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Distance to Fault
Hi all,
For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help |
Distance to Fault
On Nov 2, 11:22*pm, ashwanthh
wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. *What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh No transducer. Use an oscilloscope to measure the time between the emitted pulse and the reflection of the same. KD7HB |
Distance to Fault
On Nov 2, 11:22*pm, ashwanthh
wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. *What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh Forward and reverse power are normally resolved using a "directional coupler." Such a coupler can also be thought of as a bridge circuit. Directional couplers can be built in various ways. For example, the free-ware RFSim99 program, under tools--component--coupler shows five different ways to build a directional coupler. That program doesn't show the Wheatstone bridge form of directional coupler, but those are also common, especially where a wide range of frequencies is to be covered. Wheatstone bridges are generally used with equal (e.g. 50 ohm) arms in test and measurement equipment that runs low RF power (because such a bridge must dissipate most of the RF power fed to it), and with very unequal arms when high power is to be handled (resulting in a low coupling factor; -40dB is common). Please note that directional couplers must be designed for the impedance line they are used with (and ideally, the calibration should be verified). In your project to locate faults, you may wish to consider not only the ratio of forward and reverse power (e.g., SWR), but also the phase relationship between the two. The rate of change of phase as frequency is changed should tell you how far away a single fault is; multiple faults are more complicated. Cheers, Tom |
Distance to Fault
On 03/11/2010 15:46, KD7HB wrote:
On Nov 2, 11:22 pm, wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh No transducer. Use an oscilloscope to measure the time between the emitted pulse and the reflection of the same. KD7HB just to add to that Google Time domain Reflectometer. Jeff |
Distance to Fault
On Nov 3, 6:22*am, ashwanthh
wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. *What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh look up time domain reflectometers... they are much simpler than what you are thinking of. using a simple step input wave you measure the time to the reflection. if you get fancy you can measure the polarity and size of the reflection to determine if it is a short or open or impedance change... if you really get fancy you can hook a scope up and look at more details. the only transducer needed is a voltage probe OR a current shunt, either one will work depending on the installation needs. |
Distance to Fault
K1TTT wrote:
On Nov 3, 6:22 am, ashwanthh wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh look up time domain reflectometers... they are much simpler than what you are thinking of. using a simple step input wave you measure the time to the reflection. if you get fancy you can measure the polarity and size of the reflection to determine if it is a short or open or impedance change... if you really get fancy you can hook a scope up and look at more details. the only transducer needed is a voltage probe OR a current shunt, either one will work depending on the installation needs. Or, use the technique used in vector network analyzers.. Sweep the frequency, do the fourier transform to find time domain. If you're using just power detection, it's more like getting the autocorrelation or power spectrum of the reflected signal vs frequency. |
Distance to Fault
K1TTT wrote:
On Nov 3, 6:22 am, ashwanthh wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh look up time domain reflectometers... they are much simpler than what you are thinking of. using a simple step input wave you measure the time to the reflection. if you get fancy you can measure the polarity and size of the reflection to determine if it is a short or open or impedance change... if you really get fancy you can hook a scope up and look at more details. the only transducer needed is a voltage probe OR a current shunt, either one will work depending on the installation needs. If the school project requires him to use a Spectrum Analyzer, then the time domain may not be an option. |
Distance to Fault
On Nov 3, 10:31*pm, joe wrote:
K1TTT wrote: On Nov 3, 6:22 am, ashwanthh wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. *What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh look up time domain reflectometers... they are much simpler than what you are thinking of. *using a simple step input wave you measure the time to the reflection. *if you get fancy you can measure the polarity and size of the reflection to determine if it is a short or open or impedance change... if you really get fancy you can hook a scope up and look at more details. *the only transducer needed is a voltage probe OR a current shunt, either one will work depending on the installation needs. If the school project requires him to use a Spectrum Analyzer, then the time domain may not be an option. if the school project is requiring a spectrum analyzer to measure distance to a fault location then its a pretty dumb project. there are much better and more straight forward ways to do that. |
Distance to Fault
On Nov 3, 9:16*am, Jeff wrote:
On 03/11/2010 15:46, KD7HB wrote: On Nov 2, 11:22 pm, wrote: Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. *What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help -- ashwanthh No transducer. Use an oscilloscope to measure the time between the emitted pulse and the reflection of the same. KD7HB just to add to that Google Time domain Reflectometer. Jeff Yup, that's the default instrument, but a fast-rise-time pulse generator and a wideband scope will do it, too. I created a TDR accidentally when I thought I was just measuring rise times on the input and output of an amplifier. (I had inserted a T-connector, creating an impedance mismatch, which showed up as a reflection. When I saw the glitch on the trace, I puzzled over it for about half a minute before it dawned on me what I was seeing.) However, if this guy feeds his spectrum analyzer with a tracking generator and tee's off to the unknown cable, the reflected energy from the cable fault will generate a comb of nulls. The frequency of the first (lowest frequency) null will indicate either the quarter- wave point or the half-wave point (depending on whether the fault is a short or an open. You'd need to know the velocity factor, too. I don't see how VSWR would come into play. Yeah, I'd really rather have a TDR, hi-hi. By the way, this arrangement also works to approximate the resonant frequency of an antenna. The comb of nulls will flatten out -- be less pronounced -- at the resonant frequency, due to the incident energy being radiated, rather than reflected. "Sal" (KD6VKW) |
Distance to Fault
K1TTT wrote:
if the school project is requiring a spectrum analyzer to measure distance to a fault location then its a pretty dumb project. there are much better and more straight forward ways to do that. Not if the purpose of the school project is education, and understanding the relationship between frequency domain and time domain. |
Distance to Fault
On 11/03/2010 03:22 AM, ashwanthh wrote:
Hi all, For my project, I want to perform Distance to fault measurement in the cable connecting the antenna. The principle is that a signal should be sent to the cable and using the forward and reflected voltage VSWR should be calculated and should be viewed in a spectrum analyzer. My question is 1. What transducer is used to detect the transmitted/reflected signals from the cable? 2. Should the transducer be connected in series or parallel? Anyone, Please help Or just use a grid dip meter. 1) Determine if the fault is a short or an open circuit with any tester. 2) Connect a small loop at the input of the cable. 3) Couple the dip meter and read the minimum frequency with a dip. 4) If a short circuit === Distance(meters) = 75 * cable velocity factor / frequency (MHz) 5) If an open circuit === Distance(meters)= 150 * cable velocity factor / frequency (MHz) -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map foF2: http://1fcr.com.ar |
Distance to Fault
I have a similar practical real problem.
