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#1
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Cecil Moore wrote:
Roger wrote: Cecil Moore wrote: Where did that current come from if current cannot flow into the stub? Stored in the 1/4 WL between the short and mouth. No more current needed once stability is reached. EM RF current is stored in the stub? In what form? Come on Cecil! Let's not go around in circles! You know very well how it happens. On the remote chance that you are serious, I suggest you read CAREFULLY my other postings. If you want even more information, read your own postings from the past. For my part, I have learned from you and your examples. For better or worse, I feel much more comfortable in my knowledge base and ability to communicate to others. As to the validity of my postings, each reader will need to decide for himself, just as the reader must do for your postings. 73, Roger, W7WKB |
#2
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Roger wrote:
Cecil Moore wrote: Roger wrote: Stored in the 1/4 WL between the short and mouth. No more current needed once stability is reached. EM RF current is stored in the stub? In what form? Come on Cecil! Let's not go around in circles! You know very well how it happens. Here's an example using a circulator and load in a 50 ohm system. Please think about it. SGCL---1---2------------------------------+ \ / | 1/4 3 | WL | everything is 50 ohms | shorted R | stub Are there any reflections at point '+'? If not, how is energy stored in the stub? If so, what causes those reflections? -- 73, Cecil http://www.w5dxp.com |
#3
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Cecil Moore wrote:
Roger wrote: Cecil Moore wrote: Roger wrote: Stored in the 1/4 WL between the short and mouth. No more current needed once stability is reached. EM RF current is stored in the stub? In what form? Come on Cecil! Let's not go around in circles! You know very well how it happens. Here's an example using a circulator and load in a 50 ohm system. Please think about it. SGCL---1---2------------------------------+ \ / | 1/4 3 | WL | everything is 50 ohms | shorted R | stub Are there any reflections at point '+'? If not, how is energy stored in the stub? If so, what causes those reflections? I am not sufficiently familiar with circulators to respond. My present level of understanding is that they can only be built using ferrite inductors which have an ansiotropic (non-linear) magnetic response. If so, they could not be compared to transmission lines without adding that non-linear factor. Apparently energy is stored in these inductors only if the power is moving in one direction, so it never reaches one branch. I don't understand how a ferrite could do that. Is there such a thing as a "all transmission line" circulator? If so, where could I find the circuit? Thanks, 73, Roger, W7WKB |
#4
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Roger wrote:
Cecil Moore wrote: Are there any reflections at point '+'? If not, how is energy stored in the stub? If so, what causes those reflections? I am not sufficiently familiar with circulators to respond. If the circulator is bothering you, forget it and assume the following lossless conditions: Ifor = 1 amp -- ------------------------------+ -- Iref = 1 amp | 1/4 | WL All Z0 = 50 ohms | shorted | stub Please think about it and answer the questions above. The main point to remember is that there is no physical impedance discontinuity at '+'. -- 73, Cecil http://www.w5dxp.com |
#5
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Cecil Moore wrote:
Roger wrote: Cecil Moore wrote: Are there any reflections at point '+'? If not, how is energy stored in the stub? If so, what causes those reflections? I am not sufficiently familiar with circulators to respond. If the circulator is bothering you, forget it and assume the following lossless conditions: Ifor = 1 amp -- ------------------------------+ -- Iref = 1 amp | 1/4 | WL All Z0 = 50 ohms | shorted | stub Please think about it and answer the questions above. The main point to remember is that there is no physical impedance discontinuity at '+'. OK. Let's begin by recognizing that this circuit is identical to a straight transmission line. The purpose of identifying the stub is to clearly locate the point 1/4 wavelength from the end of the line. The line is shorted at the end. We further assume that the peak current is 1 amp. Are there reflections at point "+"? Traveling waves going in opposite directions must pass here, therefore they must either pass through one another, or reflect off one another. Is it important to decide this issue? Yes, if it will affect the answer to questions such as what is the voltage or current at this point. Will it affect the answers? No. Under the conditions described, the waves passing in opposite directions will have equal voltages and opposite currents. If they pass through one another, the voltages will add, but the currents will subtract. If they reflect, the voltage of each component (Vf and Vr) will add on itself, and the individual currents will reverse on themselves and therefore subtract. Either way, the total voltage will double, and total measured current would be zero. There is no reason to decide the issue. How is energy stored in the stub? We have defined current as entering an leaving the stub. Current is thought of as movement of charged particles, but not as a concentration of particles. A concentration of charged particles exhibits voltage. Energy is present when EITHER current or voltage are shown to be present. Here, current is defined as one amp so energy must be present some place on the line. The stub is 1/4 wavelength long physically, but it is 1/2 wavelength long electrically, so that if we have energy present in the time-distance shape of a sine wave, we would have an entire 1/2 wave's worth of energy present on the stub at all times. The location of peak voltage (or peak current) will depend upon the time-distance reference used to describe the moving wave. (We would have equal voltage(but opposite polarity) peaks located at the point {+} if we assumed the center of the forward and reflected wave each to located 90 degrees from the shorted end.) The circuit shows forward current Ifor and reflected current Iref as if each were only one current. When we consider traveling waves, we need to remember that Ifor and Iref can be measured on either of the two wires composing a transmission line. The forward wave exists on both wires, but the sides display opposite polatity and direction of current despite both moving in the same direction. It is best to consider the forward traveling wave as two waves, each carrying half the power, with one wave per wire. Does this match your own concept of the traveling waves acting at the {+} point Cecil? If not, where do we differ? 73, Roger, W7WKB Is this the kind of answer you were looking for? The answer could be given mathematically but that might be even more confusing. |
