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#261
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My challenge to you was rhetorical. Based on past experience, I had no
real expectation that you'd be able to actually calculate a current. Our educations differ a great deal. Mine enabled me to give a numerical prediction, which as anyone who has read my earlier postings, is 1. Yours has evidently not prepared you to meet this onerous challenge. Does anyone else feel up to the task of calculating the currents in a simple circuit? It used to be that you'd have to be able to do this to get a first phone license, or probably an amateur extra. Now, it appears that even American engineering education isn't always up to the task. Roy Lewallen, W7EL Cecil Moore wrote: . . . Balanis would be surprised to know that you consider the material that he teaches in his classes at ASU to be pseudo-analysis. Some of the stuff I have posted is in Balanis' book, _Antenna_Theory_ which you haven't read. In particular, he says: "Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forwards and backwards) as represented by traveling wave currents If and Ib in Figure 10.1(a)." . . . |
#262
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Roy Lewallen wrote:
My challenge to you was rhetorical. Based on past experience, I had no real expectation that you'd be able to actually calculate a current. Here's how to mask the effects that we have been discussing: 1. Choose a small inductance that replaces a very small number of degrees of the antenna. 2. Use a ferrite coil designed to minimize distributed effects. 3. Mount the coil at a place in the antenna where the slope of the current is virtually zero. That's what you have done. Here's how to showcase the effects that we have been discussing: 1. Chose a large inductance that replaces an appreciable number of degrees of the antenna. 2. Use a typical air-core loading coil like a bugcatcher that has appreciable distributed effects. 3. Mount the coil at the center of the antenna where the slope of the current curve is near maximum. When you perform your experiment with an 8 foot center-loaded bugcatcher on 75m, then you will be taken seriously. -- 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! =----- |
#263
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W5DXP wrote to W7EL:
Question: Why didn't you use a 75m bugcatcher coil for the experiment? And why did you put toroid at the base? We are talking about loading coils that are installed at half to 2/3 up the radiator. Try 12 foot radiator with loading inductor at ~65% up from the feedpoint. The results should be "magnified". Our theory is that the current drop across the inductor should be roughly proportional to the current in the radiator (in degrees) that it replaces (Cosine law). Judging by description, I would guess that there wasn't much difference. Put that coil up but don't use scope probes, they will detune the antenna, no wires, use thermocouple meters. Probes at the base they probably do not distort the measurements much. Nice disertation on engineers and modeling. The only small problem is what and how you model. If your modeling uses 0 size inductance and real measurement shows something else, maybe there is a reason to question modeling how well it reflects reality. I went to university first, I designed parts that human life depended on, and I would think twice about relying on some numbers without testing and verifying it. Space shuttle tiles modeled OK? Yuri |
#264
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Yuri Blanarovich wrote:
Judging by description, I would guess that there wasn't much difference. The feedpoint of the radiator alone is 35-j185. The impedance of the loading toroid is 0.6+j193. Assuming perfect predictability, that gives the antenna system a feedpoint impedance of 35.6+j8, i.e. it is *longer* than resonant. That moves the current maximum point inside the toroid making the current in and out even closer to equal. If a coil is installed at a current maximum point or a current minimum point, the current in and out will be the same. If a coil is installed at a place where the slope of the current envelope is positive, the current will actually increase through the coil. -- 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! =----- |
#265
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Roy Lewallen wrote:
Our educations differ a great deal. Mine enabled me to give a numerical prediction, which as anyone who has read my earlier postings, is 1. Yours has evidently not prepared you to meet this onerous challenge. Roy, I have repeated a statement three or four times earlier on this newsgroup. My statement predicts a result of 1. Here is that statement again: "If a loading coil is placed at a current maximum point, the current in and out of the coil will be equal." I have been assuming that is why your coil was placed at the current maximum point, to ensure that the currents would be equal. Depending upon where the coil is placed, the currents in and out of the coil can be equal, greater than, or less than. -- 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! =----- |
#266
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Roy, W7EL wrote:
