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Additional Line Losses Due to SWR
Various authors provide curves or formula for computing the "total
loss" in transmission lines, as opposed to the "matched-line" loss. Specifically, The ARRL Antenna Book gives an equation in Chapter 24 that seems to give results consistent with other sources (See the details at the end of this posting). However, there seems to be a fundamental flaw in the way in which the equation is applied. In essence, the equation provides a loss factor which is a function of the matched-loss attenuation and the absolute reflection coefficient. The matched-loss attenuation is the value normally expressed in dB per 100 ft. and shown in tables or shown in logarithmic plots as a function of frequency. The reflection coefficient is introduced into the expression in order to increase the total losses as the SWR on the line increases. After calculating a total loss factor it is applied to lines of any length based on the reflection coefficient at the load. In my opinion, it makes no sense whatsoever to provide an expression that is to determine the losses per unit length on a line and have it based on the reflection coefficient at the end of the line. If there is a mismatched load, and if the line has losses, then it follows that the SWR will become lower and lower the further we are from the termination. That being the case, would it not make more sense to say that the "additional" losses would be much higher at the load end of the line, where the SWR is high, than at great distance from the load, where the SWR is significantly lower? In fact, if the line is long enough, we know that the SWR approaches 1:1, and in a line with an SWR of 1:1 there should be no additional losses above the matched-line losses. Nonetheless, with that non-sensical approach, the numerical examples shown at the referenced page and also in a later article on the subject of Highly Reactive Loads makes it quite clear that the loss factor is applied uniformly to the entire length of line. If we take the expression for the total loss and apply it to small increments of line wherein the SWR is relatively constant, then it not only makes more sense, but it also predicts noticeably less total loss in longer lines. I have embarked on careful measurements of lines severely mismatched (quarter wave open circuit stubs), and I can find no correlation between my measurements and the values predicted by the "total loss" equation. My measurements always show very low losses in comparison to the model. I would be interested in corresponding with anyone who has other models for line losses, or anyone who has made measurements on quarter-wave stubs. ##########Equation and data taken directly from The ARRL Antenna Book, 17th Ed., page 24-9 ############### (Eq 10) Total Loss (dB) = 10 log [ {(Alpha * Alpha - (AbsRho * AbsRho)} / {Alpha * (1- (AbsRho * AbsRho)) } ] where Alpha = 10^(ML/10) = matched-line loss ratio AbsRho = (SWR - 1) / (SWR + 1) where ML = the matched-line loss for particular length of line, in dB SWR = SWR at load end of line The text then goes on with a numeric example using a 150 ft. length of RG-213 coax that is terminated in a 4:1 mismatch (SWR = 4:1, AbsRho = 0.6) at 14.2 MHz. The calculations for total line loss, per the above equation, results in a total line loss of 2.107 dB. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
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On Thu, 25 Nov 2004 21:11:13 GMT, Richard Clark
wrote: On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay W9DMK) wrote: I have embarked on careful measurements of lines severely mismatched (quarter wave open circuit stubs), and I can find no correlation between my measurements and the values predicted by the "total loss" equation. My measurements always show very low losses in comparison to the model. Hi Bob, Looks like our two recent postings about roughly the same topic passed like ships in the night. How about the numbers (your data) that leads to your suspicions? 73's Richard Clark, KB7QHC The only one that falls immediately to hand is probably a decent representative example. It is a piece of RG-8/U, also known by its maker, Columbia, as 9913. It is 5.334 meters long and is approximately 1/4 wave at 10.6 MHz, which is where I made the measurement. In its open circuit configuration, it measures 0.57 + j 0.3 ohms. I am still struggling with some issues of how to interpret those results, but two things seem to be clear - 1) the reactive component is because I was about a degree too long for the frequency of measurement, or there is enough stray capacitance at the open circuit to transform into a bit of inductive reactance, and 2) the very low resistive component doesn't seem to fit any math models that I'm working with. What do you think? Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
"Robert Lay W9DMK" wrote in message ... On Thu, 25 Nov 2004 21:11:13 GMT, Richard Clark wrote: On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay W9DMK) wrote: I have embarked on careful measurements of lines severely mismatched (quarter wave open circuit stubs), and I can find no correlation between my measurements and the values predicted by the "total loss" equation. My measurements always show very low losses in comparison to the model. Hi Bob, Looks like our two recent postings about roughly the same topic passed like ships in the night. How about the numbers (your data) that leads to your suspicions? 73's Richard Clark, KB7QHC The only one that falls immediately to hand is probably a decent representative example. It is a piece of RG-8/U, also known by its maker, Columbia, as 9913. It is 5.334 meters long and is approximately 1/4 wave at 10.6 MHz, which is where I made the measurement. In its open circuit configuration, it measures 0.57 + j 0.3 ohms. I am still struggling with some issues of how to interpret those results, but two things seem to be clear - 1) the reactive component is because I was about a degree too long for the frequency of measurement, or there is enough stray capacitance at the open circuit to transform into a bit of inductive reactance, and 2) the very low resistive component doesn't seem to fit any math models that I'm working with. What do you think? Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk the 1/4 wave open end coax looks like a short circuit at the feed point. so your reading makes perfect sense. |
On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay
