Cecil Moore wrote: For that statement, the length of the feedline is unknown and the load is unknown. I was commenting on this, the subject of our conversation: "Consider the following: Source---50 ohm feedline---+---1/2WL 150 ohm---50 ohm load" As I was saying, for the two boundaries as a network, and we call rho at the first boundary r12 and rho at the second boundary r23 then rho(network) = (r12 + r23)/(1 + r12*r23) = 0. Note that if we use your value for r12, the network generates a reflection. I note the utility of negative rho in this example. Seems to me, rho cannot both cause a voltage and be caused by a (voltage divided by a current) which is an impedance upon which rho is dependent. Voltages on a transmission line do not determine reflection coefficients. Reflection coefficients are determined by characteristic impedances, not virtual ones. 73, Jim AC6XG |
Jim Kelley wrote:
. . . Voltages on a transmission line do not determine reflection coefficients. Reflection coefficients are determined by characteristic impedances, not virtual ones. 73, Jim AC6XG I disagree with this. When applied to transmission lines, the (voltage) reflection coefficient is, as far as I can tell, universally defined as the ratio of reflected to forward voltage to reverse voltage at a point. So a reflection coefficient can be, and often is, calculated for every point along a line, not just at discontinuities or points of actual reflection. This can be done with nothing more than the knowledge of the values of forward and reflected voltages at the point of calculation. As it turns out, the value of the reflection coefficient at any point will be equal to (Z - Z0) / (Z + Z0), where Z is the impedance seen looking down the line toward the load at the point of calculation. I'm very leery of the use of "virtual" anything, since it often adds an unnecessary level of confusion. But if I were to calculate a reflection coefficient at some point along a continuous line, I could replace the remainder of the line and the load with a lumped load of impedance Z, and maintain exactly the same reflection coefficient and forward and reverse traveling waves in the remaining line. I wouldn't object, then, if someone would say that there was a "virtual impedance" of Z at that point when the line was intact, since all properties prior to that point are unchanged if a lumped Z of that value is substituted for the remainder. (I personally wouldn't call it "virtual" -- I'd just call it the Z at that point, since it's the ratio of V to I there.) The point is that the reflection coefficient was the same before and after the substitution of the remaining line with a real lumped Z. Before the substitution, reflection was occurring at the load. After, the reflection is occurring at the new, substituted Z load. Yet the reflection coefficient and traveling waves remain the same on the remaining line. A reflection coefficient isn't the cause of anything. It's simply a calculated quantity used for computational and conceptual convenience. Only an impedance discontinuity causes reflections, but we can calculate a reflection coefficient at any point we choose, with its value being well defined and unambiguous. Roy Lewallen, W7EL |
Jim Kelley wrote:
Cecil Moore wrote: For that statement, the length of the feedline is unknown and the load is unknown. I was commenting on this, the subject of our conversation: My statement that you objected to didn't have anything to do with the following. "Consider the following: Source---50 ohm feedline---+---1/2WL 150 ohm---50 ohm load" Reflection coefficients are determined by characteristic impedances, not virtual ones. On the contrary, the reflection coefficient, rho, at '+' in the example above, is NOT determined by the characteristic impedances. Above, rho (looking at '+' from the left) is determined by taking the square root of (Pref/Pfwd) = 0. (150-50)/(150+50) is NOT rho. I was mistaken to call that quantity "rho" in my article. That quantity that I called "rho" is actually 's11' and I need to update my article. -- 73, Cecil, W5DXP |
Roy Lewallen wrote:
Jim Kelley wrote: . . . Voltages on a transmission line do not determine reflection coefficients. Reflection coefficients are determined by characteristic impedances, not virtual ones. 73, Jim AC6XG I disagree with this. When applied to transmission lines, the (voltage) reflection coefficient is, as far as I can tell, universally defined as the ratio of reflected to forward voltage to reverse voltage at a point. That rho is equivalent to that ratio of voltages is not in dispute. I might dispute that it's 'defined' by that ratio. We agree the reflection is caused by an impedance discontinuity. It is the relationship of those impedances that determines how much of an incident voltage will be reflected. From my perspective, one builds a network of impedances in order to achieve the desired voltage relationships. But one cannot build voltage relationships in order to obtain a network of impedances. Maybe it's another chicken and egg argument. Only an impedance discontinuity causes reflections, but we can calculate a reflection coefficient at any point we choose, with its value being well defined and unambiguous. Wouldn't the most well defined and unabiguous be at a point of reflection? ;-) 73, Jim AC6XG |
Cecil Moore wrote: Jim Kelley wrote: Reflection coefficients are determined by characteristic impedances, not virtual ones. On the contrary, the reflection coefficient, rho, at '+' in the example above, is NOT determined by the characteristic impedances. I just showed you how characteristic impedances are used to calculate the reflection coefficient at '+'. But you can wish it into the cornfield if you like, Anthony. :-) (150-50)/(150+50) is NOT rho. Is it the reflection coefficient for a 50 ohm to 150 ohm impedance discontinuity? I was mistaken to call that quantity "rho" in my article. That quantity that I called "rho" is actually 's11' and I need to update my article. Since S-parameters were never even mentioned in your article, updating it seems an understatement. 73, Jim AC6XG |
On Thu, 02 Oct 2003 14:00:26 -0700, Jim Kelley
wrote: updating it seems an understatement. :-) |