#6
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Roger wrote:
Are there reflections at point "+"? Traveling waves going in opposite directions must pass here, therefore they must either pass through one another, or reflect off one another. In the absence of a real physical impedance discontinuity, they cannot "reflect off one another". In a constant Z0 transmission line, reflections can only occur at the ends of the line and only then at an impedance discontinuity. Does this match your own concept of the traveling waves acting at the {+} point Cecil? If not, where do we differ? Where we differ is that you allow traveling waves to "reflect off one another". There are no laws of physics which allow that in the absence of a physical impedance discontinuity. EM waves simply do not bounce off each other. -- 73, Cecil http://www.w5dxp.com |
#7
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On Dec 29, 2:31*pm, Cecil Moore wrote:
Roger wrote: Are there reflections at point "+"? *Traveling waves going in opposite directions must pass here, therefore they must either pass through one another, or reflect off one another. In the absence of a real physical impedance discontinuity, they cannot "reflect off one another". In a constant Z0 transmission line, reflections can only occur at the ends of the line and only then at an impedance discontinuity. Roger: an astute observation. And Cecil thinks he has the ONLY answer. Allow me to provide an alternative. Many years ago, when I first encountered this news group and started really learning about transmission lines, I found it useful to consider not only sinusoidallly excited transmission lines, but also pulse excitation. It sometimes helps remove some of the confusion and clarify the thinking. So for this example, I will use pulses. Consider a 50 ohm transmission line that is 4 seconds long with a pulse generator at one end and a 50 ohm resistor at the other. The pulse generator generates a single 1 second pulse of 50 volts into the line. Before and after the pulse its output voltage is 0. While generating the pulse, 1 amp (1 coulomb/s) is being put into the line, so the generator is providing 50 watts to the line. After one second the pulse is completely in the line. The pulse is one second long, contains 1 coulomb of charge and 50 joules of energy. It is 50 volts with 1 amp: 50 watts. Let's examine the midpoint (2 second) on the line. At two seconds the leading edge of the pulse arrives at the midpoint. The voltage rises to 50 volts and the current becomes 1 amp. One second later, the voltage drops back to 0, as does the current. The charge and the energy have completely passed the midpoint. When the pulse reaches the end of the line, 50 joules are dissipated in the terminating resistor. Notice a key point about this description. It is completely in terms of charge. There is not a single mention of EM waves, travelling or otherwise. Now we expand the experiment by placing a pulse generator at each end of the line and triggering them to each generate a 50V one second pulse at the same time. So after one second a pulse has completely entered each end of the line and these pulse are racing towards each other at the speed of light (in the line). In another second these pulses will collide at the middle of the line. What will happen? Recall one of the basics about charge: like charge repel. So it is no surprise that these two pulses of charge bounce off each and head back from where they came. At the center of the line, for one second the voltage is 100 V (50 V from each pulse), while the current is always zero. No charge crossed the mid-point. No energy crossed the mid-point (how could it if the current is always zero (i.e. no charge moves) at the mid-point. It is a minor extension to have this model deal with sinusoidal excitation. What happens when these pulses arrive back at the generator? This depends on generator output impedance. If it is 50 ohms (i.e. equal to Z0), then there is no reflection and 1 joule is dissipated in each generator. Other values of impedance result in more complicated behaviour. So do the travelling waves "reflect" off each other? Save the term "reflect" for those cases where there is an impedance discontinuity and use "bounce" for those cases where no energy is crossing a point and even Cecil may be happy. But bounce it does. ...Keith |
#8
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Cecil Moore wrote:
Roger wrote: Are there reflections at point "+"? Traveling waves going in opposite directions must pass here, therefore they must either pass through one another, or reflect off one another. In the absence of a real physical impedance discontinuity, they cannot "reflect off one another". In a constant Z0 transmission line, reflections can only occur at the ends of the line and only then at an impedance discontinuity. Cecil, this sounds more like a pronouncement from God than like an conclusion from observations. Does this match your own concept of the traveling waves acting at the {+} point Cecil? If not, where do we differ? Where we differ is that you allow traveling waves to "reflect off one another". There are no laws of physics which allow that in the absence of a physical impedance discontinuity. EM waves simply do not bounce off each other. I am not aware of any laws of physics that prevent it either. I don't see any evidence that it happens in open space, like light bouncing off light. It might happen on transmission lines however. I just cannot find any convincing evidence either way. What I have deduced so far indicates that it makes no difference which happens. Maybe both things happen (both reflect and pass). This because the EM field travels very close to the speed of light. It is a little hard to see how one wave could "see" the other coming. On the other hand, the charges move slowly, far below the speed of light. It is easy to see how they might "see or feel" each other coming. 73, Roger, W7WKB |
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