W9DMK) wrote: Bob, You might want to look at this paper: http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf |Various authors provide curves or formula for computing the "total |loss" in transmission lines, as opposed to the "matched-line" loss. |Specifically, The ARRL Antenna Book gives an equation in Chapter 24 |that seems to give results consistent with other sources (See the |details at the end of this posting). However, there seems to be a |fundamental flaw in the way in which the equation is applied. | |In essence, the equation provides a loss factor which is a function of |the matched-loss attenuation and the absolute reflection coefficient. |The matched-loss attenuation is the value normally expressed in dB per |100 ft. and shown in tables or shown in logarithmic plots as a |function of frequency. The reflection coefficient is introduced into |the expression in order to increase the total losses as the SWR on the |line increases. | |After calculating a total loss factor it is applied to lines of any |length based on the reflection coefficient at the load. In my opinion, |it makes no sense whatsoever to provide an expression that is to |determine the losses per unit length on a line and have it based on |the reflection coefficient at the end of the line. If there is a |mismatched load, and if the line has losses, then it follows that the |SWR will become lower and lower the further we are from the |termination. That being the case, would it not make more sense to say |that the "additional" losses would be much higher at the load end of |the line, where the SWR is high, than at great distance from the load, |where the SWR is significantly lower? In fact, if the line is long |enough, we know that the SWR approaches 1:1, and in a line with an SWR |of 1:1 there should be no additional losses above the matched-line |losses. | |Nonetheless, with that non-sensical approach, the numerical examples |shown at the referenced page and also in a later article on the |subject of Highly Reactive Loads makes it quite clear that the loss |factor is applied uniformly to the entire length of line. | |If we take the expression for the total loss and apply it to small |increments of line wherein the SWR is relatively constant, then it not |only makes more sense, but it also predicts noticeably less total loss |in longer lines. | |I have embarked on careful measurements of lines severely mismatched |(quarter wave open circuit stubs), and I can find no correlation |between my measurements and the values predicted by the "total loss" |equation. My measurements always show very low losses in comparison to |the model. | |I would be interested in corresponding with anyone who has other |models for line losses, or anyone who has made measurements on |quarter-wave stubs. | |##########Equation and data taken directly from The ARRL Antenna Book, |17th Ed., page 24-9 ############### |(Eq 10) Total Loss (dB) = 10 log [ {(Alpha * Alpha - (AbsRho * |AbsRho)} / {Alpha * (1- (AbsRho * AbsRho)) } ] | |where | | Alpha = 10^(ML/10) = matched-line loss ratio | | AbsRho = (SWR - 1) / (SWR + 1) | |where | ML = the matched-line loss for particular length of line, in |dB | | SWR = SWR at load end of line | |The text then goes on with a numeric example using a 150 ft. length of |RG-213 coax that is terminated in a 4:1 mismatch (SWR = 4:1, AbsRho = |0.6) at 14.2 MHz. The calculations for total line loss, per the above |equation, results in a total line loss of 2.107 dB. | | | | |Bob, W9DMK, Dahlgren, VA |http://www.qsl.net/w9dmk |
On Fri, 26 Nov 2004 00:24:13 -0000, "Dave" wrote:
the 1/4 wave open end coax looks like a short circuit at the feed point. so your reading makes perfect sense. Dear Dave, Yes, I believe it does - that is, it makes perfect sense to have a low resistance and to have a near zero reactive component. What does not make sense is that the high SWR is supposed to produce outrageous losses. I don't see values that I can interpret as high losses - quite the opposite. Maybe I just don't interpret it correctly, but I would expect it to be several ohms - not 0.57 ohms. In fact, and this is where it gets ridiculous, the examples in the ARRL Antenna Book would lead me to believe that the above quarter wave line would exhibit 20 dB of total losses. In order to get those numbers the SWR at the load of say 8000 would have to decrease to 1.01:1 at the source end in order to account for 20 dB in losses. (See the example on page 24-9 of the 17th Edition.) Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Thu, 25 Nov 2004 23:16:21 GMT, (Robert Lay
W9DMK) wrote: The only one that falls immediately to hand is probably a decent representative example. It is a piece of RG-8/U, also known by its maker, Columbia, as 9913. Don't ask me where I got that number - it's Columbia's number 1198 - not 9913. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
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"Robert Lay W9DMK" wrote in message ... On Fri, 26 Nov 2004 00:24:13 -0000, "Dave" wrote: the 1/4 wave open end coax looks like a short circuit at the feed point. so your reading makes perfect sense. Dear Dave, Yes, I believe it does - that is, it makes perfect sense to have a low resistance and to have a near zero reactive component. What does not make sense is that the high SWR is supposed to produce outrageous losses. I don't see values that I can interpret as high losses - quite the opposite. Maybe I just don't interpret it correctly, but I would expect it to be several ohms - not 0.57 ohms. In fact, and this is where it gets ridiculous, the examples in the ARRL Antenna Book would lead me to believe that the above quarter wave line would exhibit 20 dB of total losses. In order to get those numbers the SWR at the load of say 8000 would have to decrease to 1.01:1 at the source end in order to account for 20 dB in losses. (See the example on page 24-9 of the 17th Edition.) the cases they talk about in there are figuring the loss in power that you would be supplying to a load. in your case the load is an infinite resistance so it receives zero power which is what the arrl book says... in this case all the power that is sent down the line is reflected back minus a little bit of heating so the swr at the feedpoint should be near infinite, but not quite. the actual loss in the wave going down and coming back is very small hence the very low impedance. this is an effect that is used to make coaxial stub filters and transformers. |
On Thu, 25 Nov 2004 20:51:16 -0700, Wes Stewart
wrote: On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay W9DMK) wrote: Bob, You might want to look at this paper: http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf Dear Wes, I have downloaded the pdf file and printed it out. It's tough reading. I hope that MacAlpine agrees with what Dave and Richard are telling me, because their responses seem to be correct and are exactly what I was afraid of - that I've been sucked into another example of the strange terminology used to describe "losses". I have always thought of "loss" as a conversion to another form of energy (typically heat energy) which is lost from the system. Apparently, the kind of "loss" being described in the example that I gave is not a loss at all. It's more like "return loss", which is also not a true "loss" in my thinking. In other words, it seems that the "Additional Losses Due to SWR" are not losses at all, but are simply a measure of the power that "could" have been delivered to the load were it not for the mis-match. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
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On Fri, 26 Nov 2004 16:12:34 GMT, (Robert Lay