Jim Kelley wrote:
Roy Lewallen wrote: Jim Kelley wrote: . . . Voltages on a transmission line do not determine reflection coefficients. Reflection coefficients are determined by characteristic impedances, not virtual ones. 73, Jim AC6XG I disagree with this. When applied to transmission lines, the (voltage) reflection coefficient is, as far as I can tell, universally defined as the ratio of reflected to forward voltage to reverse voltage at a point. That rho is equivalent to that ratio of voltages is not in dispute. I might dispute that it's 'defined' by that ratio. I stand corrected on that point. I did a quick scan of ten electromagnetics references. Seven quite clearly defined it that way. One (Magnusson) was vague, and one (Jordan & Balmain) gave pretty equal weight to both V and Z ratios as a definition. Holt, whom I consider one my best references, clearly defines reflection coefficient in terms of impedances. Regardless of definition, virtually all give both the V and Z relationships which are, of course, mathematically equivalent. We agree the reflection is caused by an impedance discontinuity. It is the relationship of those impedances that determines how much of an incident voltage will be reflected. From my perspective, one builds a network of impedances in order to achieve the desired voltage relationships. But one cannot build voltage relationships in order to obtain a network of impedances. Maybe it's another chicken and egg argument. I think so. One sees different impedances, i.e., V/I ratios, along a transmission line, so it could be argued that impedances have been created from voltages and currents. But I'll leave the arguing to those people more interested in philosophy than engineering. As long as the principles are understood and communicated, the point of view is largely a matter of taste. Only an impedance discontinuity causes reflections, but we can calculate a reflection coefficient at any point we choose, with its value being well defined and unambiguous. Wouldn't the most well defined and unabiguous be at a point of reflection? ;-) I don't think so. It's completely well defined and unambiguous at any point along the line. At any point, there's only one value of Vf and Vr, and only one value of Z and Z0, so there can only be one value of reflection coefficient. Knowing the reflection coefficient and Z0, for example, one knows for sure what the value of Z is at that point. I find that a number of the text authors freely use the concept of reflection coefficient at any point along a line, not necessarily a point of reflection. I don't have any problem with it, either. I see requiring a reflection in order to have a reflection coefficient as sort of like requiring dissipation in order to have a resistance. It's not really necessary, and the calculated value can still have meaning. Please don't let this detract from the ongoing discussion. It's really a minor point. Roy Lewallen, W7EL |
I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as the ratio of reflected to forward voltage to reverse voltage at a point. So a reflection coefficient can be, and often is, calculated for every point along a line, not just at discontinuities or points of actual reflection. This can be done with nothing more than the knowledge of the values of forward and reflected voltages at the point of calculation. ============================= Sorry! Just to continue and further confuse the haggling, the forward voltages are unknown because one does not know, in the case of amateur systems, what is the internal voltage and internal impedance of the transmitter. It is this unknown voltage and internal impedance which the so-called SWR (Rho) meter merely ASSUMES. |
Are you disagreeing with something I said, or just adding a note? If
disagreeing, what again was it that you disagree with? The source voltage and internal impedance have nothing to do with the reflection coefficient at any point. The forward and reverse voltages are indeed known if one knows, for example, the line Z0 and the load impedance and the load power, voltage, or current. It's not necessary to know the source impedance to find these values, the forward and reverse voltages, or the reflection coefficient. Roy Lewallen, W7EL Reg Edwards wrote: I disagree with this. When applied to transmission lines, the (voltage) reflection coefficient is, as far as I can tell, universally defined as the ratio of reflected to forward voltage to reverse voltage at a point. So a reflection coefficient can be, and often is, calculated for every point along a line, not just at discontinuities or points of actual reflection. This can be done with nothing more than the knowledge of the values of forward and reflected voltages at the point of calculation. ============================= Sorry! Just to continue and further confuse the haggling, the forward voltages are unknown because one does not know, in the case of amateur systems, what is the internal voltage and internal impedance of the transmitter. It is this unknown voltage and internal impedance which the so-called SWR (Rho) meter merely ASSUMES. |
Jim Kelley wrote:
I just showed you how characteristic impedances are used to calculate the reflection coefficient at '+'. But you can wish it into the cornfield if you like, Anthony. :-) Absolutely no chance that you are simply wrong? (150-50)/(150+50) is NOT rho. Is it the reflection coefficient for a 50 ohm to 150 ohm impedance discontinuity? It is the 's11' reflection coefficient for that impedance discontinuity. It is NOT the 'rho' at '+' unless the signals are orthogonal to each other at '+'. Chances are they are not orthogonal. I was mistaken to call that quantity "rho" in my article. That quantity that I called "rho" is actually 's11' and I need to update my article. Since S-parameters were never even mentioned in your article, updating it seems an understatement. In my article, I called (Z1-Z0)/(Z1+Z0) the RHO(fv) and said it was equal to S11. I should just have called it 'S11'. And I just checked my web page. S11 is definitely mentioned in my article. -- 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! =----- |
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