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Cecil Moore wrote:
Tom Donaly wrote: There are lots of ways to make inductors, (coils, transmission lines, meander lines, etc.) but there is only one inductance. Have you ever seen the equivalent circuit of a transmission line presented with "only one inductance"? ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- I don't hook equivalent circuits up to my antennas, Cecil, and I'm surprised that you do. Most of the transmission lines I've ever dealt with had one inductance which the theorists wouldn't think was as important as the inductance per unit length. Besides, you missed the point again. 73, Tom Donaly, KA6RUH |
Richard Clark wrote:
On Wed, 20 Oct 2004 04:49:52 GMT, " wrote: Actually I modelled the coil to real world dimensions with a NEC program with copious segments that supplied current levels at multiple points around each turn to get my answers which this thread now suggests that NEC answers could me incorrect thus I am following the thread but not partaking in it. Hi Art, Read the thread where it is correctly described and stick with a winner. After all, the difference between the point inductor, and the helical one offers barely half a dB difference in the outcome. No one here could possibly measure that spread accurately (about the quarter of the width of an S-Meter's needle). 73's Richard Clark, KB7QHC Hi Richard, you've hit the nail squarely on the head. The validity of the whole argument boils down to whether or not you can safely neglect the effects of the physical dimensions of the inductor on the behavior of the antenna. It looks to me as if you can, but some of the other fellows on this newsgroup seem to be as much interested in characterizing Tom Rauch as a rat as they are in verifying some antenna effects due to the properties of real loading coils. 73, Tom Donaly, KA6RUH |
A good way to check the validity of an EZNEC helix model is to create
the helix by itself, run a wire from one end of the helix to the other right through the middle of the helix, and put a source in the middle of the wire. Specify a low enough frequency that the helix will be small in terms of wavelength. I've found the source reactance to compare quite favorably with the reactance of the inductance calculated by Reg's program for the same physical dimensions. The self resonant frequency comes out quite close, also. The Q should be in the same ballpark, provided wire loss is included in the model, although radiation will lower it some in the EZNEC model. (The source resistance with wire loss set to zero is the radiation resistance. As long as it's much lower than the resistance with wire loss included, the effect of radiation will be small. If it's not much lower, reduce the frequency.) As I mentioned before, EZNEC doesn't model proximity effect (significant only when the turns are pretty closely spaced) but I don't think Reg's program includes proximity effect, either. Roy Lewallen, W7EL |
Richard Harrison wrote:
Tom Donaly wrote: "You and Richard need a refresher course in electromagnetics." This Richard agrees a refresher course could help. I no longer read the Proceedings of the IRE for fun. It`s been over 50 years since I last darkened the halls of academia. I remember a few things. One hangup I see here is the rule that the current in a series circuit is always the same everywhere. True for circuits small in terms of wavelength. False for unmatched (not terminated in Zo) circuits of a size significant in terms of wavelength. The standing-wave antennas of this thread are less than 1/4 wavelength, but they`re still significant in terms of wavelength with or without loading coils. They are open-circuited and make a big reflection from their open-circuited ends. This reflection causes a current which varies from zero at the open-circuit end to something substantial back a ways from the open-circuit. Between substantial and zero is a current drop. My favorite author. F.E. Terman depicts this current distribution in a dipole (two 1/4-wave antennas back to back) in Fig. 23-2 on page 866 of his 1955 edition. For a whip worked against ground, the current distribution is either half of the dipole representation. Look at Fig.23-2. Anyone can see the current drops to zero at the antenna tips. Loading coils won`t change that. Coils added to bring total antenna inductance to resonance with its capacitance occupy space. Current through a loading coil takes time to traverse the coil. By the time current has made the trip through the coil, current arriving from the source is out of phase to some extent with that arriving through the coil. The delays in transit to both ends of the coil are likely not equal. The inequality in phase results in a difference in volts, amps, and impedance at the two coil ends. Recall, we are discussing r-f, traveling as a wave from both terminals of a generator. It is not d-c emerging from one battery terminal and entering another. Cecil is exactly correct in his characterization of how waves supeerpose to produce standing-wave variations in voltage, current, and impedance in the total series circuit. Proof has been offered by modeling, and measurement. You may accept or reject the observations of others. You could also make your own. Best regards, Richard Harrison, KB5WZI Hi Richard, current is the same everywhere in a series circuit only when you can neglect the length of the components. Network theory is supposed to be an abstraction that is close to being accurate only at low frequencies and short dimensions. So, I agree with you. I don't agree with the term "current drop" because, even in a transmission line, current, or more properly, current density, doesn't act like a potential of any sort to which you could ascribe a "drop." You probably think this is nitpicking. I don't think it is, any more than Yuri having a fit over how much the shape of the current is changed in a short, inefficient antenna by a slight change in where the current bends, ie whether it's at the beginning or end of the loading coil. 73, Tom Donaly, KA6RUH |