W9DMK) wrote: |On Thu, 25 Nov 2004 20:51:16 -0700, Wes Stewart |wrote: | |On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay |W9DMK) wrote: | |Bob, | |You might want to look at this paper: | |http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf | | |Dear Wes, | |I have downloaded the pdf file and printed it out. It's tough reading. Yes. But the ITT Reference Data For Radio Engineers uses this paper as a reference. If you have Mathcad, a sheet that implements some of the equations was included as a reference in my Balanced Transmission line paper. http://users.triconet.org/wesandlinda/LineCalc.mcd |I hope that MacAlpine agrees with what Dave and Richard are telling |me, because their responses seem to be correct and are exactly what I |was afraid of - that I've been sucked into another example of the |strange terminology used to describe "losses". | |I have always thought of "loss" as a conversion to another form of |energy (typically heat energy) which is lost from the system. |Apparently, the kind of "loss" being described in the example that I |gave is not a loss at all. Yes it is. A simple-minded way of looking at it is if the SWR is greater than unity then increased current is flowing in the line. The line has resistive loss, so the I^2*R loss increases. The current isn't constant (there is a current standing ratio, ISWR, just like a VSWR) so there are peaks and valleys in the current and as you have figured out, the longer the line and the higher its nominal loss, the lower the ISWR is at the line input. So the loss per unit length is non-linear and varies with distance from the mismatch, but it is a real dissipative loss. For those interested in the loss in the shorted or open stub case, maybe this will be of interest: http://users.triconet.org/wesandlind...ching_Loss.pdf |
Keep in mind that real ohmic and dielectric losses measured in watts depend
upon sqrt(SWR). Thus, the higher the SWR (load mismatch) the greater the I^2R losses in the conductors and similarly in the dielectric. So, to me, a non-unity SWR connotes real power loss measurable in watts and attributable to well-known loss mechanisms. Of course, any real power lost in the line materials represents power not delivered to the load, so this fits somewhat with the viewpoint that Line Loss is in fact the magnitude of power undelivered to the load due to the mismatch. But, I think that we are looking at real watts of loss here. Another confusing factor is that one is usually interested in the total loss attributable to the use of a mismatched line and not especially in how that loss is distributed along the line from load to source. But there are applications where the loss distribution with line length is of concern. An example is the case of a complex Zo with rho unity in which the majority of the power loss occurs in the section of the line nearest the load and decreases toward the source. In that case of probably limited application, the line nearest the load might be required to handle more power than that further toward the source. A somewhat related example concerns the W2DU balun in which is it observed that the beads nearest the mismatched load endure the largest heat dissipation and are commonly larger that the remainder further toward the source. However, since complex Zo is an issue of magnitude usually only at low r-f and more so at audio frequencies, this is seldom a practical consideration. Thanks for bringing this topic to light, Bob. Like most engineers, I have been guilty of looking at "line loss" as a monolithic phenomenon and not being concerned with the micro-structure of its distribution. -- 73, George W5YR Fairview, TX http://www.w5yr.com "Robert Lay W9DMK" wrote in message ... On Thu, 25 Nov 2004 20:51:16 -0700, Wes Stewart wrote: On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay W9DMK) wrote: Bob, You might want to look at this paper: http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf Dear Wes, I have downloaded the pdf file and printed it out. It's tough reading. I hope that MacAlpine agrees with what Dave and Richard are telling me, because their responses seem to be correct and are exactly what I was afraid of - that I've been sucked into another example of the strange terminology used to describe "losses". I have always thought of "loss" as a conversion to another form of energy (typically heat energy) which is lost from the system. Apparently, the kind of "loss" being described in the example that I gave is not a loss at all. It's more like "return loss", which is also not a true "loss" in my thinking. In other words, it seems that the "Additional Losses Due to SWR" are not losses at all, but are simply a measure of the power that "could" have been delivered to the load were it not for the mis-match. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
Modeling a free space dipole made from a lossless conductor, 100 ft in
length, at 1.8 MHz shows an input impedance of 6.694 - j1621 Ohms. As expected the radiation efficiency is 100%. Adding 300 ft of 600 Ohm, 6" spaced, copper open wire transmission line degrades the radiation efficiency to 16.75 %. The result, therefore indicates a transmission line loss of 7.76 dB. The input impedance is calculated as 11 - j619.7 Ohms. The ARRL, DOS based program, "TL" computes, for 300 ft of 600 Ohm line terminated with 6.694 - j1621 Ohms, a loss of 8.19 dB, and an input impedance of 18.35 - j805 Ohms. Realizing that 6" spaced, #14 AWG, is not exactly 600 Ohms, and NEC's computation of parallel wire transmission lines is not 100% accurate; the results do seem to confirm the validity of the ARRL's program. Another interesting experiment with the ARRL's program also seems to verify its accuracy: RG8, 1000 ft, frequency 100 MHz. Matched line loss = 24.82 dB. Load impedance 1 - j1000 Ohms. Total line loss = 61.82 dB. The program computes the input impedance to by: 50.3 - j0.2 Ohms. 73, Frank "George, W5YR" wrote in message ... Keep in mind that real ohmic and dielectric losses measured in watts depend upon sqrt(SWR). Thus, the higher the SWR (load mismatch) the greater the I^2R losses in the conductors and similarly in the dielectric. So, to me, a non-unity SWR connotes real power loss measurable in watts and attributable to well-known loss mechanisms. Of course, any real power lost in the line materials represents power not delivered to the load, so this fits somewhat with the viewpoint that Line Loss is in fact the magnitude of power undelivered to the load due to the mismatch. But, I think that we are looking at real watts of loss here. Another confusing factor is that one is usually interested in the total loss attributable to the use of a mismatched line and not especially in how that loss is distributed along the line from load to source. But there are applications where the loss distribution with line length is of concern. An example is the case of a complex Zo with rho unity in which the majority of the power loss occurs in the section of the line nearest the load and decreases toward the source. In that case of probably limited application, the line nearest the load might be required to handle more power than that further toward the source. A somewhat related example concerns the W2DU balun in which is it observed that the beads nearest the mismatched load endure the largest heat dissipation and are commonly larger that the remainder further toward the source. However, since complex Zo is an issue of magnitude usually only at low r-f and more so at audio frequencies, this is seldom a practical consideration. Thanks for bringing this topic to light, Bob. Like most engineers, I have been guilty of looking at "line loss" as a monolithic phenomenon and not being concerned with the micro-structure of its distribution. -- 73, George W5YR Fairview, TX http://www.w5yr.com "Robert Lay W9DMK" wrote in message ... On Thu, 25 Nov 2004 20:51:16 -0700, Wes Stewart wrote: On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay W9DMK) wrote: Bob, You might want to look at this paper: http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf Dear Wes, I have downloaded the pdf file and printed it out. It's tough reading. I hope that MacAlpine agrees with what Dave and Richard are telling me, because their responses seem to be correct and are exactly what I was afraid of - that I've been sucked into another example of the strange terminology used to describe "losses". I have always thought of "loss" as a conversion to another form of energy (typically heat energy) which is lost from the system. Apparently, the kind of "loss" being described in the example that I gave is not a loss at all. It's more like "return loss", which is also not a true "loss" in my thinking. In other words, it seems that the "Additional Losses Due to SWR" are not losses at all, but are simply a measure of the power that "could" have been delivered to the load were it not for the mis-match. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Fri, 26 Nov 2004 07:33:04 GMT, Richard Clark
