrote: I'm sorry. If I'm bothering the readers, I'll be glad to bow out. My postings aren't really directed to Cecil -- I know much better than to imagine that I'll ever change his mind, and I'm a firm believer in not wasting time on things I can't change. No, you and the other readers are really the audience, and the whole reason for the postings. If you and the other readers would rather I shut up, I'll be more than happy to spend my time at more productive pursuits. Just let me know. Roy Lewallen, W7EL Roy: You and a handfull others are on my "must read" list. Keep up the good work! Jack K8ZOA |
On Mon, 03 Nov 2003 19:55:33 -0600, Cecil Moore
wrote: |Wes Stewart wrote: | You are comparing a mess of wire with a ideal lumped inductor. Apples | and oranges. | |Not my fault. Why does EZNEC treat these two inductances so differently? Uh... maybe because they're different? | | If you really want to model this stuff accurately take a few hundred | $K out of your next retirement check and buy a high frequency | structure simulator. | |No thanks, Wes. I can live without that. But let me ask you a question. |If there is a one degree delay through one foot of copper wire, how |can there be a zero delay through 20 feet of wire coiled into a one |foot coil? Given the pressure on the electrons, one would expect |20 times the delay through the coil as through the one foot wire. When I was working my way to engineering I was a senior associate engineer working with a PhD who had tired of the politics in the university environment, given up his tenure, and come to work for Hughes. We were putting together a bench top setup to evaluate something or the other in the mixers from the early version of the Phoenix missile. The front end of this thing, from the waveguide slot planar antenna back the the mixer inputs, was all waveguide. Whatever we were doing required the use of a "magic" T. I, the ever inquisitive student, asked; "Doc, how does a magic T work?" Doc, former professor and the author of "Intermediate Mathematics of Electromagnetics", replied, "It's magic." |
I'll choose this one to respond to, since it talks directly about the
models. Cecil has been kind enough to provide me with the models, and given me permission to post them on my web site. You can download them from ftp://eznec.com/pub/ as Cecils_Models.zip, and run them with the standard version of EZNEC. If you only have the demo version, a reduced-segment model of the verticals at least should work well enough to illustrate the subject matter. I looked particularly at the two models of a vertical, since they're the simplest. They're both 51 feet high, on 75 meters. One has a single lumped "load" of +j335 ohms between 25 and 26 feet from the ground, and the other has a one-foot-spaced horizontal stub protruding horizontally from the same point. One point that seems to be drawing attention, if not to say some creative theories, is that the current at the bottom terminal of the stub isn't equal to the current at the top terminal. They are, EZNEC reports, 0.846 and 0.581 amps respectively (at the stub end segments). EZNEC shows them to have very little phase shift along the stub, and very nearly 180 degrees out of phase on the two wires. (People looking at the model should be aware that stub wires 2 and 4 are defined with end 1 of one opposite end 2 of the other. So the phase angles reported by EZNEC are referenced in opposite physical directions. EZNEC reports the phase angles of the currents on the two wires as being nearly identical. So that means that the currents are flowing in phase in opposite directions -- or nearly exactly out of phase if you define positive as the same direction for both wires.) The fact that the currents at the stub terminals aren't the same means that there can be no doubt that the stub is radiating. The difference constitutes a common mode current. Because the currents are almost exactly out of phase, we can simply subtract them to find the common mode current. At the antenna end of the stub, it's about 0.27 amp. At the output end, it's zero (EZNEC reports a 0.03 amp difference for the segments nearest the short). Taking a simple-minded average, we can say it's very roughly 0.15 amp. This is the equivalent single-wire common mode current. That is, it will radiate as though that amount of current were flowing on a single conductor of the same length. Field strength is proportional to the current flowing on a conductor, and the length of the conductor. It's not at all valid (using this sort of analysis at least) to apportion radiation to being so much from this part of the antenna or so much from that. For example, the field from one part of the antenna can interfere with the field from others, resulting in little or no contribution from those sections in certain directions, or maybe in nearly all directions. But to get an idea of the potential radiation from the stub, we can look at the 0.15 amp approximate average current flowing along the ten foot stub, and compare it to the roughly 0.5 amp average over 51 feet for the vertical itself. From that, we see it probably won't be a big contributor to the total field. But that's not at all a criterion for imbalancing the stub current -- which does affect the feedpoint impedance and potentially the pattern. In fact, the stub can cause more disturbance by modifying the current in the main radiator than by its own radiation. That's definitely true in something like a collinear with phasing stubs. The stub common mode current (that it, the imbalance between currents at the stub terminals) is due to mutual coupling between the stub conductors and the vertical portion of the antenna. It shouldn't be a surprise, and it doesn't require any new theory, reflected currents or powers, or hocus-pocus to explain. It's exactly the same phenomenon that induces current in a Yagi parasitic element, and countless other familiar everyday examples. And EZNEC does the calculation exactly the same (from very basic principles) for all conductors -- it doesn't know or care if you regard some of them as being a "stub". I'll make a prediction here without having actually tried it. So here's a chance to show just how full of BS I am. Convert the model to a dipole of double the length, in free space, but with a stub on only one side. Move the stub inward toward the center. As you do, I predict that the currents will become more and more balanced. That is, the currents on the two terminals of the stub should become more and more equal. Why? Because as you get it closer to the center, the mutual coupling from the two halves of the antenna to the stub becomes more equal. Exactly at the antenna center, they cancel out. At that point, you can replace the stub with a lumped inductor and find no change. I restricted this to one stub because if there were two, coupling from one stub to the other would create imbalance even near the center. To answer an earlier question of whether you should expect a ten foot stub to behave like a lumped inductor, the answer is, of course not. At least not if it's in the field of other current-carrying conductors. What's the mystery? What's the big deal? Roy Lewallen, W7EL Cecil Moore wrote: . . . Roy suggested the stubs might be radiating. EZNEC says they are not. You can add two inches of vertical wire to the ends of the antenna and see the red vertical radiation. The stubs are 0.04WL, #14 wire, and 6 inch spacing. How much could they radiate on 75m? The difference in current is not due to radiation. It is due to the phase shift between Ifwd and Iref through the stub. |
Here's an experiment to try.
Take Cecil's model of the vertical with the loading coil. Add a single horizontal wire, 10 feet long, connected at the top of the loading coil. That is, make the new wire go from 0, 0, 26 to 10, 0, 26. Notice how much current there is in the horizontal wire. Notice how much different the current is in the vertical below the wire compared to above the wire. Look familiar? We can work our magic without either an inductor or a stub. Roy Lewallen, W7EL |
| All times are GMT +1. The time now is 02:34 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com