Jim Kelley wrote:
Tom Donaly wrote: Next, Cecil, you're going to be talking about a "current gradient" and a "scalar current field." Here's a question for you, Cecil, and Richard Harrison, and Yuri, too: how do you take the gradient of the current at a point on a transmission line, and, if were possible to do so, what is the physical significance of the result? 73, Tom Donaly, KA6RUH The standing wave current profile along, for example, a quarter wave radiator is a cosine function. The gradient then would be the derivative of the cosine function which is a -sine function. 73, ac6xg Jim, current, in a wire, is the total current density integrated across a cross section of the wire. It's a vector, as is the current density. Now tell me, how do you take the gradient of a vector? David K. Cheng, in his book Field and Wave Electromagnetics, defines the gradient operation this way: "We define the vector that represents both the magnitude and the direction of the maximum space rate of increase of a scalar as the gradient of that scalar." He wrote "scalar," not "vector," Jim. You and the rest of the boys are acting as if current had magnitude but no direction, whereas it has both. 73, Tom Donaly, KA6RUH |
Cecil Moore wrote:
Jim Kelley wrote: The standing wave current profile along, for example, a quarter wave radiator is a cosine function. The gradient then would be the derivative of the cosine function which is a -sine function. Yep, the feedpoint is at a current loop (max). The open end of the quarter wave radiator is obviously at a current node (min). There are electrically 90 degrees of signal between the current loop and the current node on a standing-wave antenna or on a transmission line with standing waves. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- You're both wrong for reasons I've given in another post. 73, Tom Donaly, KA6RUH |
Tom Donaly wrote: Jim Kelley wrote: Tom Donaly wrote: Next, Cecil, you're going to be talking about a "current gradient" and a "scalar current field." Here's a question for you, Cecil, and Richard Harrison, and Yuri, too: how do you take the gradient of the current at a point on a transmission line, and, if were possible to do so, what is the physical significance of the result? 73, Tom Donaly, KA6RUH The standing wave current profile along, for example, a quarter wave radiator is a cosine function. The gradient then would be the derivative of the cosine function which is a -sine function. 73, ac6xg Jim, current, in a wire, is the total current density integrated across a cross section of the wire. It's a vector, as is the current density. Now tell me, how do you take the gradient of a vector? David K. Cheng, in his book Field and Wave Electromagnetics, defines the gradient operation this way: "We define the vector that represents both the magnitude and the direction of the maximum space rate of increase of a scalar as the gradient of that scalar." He wrote "scalar," not "vector," Jim. You and the rest of the boys are acting as if current had magnitude but no direction, whereas it has both. 73, Tom Donaly, KA6RUH Not sure why you don't like gradients, Tom. I'm sure Mr. Cheng is undoubtedly correct, but I'm just as sure he didn't intend that sentence as any sort of definition of the term "gradient". That's something you have apparently read into it. The gradient in our case (since you proposed the question) would be expressed as the superposition of forward and reverse currents, with magnitude and phase (or direction if you prefer) written as a function of either position or angle *along* the radiator. It's nothing fancy. Honest. It's simply the rate of change of current as a function of position. The gradient across the radiator at any given point along the radiator could then be determined using some additional parameters - if someone were really that interested in it (which I'm not). 73, ac6xg |
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EZNEC doesn't model proximity effect (significant only when the
turns are pretty closely spaced) but I don't think Reg's program includes proximity effect, either. Roy Lewallen, W7EL ====================================== Yes it does! But you can forget it. It doesn't matter except when calculating efficiency. It has no affect on how the thing works which is what you are all so-aggressively fighting about. You'll soon be using assault weapons. Program "Loadcoil" also includes the ALL-IMPORTANT COIL CAPACITANCE (which I suspect Eznec does not - I never use it) - the existence of which the whole set of you block-heads, so-called electrical engineers, appear to be entirely ignorant. We ARE dealing with alternating currents. Oh Boy - I enjoyed typing that! ;o) ---- Reg, G4FGQ |
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