wrote: .....snip Reference Data for Radio Engineers, "Mismatch and Transducer Loss," "One End Mismatched," pg. 22-12: Transducer Loss = A0 + 10 · log (Pm/P) decibels where A0 = normal attenuation of the line Pm = power that would be delivered were system matched P = power delivered to the load Of particular note is that this is one of my references as to the nature of Source Z which is often neglected in academic treatments with the presumption that the engineer has already been schooled in the nature of Real sources (this may shock some complaisant readers here). However, this citation offers that explicit lesson in figure 10 and makes use of this commonplace characteristic in illustrations of Mismatch Uncertainty. They go as far as to explicitly offer a section entitled "Generator and Load Mismatched." You may wish to review this treatment as it offers the math that would present the most loss available in a line, above and beyond the typical charts offered for line loss (which are confined to both ends being matched). Dear Richard, I'm finally ready to comment on the above - it is my great fortune to be blessed with copies of both the Fourth and Fifth Editions of the ITT Handbook. I studied over the first 13 pages of Chapter 22 and found that, just as Wes said, it's entirely the work of MacAlpine as published in 1953. I went over Equations (1) through (4) in the Mismatch section very carefully and found no heartburn with anything in that section. This is NOT to say that I LIKE it, but I do understand it and have no problem with the math model and the figures. My problems with the two mismatch topics is simply that I just don't like to call it a loss when energy that COULD have been delivered to the load does NOT get delivered to the load as a result of mismatch. For me, lost energy in a transmission line problem is energy actually lost in the transmission line, not energy that is being lost elsewhere as a result of the transmission line not being matched properly. I realize that I'm probably alone in that thinking, but I like to feel that such terms as efficiency and losses should be associated strongly with the item under evaluation, namely the transmission line, and not the ancillary equipment which feed it or terminate it. Those items get their own hearings relative to efficiency and losses and those evaluations do not require the presence of the transmission line. In fact, those items are usually evaluated as to their performance in ways that do not in any way relate to how well some transmission line is or is not working. However, this is not the nub of the problem that I was encountering - a problem which has now been partly resolved. At least I think I have a far, far better understanding of the problem now than I had a few days ago. The problem centers on the Additional Losses Due to SWR and how to model them. Since it is, perhaps, more appropriate to continue that topic under the responses from Wes, I will not go into it here. I want to thank you and Wes, both, for leading me to Chapter 22 - it is much more readable than MacAlpine's original paper. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Fri, 26 Nov 2004 10:57:25 -0700, Wes Stewart
wrote: Yes. But the ITT Reference Data For Radio Engineers uses this paper as a reference. If you have Mathcad, a sheet that implements some of the equations was included as a reference in my Balanced Transmission line paper. http://users.triconet.org/wesandlinda/LineCalc.mcd Dear Wes, I was happy to find that the MacAlpine paper is the first part of Chapter 22 of the ITT Handbook, as the latter is much more readable. I did not pick up on the MathCad files, because I do not have MathCd - however, the material from MacAlpine and Ricardi have answered most of my concerns. |I hope that MacAlpine agrees with what Dave and Richard are telling |me, because their responses seem to be correct and are exactly what I |was afraid of - that I've been sucked into another example of the |strange terminology used to describe "losses". | |I have always thought of "loss" as a conversion to another form of |energy (typically heat energy) which is lost from the system. |Apparently, the kind of "loss" being described in the example that I |gave is not a loss at all. I was premature in those two paragraphs, above. I can see now that the Additional Losses Due to SWR really are dissipative and are unrelated to the "Mismatch Losses" and "Transducer Losses" defined on page 22-12 of the ITT Handbook, 5th Ed. Yes it is. A simple-minded way of looking at it is if the SWR is greater than unity then increased current is flowing in the line. The line has resistive loss, so the I^2*R loss increases. The current isn't constant (there is a current standing ratio, ISWR, just like a VSWR) so there are peaks and valleys in the current and as you have figured out, the longer the line and the higher its nominal loss, the lower the ISWR is at the line input. My interpretation of your "Yes it is." is that you mean that the Additional Losses Due to SWR are truly heat losses and are due to the ohmic losses in the hot spots of the line. Then we agree on that point. Your paragraph above is much more succinct than the papers by MacAlpine and Ricardi, but it certainly tells the story. So the loss per unit length is non-linear and varies with distance from the mismatch, but it is a real dissipative loss. I don't know that I would have used the term "non-linear", but I would certainly agree that it varies along the line in accordance with the current loops. For those interested in the loss in the shorted or open stub case, maybe this will be of interest: http://users.triconet.org/wesandlind...ching_Loss.pdf I took that pdf and added it to the collection. There were several things about that paper that filled-in gaps of detail in MacAlpine. However, neither paper gives us much hope for a simple model of these losses. Nonetheless, it makes hash out of the material in The ARRL Antenna Book. In all fairness, the Antenna Book cannot cover all aspects of these topics in detail. Unfortunately, the material in the Antenna Book is, in my opinion, very misleading in several specific areas, as follows: - The Antenna Book gives only one expression for Total Line Loss (combining ML loss and the Additional Loss Due to SWR). If we accept Macalpine's model, there are different relationships for the range of SWR from 0 to 6 and for the range from 6 upwards. - Antenna Book does not explain that the hot spots are very localized and that the additional losses can be quite dependant upon the length of the line in wavelengths. For example, the losses in a segment of line less than 1/3 wavelength might be insignificant in comparison with a segment of line greater than 1/3 