E&M Question

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PsiStar

Golden Member
Dec 21, 2005
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TecHNooB ... use search for "bounce diagrams" also. I didn't fine one that I liked but I liked the combination of several ... as if that is not confusing.

Bounce diagrams show how a step function generator's signal can be additive & subtractive as a signal bounces between ends of mismatched terminations of a transmission line.

A step function is like a DC voltage source being rapidly turned on. Rapid being fractions of a second ... picoseconds from 0 V to the on voltage, for instance.
 

PsiStar

Golden Member
Dec 21, 2005
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Comments in red. I am not going to pretend to provide the most comprehensible description in a couple of posts, but this is pretty on the money.

I have also been trying to meticulously stick to using the full terms to avoid confusion and ambiguity without writing a book. If I knew that you knew ... we would save a lot of words. I also threw in a couple more terms which you might have been dancing around accidently; they are real effects and you are close to "knowing".

Okay, I think this is what I'm trying to say. Say you have one co-ax cable. You turn on the generator, and the wave begins to propagate down the cable.this *is* time transient As the wavefront moves, the capacitance along the cable gets charged little by little. Imagine a bajillion capacitors in parallel along the cable.right on If there is no resistance, they each get charged up as the wave propagates across them. If there is resistance, the rate at which all the little caps get charged isn't dependent only on the speed of the wave.well, ok. But don't confuse this resistance to Ohmic resistance. This increased in charge time would be due to the dielectric constant slowing the propagation of the wave... which I think is what you are saying. So now as you charge the caps further and further down the line, your current decreaseshaha, you are right, but I wonder if for the wrong reason? The instaneous current at any particular location will be less because the wave is spreading out. This is known as dispersion & can only occur with lossy line. and so does the rate of charge. THAT is effect of the dielectric constant In fact, the rate of charge may not even keep up with the speed that the wave propagates oops! That is where your analogy just fell apart. The sequence of Ls & Cs is the model of the tx line. The wave ... is *the* wave which is what is doing all of the charging.and you may end up charging several caps in parallel rather than just the one nearest the wavefront. As the length over which the wave occupies increases, so does the resistance.the resistance doesn't change ... in any of your usages ... assuming a uniform tx line So as the wave moves across the line, your overall current is dropping. This causes a change in the circular H-field. Would this not cause transmission all over the place?This would be true if the wave could get ahead of itself. haha I'll throw another term at you, "causal" or causality. Which simply defined means that the coax line cannot anticipate what is coming down the tube.

I realize this is not reflection, but my real question was actually the mechanism which alters the field such that it reflects the currenta reflection is only due to a change in impedance be it the end of the tx line is not perfectly terminated or along the line due to defect or intent that should be in the line. I thought some kind of reflection was the source of the field correction. Cuz ultimately, I should be able to apply ampere's law around the cable and find the current.you can on the forward wave and include the reflected wave ... if it exists But with there being no reflection, it almost didn't seem like the field was changing. I don't even want to think about the transient behavior of the field in the conductor itself :(that statement is why my 1st comment was that this is transient behavior