"And, Richard (Harrison), who said something like "an inductor without phase shift is like"...I don`t recall ; hot dog without ketchup or something." My analogy may not have been apt, but fact is that you don`t have an inductor without phase shift. The current lags the voltage in an inductor. My dictionary says that phase is a particular stage or point of adbvancement in a cycle; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Apply a voltage or the voltage across the inductance. Current does not change instantaneously in an inductance, but it lags the imposed voltage change. Lag is to move slowly or fall behind. In a circuit containing resistance and inductance, almost all real world circuits, current lags the voltage. This is phase shift by definition. We correct power factor to overcome phase lag and to eliminate the excess current and loss from the inductive charging and discharging current of an inductive circuit. Reactance only stores energy and does no useful work. I reiterate the accuracy of my postings in this thread, and indeed, inductance and phase shift are inseparable. Please note that inductance can be neutralized with capacitance. Best regards, Richard Harrison, KB5WZI |
#267
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Yuri wrote,
Our theory is that the current drop across the inductor should be roughly proportional to the current in the radiator (in degrees) that it replaces (Cosine law). That's a pretty good theory, Yuri. I'd like to know where you got this "Cosine law" you keep talking about. I can't seem to find mention of any such _law_ anywhere but on this newsgroup. Does that mean I should throw away my method of moments software because I don't need it any more? And what is a current drop? I've heard of voltage drops and cough drops but never current drops. Finally, how do you measure the "current in the radiator (in degrees)?" Why not use amperes like everyone else? I won't believe your theory, Yuri, until you and Cecil take the time to present it in terms of field theory. Since you guys have taken EM classes in college you should have no trouble doing this, right? 73, Tom Donaly, KA6RUH |
#268
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Tdonaly wrote:
And what is a current drop? I've heard of voltage drops and cough drops but never current drops. It's is the decrease in current due to the attenuation (alpha) factor in equation 1.22 (2) in Ramo, Whinnery, & Van Duzer. It's all covered in any distributed networks course. According to Balanis, antennas have an attenuation factor due to radiation and is similar to (slightly more complicated than) this familiar transmission line equation for lossy lines. I = Im(e^-az)d^j(wt-bz) where a is alpha, w is omega (2*pi*f), and b is beta (phase factor). I won't believe your theory, Yuri, until you and Cecil take the time to present it in terms of field theory. Since you guys have taken EM classes in college you should have no trouble doing this, right? Please reference Chapter 1 of _Fields_and_Waves_... by Ramo, Whinnery, and Van Duzer. Start with equations 1.18 (4)&(5) and 1.22 (1) & (2). Also _Antenna_Theory_ by Balanis, equations 4-81 and 10-1 and one other that I cannot locate right now. :-) The one I cannot locate is the simplified one for a 1/2WL dipole. -- 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! =----- |
#269
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I won't believe your theory, Yuri, until you and Cecil take the time to present it in terms of field theory. Since you guys have taken EM classes in college you should have no trouble doing this, right? Please reference Chapter 1 of _Fields_and_Waves_... by Ramo, Whinnery, and Van Duzer. Start with equations 1.18 (4)&(5) and 1.22 (1) & (2). Also _Antenna_Theory_ by Balanis, equations 4-81 and 10-1 and one other that I cannot locate right now. :-) The one I cannot locate is the simplified one for a 1/2WL dipole. -- 73, Cecil http://www.qsl.net/w5dxp I don't have Ramo et al's book, but I do have Balanis' book. I think anyone who wants to understand equation 4-81 should read the whole section: 4.5.6 where he makes it clear these equations are approximations that are pretty good under some circumstances and lousy under others. The standards he judges them on are the techniques of Integral Equations and Moment Method which he explains in another part of the book. He doesn't say a single thing about a "cosine law" for a real antenna, as Yuri does. I think I'll keep my EZNEC. 73, Tom Donaly, KA6RUH (P.S. I looked, in Balanis, for a section on inductively loaded antennas and couldn't find one. That doesn't mean it doesn't exist. If anyone knows where to look in that book for information on such I'd be obliged for the information.) |
#270
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Tdonaly wrote:
He doesn't say a single thing about a "cosine law" for a real antenna, ... I didn't say a single thing about a "cosine law" either. If anyone knows where to look in that book for information on such I'd be obliged for the information.) Information on inductively loaded antennas seems to be sadly lacking. But I have found one in _Antennas_ by Kraus & Marhefka, third edition. I trust that will be a good enough reference for everyone. On page 823 under "23-13 TRAPS", what he says about traps is not relevant. But what he says about traps on half their resonant frequency is absolutely choice. "At frequency F1, for which the dipole is 1/2WL long, the traps introduce some inductance so that the resonant length of the dipole is reduced." On the next page, the current distribution is shown for the trapped dipole on 1/2 the trap's resonant frequency. Needless to say, it clearly shows a current drop through the inductive trap. And talking about phasing using coils: "A coil can also act as a 180 degree phase shifter as in the collinear array of 4 in-phase 1/2WL elements in Fig. 23-21b. ... THE COIL MAY ALSO BE THOUGHT OF AS A COILED-UP 1/2WL ELEMENT." Emphasis mine. Now you guys can stop pulling our legs and confess that it was all a joke. -- 73, Cecil http://www.qsl.net/w5dxp "One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike ..." Albert Einstein -----= 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! =----- |
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