wavelength simply because the shorter segment may not contain a hot spot. In other words, one cannot apply the Antenna Book equations, blindly, because of several factors that are not even mentioned, and for short line segments it is quite possible that there would be no signicant losses due to SWR. - The most misleading information in The Antenna Book is on pages 24-11 and 24-12 where it is shown that a 100 foot RG-213 feedline will suffer 25 dB of Additional Loss Due to SWR at 1.83 MHz because of the very short antenna. I believe that when the equations from the ITT Handbook are used instead, that the actual losses will be far, far less. Just today, I made a careful measurement on an RG-8/U line of 5.33 meters length at 30 MHz and terminated with a 4700 + j 0 load. The Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and its Velocity Factor is between 0.75 and 0.80 The input impedance was actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25. The SWR at the load end was 94. Those two SWR's establish a total loss on the line of 0.15 dB. If one were to blindly apply the formula in The Antenna Book on page 24-9, the result obtained would be 4.323 dB. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
Robert Lay W9DMK wrote:
Just today, I made a careful measurement on an RG-8/U line of 5.33 meters length at 30 MHz and terminated with a 4700 + j 0 load. The Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and its Velocity Factor is between 0.75 and 0.80 The input impedance was actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25. The SWR at the load end was 94. Those two SWR's establish a total loss on the line of 0.15 dB. If one were to blindly apply the formula in The Antenna Book on page 24-9, the result obtained would be 4.323 dB. For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. -- 73, Cecil http://www.qsl.net/w5dxp |
On Sun, 28 Nov 2004 09:38:28 -0600, Cecil Moore
wrote: Robert Lay W9DMK wrote: Just today, I made a careful measurement on an RG-8/U line of 5.33 meters length at 30 MHz and terminated with a 4700 + j 0 load. The Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and its Velocity Factor is between 0.75 and 0.80 The input impedance was actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25. The SWR at the load end was 94. Those two SWR's establish a total loss on the line of 0.15 dB. If one were to blindly apply the formula in The Antenna Book on page 24-9, the result obtained would be 4.323 dB. For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. -- 73, Cecil http://www.qsl.net/w5dxp Dear Cecil, I hope I'm not misinterpreting your values - I assume that you are starting with a theoretical open circuit and a theoretical RG-8 line and calculating a theoretical impedance seen looking into that line of 0.57 + j 0. From that you then calculate a theoretical 0.2 dB. When I say calculate, I assume that you may instead by using a nomogram. Anyway, based on all of that being the situation up to but not including the loss figure, when I take the 0.57 + j0 and calculate the SWR as 87.7 I get a loss more like .05 dB, theoretically, so I'm not sure in what ways we are coming up with these numbers. I can explain exactly how I got mine, which was via measurements followed by a theoretical cacluation of loss based on the two SWR's formula which is built into all Smith Charts. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
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Robert Lay W9DMK wrote:
..For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. I hope I'm not misinterpreting your values - I assume that you are starting with a theoretical open circuit and a theoretical RG-8 line and calculating a theoretical impedance seen looking into that line of 0.57 + j 0. From that you then calculate a theoretical 0.2 dB. When I say calculate, I assume that you may instead by using a nomogram. Not using a nomogram but everything is 100% theoretical. It doesn't matter what line is being used as long as it's Z0 is 50 ohms. Matched line loss didn't enter into my calculations. It's only total loss. Anyway, based on all of that being the situation up to but not including the loss figure, when I take the 0.57 + j0 and calculate the SWR as 87.7 I get a loss more like .05 dB, theoretically, so I'm not sure in what ways we are coming up with these numbers. Is that the additional loss due to SWR or the total loss? My theoretical loss is total loss and the matched line loss need not be known. The measured resistance of the resonant stub is all one needs to know besides Z0. I can explain exactly how I got mine, which was via measurements followed by a theoretical cacluation of loss based on the two SWR's formula which is built into all Smith Charts. I can't remember where the following formula came from. I think it was from an RF guru at Intel, but I can't be sure. I have a hand- written notebook of useful formulas covering 25 years but I didn't record where they all came from. The formula for theoretical TOTAL losses in a *resonant* stub: Total loss = 10*log{[(Z0-R)/(Z0+R)]^2} where R is the measured resistance of the resonant stub and Z0 is the characteristic impedance of the stub material. You can see the [(Z0-R)/(Z0+R)]^2 term is akin to a virtual rho^2 at the mouth of the stub. Since rho^2 = Pref/Pfor, the losses in the stub are equivalent to the losses in an equivalent resistance equal to the measured virtual resistance at the mouth of the stub. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote:
The formula for theoretical TOTAL losses in a *resonant* stub: Total loss = 10*log{[(Z0-R)/(Z0+R)]^2} where R is the measured resistance of the resonant stub and Z0 is the characteristic impedance of the stub material. You can see the [(Z0-R)/(Z0+R)]^2 term is akin to a virtual rho^2 at the mouth of the stub. Since rho^2 = Pref/Pfor, the losses in the stub are equivalent to the losses in an equivalent resistance equal to the measured virtual resistance at the mouth of the stub. In other words, replace the stub with a resistor having the same value of measured resistance as the stub, and calculate the I^2*R losses in the resistor. That will be the same value as the total losses in the stub. -- 73, Cecil http://www.qsl.net/w5dxp |
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On Sun, 28 Nov 2004 18:41:44 GMT, Richard Clark
wrote: On Sat, 27 Nov 2004 21:43:14 GMT, (Robert Lay W9DMK) wrote: I can see now that the Additional Losses Due to SWR really are dissipative and are unrelated to the "Mismatch Losses" and "Transducer Losses" defined on page 22-12 of the ITT Handbook, 5th Ed. Hi Bob, I've let this simmer for a while, but I have to return to this because you've erred in interpretation of this particular page and those particular subjects. They are entirely caloric losses, not what you dismiss as the myth of mismatch loss. You need only review the math offered to observe they use the conventional "real" line loss and add more "real" line loss in proportion to the reflections at either one or two interfaces. The equations are quite literal to this and explicitly state: A0 = normal attenuation of line I goofed on the part that is talking about transducer loss. I should NOT have included the "Transducer Losses" in my statement above. The Transducer losses do, as you say, include the normal attenuation of the line, which is indeed a dissipative loss. If you want deeper math, one source can be found in Chipman's (as unread as any here) "Transmission Lines." This is yet another of my references that attend to my recent, short thread on the nature of power determination error, and mismatched loads AND sources. In fact ALL of these references I've offered explicitly describe that the source MUST be matched for ANY of these equations about transmission lines bandied about to accurately offer true answers. The naive presumptions that Source Z is immaterial to the outcome of analysis is quite widespread here. Chipman offers the rigorous math that attends explicitly to the Smith Chart loss nomograph you reference elsewhere in this thread. If you lack access to this work, I can munge up the equations here for you. I will add, this math is for "lossless" lines, as is the implication of the Smith Chart nomograph; but it only requires you to add that in for yourself by restructuring the math to include loss. At that level of granularity, it won't be pretty; but you can rest assured it will be complete. I'm not sure what you are saying about the loss nomograph on the Smith Chart. If that's wrong too, then we're in big trouble. Everything ever written that I have seen about the Smith Charts agrees that the actual losses in the transmission line are indicated by the collapsing of the circle as one traverses the transmission line. All you have to do is read the "loss in 1 dB steps" scale to determine those losses. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Sun, 28 Nov 2004 16:53:59 -0700, Wes Stewart
wrote: If you wouild cite the pages to which you refer, I would gladly scan then to pdf and post them for all to reference. Hi Wes, The math is on the bottom of pg. 203 which is supporting Fig. 9-26. There is also a section entitled 8.8 Multiple reflections on ppg 174...176. Then there is the specific math of fully specified matches at both ends, that is at the source and the load, found in Fig. 10-7 that is supported by discussion on ppg. 225...227. All of this bears on discussion around and about the necessary treatment of the Z of the Source, but I haven't supplied all the citations within this one reference by any means. Thanx, Wes. You needn't do all these scans. The group needs to do their own heavy lifting to escape their naivety about source Z. 73's Richard Clark, KB7QHC |
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Just a few meandering thoughts -
For anyone assembling a collection of exact transmission line formulae, of which there are dozens, some more applicable to practical problems than others, it is best to begin with the most commonly used and the most complicated formulae. Don't incorporate approximate formulae or you will later wish that you hadn't. Clear your minds of superfluous Smith Charts, standing waves, reflected power, virtual short circuits, conjugate matches, etc. A clear understanding of how transmission lines work is essential. You should be familiar with complex hyperbolic functions. Only metric line dimensions should be used. What are required are calculating procedures which accepts all possible input data and finish with preferably a single number. In some cases, if not needed, input data can be set to zero but the facility must exist. You will then have designed a set of step-by-step routines as in computer programs but which can be tediously and logically worked through with a pocket calculator. The number of intermediate variables can be large. But there can be only one unambiguous straight-line path through subroutines. For example, in a large number of cases the single output quantity is related to line loss, such as insertion loss in dB, or load power in watts, or transmission efficiency in percent, or percent of input power lost in the line itself. But before this can be calculated it is essential to calculate input impedance Rin+jXin for given attenuation in dB or nepers, given phase shift in radians and given terminating impedance Rt+jXt. Then include generator impedance Rg+jXg and internal generator volts. Having done this you are half-way through. Some intermediate results may be useful. Such as input impedance which terminates a tuner or provides a source for a receiver. Such intermediate results as reflection coefficient magnitudes and angles may be explicitly available but may be of no practical use. What can you do with them? The calculation is already complete. You might find an SWR somewhere in there if you recognise it but who cares. Well, you get the idea. But if you had the source codings of some of my programs I can assure you they would not be of the slightest use. You may just as well start at the begining. There are many ways of accomplishing the same task. A mathematical program is a work of art as much as it is a set of logical rules. But only the programmer can fully appreciate the beauty. First prepare a list of proposed interrelated calculating formulae or routines. Then write the routines on paper. Then test them on a computer. Then spend the next 12 months removing the bugs. Ditto, removing the bugs caused by the debugging operations. Then publish them in the ARRL Handbook, 2009 edition, using a better printer. ---- Reg. |
On Mon, 29 Nov 2004 04:09:50 +0000 (UTC), "Reg Edwards"
wrote: But only the programmer can fully appreciate the beauty. Hi Reggie, Nice posting. Once, some many (few in your perspective) years ago, you once retorted to my style with "this is not rec.radio.amateur.antenna.poetry." There is more than the bouquet of that romantic tendency in your last posting. However, neither of us is really surprised to find that in the other. ;-) 73's Richard Clark, KB7QHC |
On Mon, 29 Nov 2004 00:28:22 GMT, Richard Clark
wrote: |On Sun, 28 Nov 2004 16:53:59 -0700, Wes Stewart |wrote: | |If you wouild cite the pages to which you refer, I would gladly scan |then to pdf and post them for all to reference. | |Hi Wes, | |The math is on the bottom of pg. 203 which is supporting Fig. 9-26. | |There is also a section entitled 8.8 Multiple reflections on ppg |174...176. | |Then there is the specific math of fully specified matches at both |ends, that is at the source and the load, found in Fig. 10-7 that is |supported by discussion on ppg. 225...227. | |All of this bears on discussion around and about the necessary |treatment of the Z of the Source, but I haven't supplied all the |citations within this one reference by any means. | |Thanx, Wes. You needn't do all these scans. The group needs to do |their own heavy lifting to escape their naivety about source Z. Hi Richard, I did it anyway. [g] Hope this covers it: http://users.triconet.org/wesandlind...rdClarkRef.pdf Regards, Wes |
On Mon, 29 Nov 2004 10:27:37 -0700, Wes Stewart
wrote: I did it anyway. [g] Hope this covers it: http://users.triconet.org/wesandlind...rdClarkRef.pdf Hi Wes, Thanx very much. I can see one of two results from this general availability. The readership here can: 1. Avoid it in stunned shame (the embarrassment in coming of age); 2. Accept it as a remarkable revelation (because it's on the web). I would hope for a third response from those who could argue what follows from these first principles, but the lazier ones would complain of my "attitude" and hobble back to their beauty contests on their crutches. ;-) To quote one of my favorite authors, Raymond Chandler, when in "The Big Sleep" Doghouse Reilly is admonished about the same defect, he avers "I don't mind if you don't like my manners. They're pretty bad. I grieve over them during the long winter evenings." 73's Richard Clark, KB7QHC |
I offer a third third response.
On p. 175, Chipman states: "Equation (8.27) demonstrates explicitly that the shape of a standing wave pattern representing |V(d)| as a function of d on a transmission line is in no way affected by the quantities Vs, Zs and [rho]s at the source." And equation 8.29 on p. 176, the calculation of reflection coefficient, contains no source-dependent terms. I'm sure that somewhere in the book, the author derives SWR in terms of reflection coefficient. These are the facts: 1. The SWR, positions of the standing waves, reflection coefficient seen looking into the line, impedance seen looking into the line, and dB line loss are independent of source impedance. 2. The actual amount of power delivered to a line for a given Thevenin source voltage will, of course, depend on the source impedance, just as it would if the source were directly connected to a load. Therefore, the absolute amount of power dissipated in the load depends on source impedance. The dB line loss, however, doesn't. Also, the length of time the line requires to reach equilibrium after initially turning on the source depends on the source impedance. These can be found, explicitly stated and/or in easily interpreted equation form, in a host of references. I see nothing in the text Wes has kindly posted which contradicts these facts, and I'm sure there's nothing elsewhere in the text that does. I often have a hard time understanding Richard's postings, so it's possible that he's not disagreeing with the statements I've made, either. If so, I apologize for the misinterpretation. Roy Lewallen, W7EL Richard Clark wrote: On Mon, 29 Nov 2004 10:27:37 -0700, Wes Stewart wrote: I did it anyway. [g] Hope this covers it: http://users.triconet.org/wesandlind...rdClarkRef.pdf Hi Wes, Thanx very much. I can see one of two results from this general availability. The readership here can: 1. Avoid it in stunned shame (the embarrassment in coming of age); 2. Accept it as a remarkable revelation (because it's on the web). I would hope for a third response from those who could argue what follows from these first principles, but the lazier ones would complain of my "attitude" and hobble back to their beauty contests on their crutches. ;-) To quote one of my favorite authors, Raymond Chandler, when in "The Big Sleep" Doghouse Reilly is admonished about the same defect, he avers "I don't mind if you don't like my manners. They're pretty bad. I grieve over them during the long winter evenings." 73's Richard Clark, KB7QHC |
Thanks.
So between equations 8.29 and 8.30 the author calculates VSWR without any source-related terms -- as every other textbook author does. Roy Lewallen, W7EL Wes Stewart wrote: On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. Indeed he does---on the next page. http://users.triconet.org/wesandlind...manPage177.pdf Equation (8.30) 1 + |rho| VSWR ------------- 1 - |rho| |
On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. | |These are the facts: | |1. The SWR, positions of the standing waves, reflection coefficient seen |looking into the line, impedance seen looking into the line, and dB line |loss are independent of source impedance. |2. The actual amount of power delivered to a line for a given Thevenin |source voltage will, of course, depend on the source impedance, just as |it would if the source were directly connected to a load. Therefore, the |absolute amount of power dissipated in the load depends on source |impedance. The dB line loss, however, doesn't. Also, the length of time |the line requires to reach equilibrium after initially turning on the |source depends on the source impedance. | |These can be found, explicitly stated and/or in easily interpreted |equation form, in a host of references. | |I see nothing in the text Wes has kindly posted which contradicts these |facts, and I'm sure there's nothing elsewhere in the text that does. | |I often have a hard time understanding Richard's postings, so it's |possible that he's not disagreeing with the statements I've made, |either. If so, I apologize for the misinterpretation. In an earlier post to this thread, Richard stated: |"This is yet another of my references that attend to my recent, short |thread on the nature of power determination error, and mismatched |loads AND sources. In fact ALL of these references I've offered |explicitly describe that the source MUST be matched for ANY of these |equations about transmission lines bandied about to accurately offer |true answers. The naive presumptions that Source Z is immaterial to |the outcome of analysis is quite widespread here." I almost demurred, much as Roy did, because this statement is not universal, but I held off because I believe (and hope) that Richard is talking only about *power* measurement errors. Wes |
On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. Indeed he does---on the next page. http://users.triconet.org/wesandlind...manPage177.pdf Equation (8.30) 1 + |rho| VSWR ------------- 1 - |rho| |
Once again, it's not clear to me just what you're trying to prove.
Do you disagree with either of the two numbered statements made in my posting? If so, which part(s) of which one(s) -- I'm sure I can demonstrate their correctness. If not, we probably don't disagree. Roy Lewallen, W7EL Richard Clark wrote: On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen wrote: I'm sure that somewhere in the book, the author derives SWR in terms of reflection coefficient. Hi Roy, No doubt. As I am not immediately interested in the reductionist art of SWR, then the remainder seemed of little relevance to line losses and the contribution of source Z to power determination error. You continue, in part quote of a part quote: representing |V(d)| as a function of d which given you offer no more comment upon it, gives me the impression you are unaware of what function d is. The point of the matter is that this very equation you chose is examined in isolation, by you, but is returned to on several occasions by Chipman where he quite "explicitly" exhibits how V(d) ranges wildly for situations where both ends of the line are terminated by forced mismatches. This is a uncommon technique for determining SWR (still not my point, but nonetheless an obvious example). And yes, I realize I often have a hard time understanding Richard's postings and I often grieve over this on long winter evenings. Roy, you are too coy by half. ;-) However, your aside into SWR shape and the focus on reductions to typical applications (source matches line) does leave a dilemma because there is now conflict between your isolated quote of Chipman and the demonstration of EZNEC as reported by my late, short lived thread. I would offer that EZNEC fully supports Chipman's other comments on this same quoted material you drew from him, and goes to the matter I offered of an uncommon technique for SWR determination. My EZNEC reports are also supported by bench results, and other sources also recited here. All this seems to leave you on the outside looking in. As it broaches upon topics that you have long cautioned me that discussion would not "change your mind," I doubt this will go any further. 73's Richard Clark, KB7QHC |
Roy Lewallen wrote:
I see nothing in the text Wes has kindly posted which contradicts these facts, and I'm sure there's nothing elsewhere in the text that does. None of us are perfect. All of us (except Jim Kelley :-) will admit to being human, i.e. capable of making a mistake. The Z(s) of the source, no doubt, has an effect on the power sourced by the source. But the "power sourced by the source" has no effect on SWR, which is independent of source impedance. Given a steady-state forward power, nothing else depends upon source impedance. If a one ohm source is capable of supplying the same voltage as a one megohm source, the steady-state results will be identical. Given any source with any source impedance, there exists a forward power. Given any forward power, the source impedance during steady- state is completely irrelevant. -- 73, Cecil http://www.qsl.net/w5dxp |
On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: I'm sure that somewhere in the book, the author derives SWR in terms of reflection coefficient. Hi Roy, No doubt. As I am not immediately interested in the reductionist art of SWR, then the remainder seemed of little relevance to line losses and the contribution of source Z to power determination error. You continue, in part quote of a part quote: representing |V(d)| as a function of d which given you offer no more comment upon it, gives me the impression you are unaware of what function d is. The point of the matter is that this very equation you chose is examined in isolation, by you, but is returned to on several occasions by Chipman where he quite "explicitly" exhibits how V(d) ranges wildly for situations where both ends of the line are terminated by forced mismatches. This is a uncommon technique for determining SWR (still not my point, but nonetheless an obvious example). And yes, I realize I often have a hard time understanding Richard's postings and I often grieve over this on long winter evenings. Roy, you are too coy by half. ;-) However, your aside into SWR shape and the focus on reductions to typical applications (source matches line) does leave a dilemma because there is now conflict between your isolated quote of Chipman and the demonstration of EZNEC as reported by my late, short lived thread. I would offer that EZNEC fully supports Chipman's other comments on this same quoted material you drew from him, and goes to the matter I offered of an uncommon technique for SWR determination. My EZNEC reports are also supported by bench results, and other sources also recited here. All this seems to leave you on the outside looking in. As it broaches upon topics that you have long cautioned me that discussion would not "change your mind," I doubt this will go any further. 73's Richard Clark, KB7QHC |
On Mon, 29 Nov 2004 15:14:42 -0800, Roy Lewallen
wrote: Once again, it's not clear to me just what you're trying to prove. Hi Roy, Troubles me too.... In the interim I have recognized where I went off the deep end in the thread I have alluded to. The attempt there was to reproduce, in EZNEC, results obtained at the bench and offered through various references which demonstrate Mismatch Uncertainty. Although the model of the transmission line was certainly of the highest quality, EZNEC lacks the capacity to separate forward and reverse waves for analysis. In that regard I mistakenly attributed the results I obtained to my bench results. Situation is that with a SWR meter moving along a line mismatched at both ends, there is a distinct variation in the computed Power. When I did this at the bench, I could evidence about a 30% variation which was consistent with theory and clearly exhibits the contribution of Source Z when it is other than 50 Ohms. When I recently attempted to model this in EZNEC, I again saw the wild fluctuation of power that seduced me with its complementary results into thinking I had achieved the same results. When I revisited those results (EZNEC ones that is), all I had done was prove the mismatch through the abstraction of the power reported at my moving test load. When I reverse engineered the voltages from the known R, it became obvious that the VSWR corresponded to every expectation - or was close enough given the degree of resolution I had available with 20 test points distributed along the line. Chipman describes this in his late chapters - one of which Wes has provided. However, this does nothing to detract from Chipman's work that includes the FULL treatment of all variables that lead to the common usages. Such treatments include the Source with full characterizations and goes on to discuss the R of the Source and not just its Z. That so many texts disregard this level of examination does not deny its importance to issues that go beyond SWR. Those lesser texts presume Source Z unlike Chipman, and its significance is lost to the student who hasn't been grounded in the fundamentals. And thus we arrive at the topic at hand and listed in the Subject Line: Additional Line Losses Due to SWR. In that regard, the Source Z is entirely an active player and any additional mismatch that it presents to the system eventually finds additional loss (caloric) injected into it. Chipman's work in that regard is complete enough to offer Bob the framework to render a complete solution and to explain how and why this additional loss appears. It may not bear on his measurements directly (the measure of a stub's Q as it eventually turns out), but it is related closely enough to provide tangible leads. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
Chipman's work in that regard is complete enough to offer Bob the framework to render a complete solution and to explain how and why this additional loss appears. It may not bear on his measurements directly (the measure of a stub's Q as it eventually turns out), but it is related closely enough to provide tangible leads. The fact remains that a transmission line/antenna system has the same characteristics whether one watt, 100 watts, or 1000 watts are input to it, i.e. the efficiency and SWR of the antenna system does not depend upon the source impedance. -- 73, Cecil http://www.qsl.net/w5dxp |
On Mon, 29 Nov 2004 22:18:56 -0600, Cecil Moore
wrote: the efficiency and SWR of the antenna system does not depend upon the source impedance. Life's like a box of chocolates, hmmm? Get off the bench. |
Roy, W7EL wrote:
"I`m sure that somewhere in the book the author (Chipman) derives SWR in terms of reflection coefficient." I`ve read, here I believe, that Chipman`s book is one of the "Schaum`s Outline" series. It would need to derive SWR in terms of reflection coefficient to be complete. Attenuation of a uniform transmission line is a function of the loss per unit length and the total length. A line with SWR has higher voltage and current in spots than a matched line. Thus it has higher loss. Analysis is complicated in lossy lines due to decline in SWR back from the reflection point. If SWR at the reflection point is less than 2:1, added loss due to SWR is hardly detectable. Such a line is considered perfect. The ARRL Antenna Book has graphs relating SWR to fopward and reflected power readings as given by a Bird wattmeter. The "Antenna Book" also has a graph of additional loss due to standing waves versus the SWR at the load for values between SWR=1.5 and SWR=20. I don`t have a dog in this fight. There is nothing particularly strange about solutions to equations which describe particular relations on a transmission line. The functions are not erratic but continuous and predictable. Selection of the right formulas is all that`s needed to get the right answer from the right data. Best regards, Richard Harrison, KB5WZI |
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