A vertical 120 ft working 75 ohm coax line hung from the top. The soft foamed dielectric MAY be crushed under the cable's own weight 10 ft from the top end. If it is I must change the hanging system. ASSUMPTIONS - the dielectric is crushed to 1/2 the original thickness over 2 inches length 10 ft from the far end - I can put a good quality non-reflecting 75 ohm load at the far end. QUESTION Can I expect such a defect to generate a detectable reflection at 2GHz? |
Distance to Fault
A good time-domain reflectometer Thank you Barry! Would TV/FM transmitter and cell BTS installers normally have such a device? I don't know any ham who has one but these days I am talking to professional antenna people. And, by the way, if one thing I am working on pans out, they'll come to install a BTS right atop my condo. (The challenge will be to convince them to let me hang HF wires from the same tower :-) |
Distance to Fault
spamhog wrote:
A good time-domain reflectometer Thank you Barry! Would TV/FM transmitter and cell BTS installers normally have such a device? I don't know any ham who has one but these days I am talking to professional antenna people. And, by the way, if one thing I am working on pans out, they'll come to install a BTS right atop my condo. (The challenge will be to convince them to let me hang HF wires from the same tower :-) If you have an oscilloscope, a pulse generator and a calculator you have a TDR. If you don't have a pulse generator it takes about 2 IC's to build one. -- Jim Pennino Remove .spam.sux to reply. |
Distance to Fault
wrote in message
... spamhog wrote: A good time-domain reflectometer Thank you Barry! Would TV/FM transmitter and cell BTS installers normally have such a device? I don't know any ham who has one but these days I am talking to professional antenna people. And, by the way, if one thing I am working on pans out, they'll come to install a BTS right atop my condo. (The challenge will be to convince them to let me hang HF wires from the same tower :-) If you have an oscilloscope, a pulse generator and a calculator you have a TDR. If you don't have a pulse generator it takes about 2 IC's to build one. -- Jim Pennino You would need an extremely fast rise-time pulse generator and an exceptionally fast oscilloscope to match a typical commercial TDR. The Tek 1502 unit I used when at Eastman's Research Labs had a pulse rise-time of 140 picoseconds, and the internal display was a sampling scope with a bandwidth of over 3 GHz. I now have a Tek 1S2 plug-in for my Tek 585A oscilloscope at home. It's fine for amateur use, but don't expect to see the tiny impedance "burbles" of BNC connectors with it. {And carrying around a 1502 is _FAR_ easier than the 585!} Quoting from a Tektronix application note on TDR resolution: the resolution limit wherein two discontinuities or changes on the transmission line begin to merge together Per this definition, the resolution limit is: half the 10% to 90% risetime or 90% to 10% fall time (depending on whether the TDR response is calibrated with a short or open circuit). To convert this to distance, you need to know the velocity factor for the cable you are testing and the speed of light. Your installers may not always carry a TDR with them, but I would bet they have access to one. 73, Barry WA4VZQ |
Distance to Fault
Barry wrote:
wrote in message ... spamhog wrote: A good time-domain reflectometer Thank you Barry! Would TV/FM transmitter and cell BTS installers normally have such a device? I don't know any ham who has one but these days I am talking to professional antenna people. And, by the way, if one thing I am working on pans out, they'll come to install a BTS right atop my condo. (The challenge will be to convince them to let me hang HF wires from the same tower :-) If you have an oscilloscope, a pulse generator and a calculator you have a TDR. If you don't have a pulse generator it takes about 2 IC's to build one. -- Jim Pennino You would need an extremely fast rise-time pulse generator and an exceptionally fast oscilloscope to match a typical commercial TDR. Yeah but in the real world you don't usually need to match a commercial TDR to find a fault. Back in the days of ethernet over RG-58 I was able to find lots of poorly attached connectors and cables crushed by the electricians that installed them (there were no network engineers and installers in those days) with a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50 Ohm line driver. The accuracy of the measurement for fault finding doesn't need to be much better than a few feet to be able to find the fault visually once you know about where it is and faults at the end are immediately obvious. If the task is to find faults, a 'scope and simple pulse generator works just fine. If the task is to certify 6 inches of hard line to GHz to some mil-spec, you probably want something more sophisticated. -- Jim Pennino Remove .spam.sux to reply. |
Distance to Fault
On Tue, 9 Nov 2010 22:01:27 -0500, "Barry" wrote:
You would need an extremely fast rise-time pulse generator and an This can be easily be obtained (provided you can find one) from a mercury wetted switch. Typical rise times in the sub-nano to nanoseconds. exceptionally fast oscilloscope Which, is something you would have to spend money on - most assuredly. OR Use a wide field microscope against the CRT being driven at the deflection plates (the early design for these applications of extremely fast pulse measurements) which can be far more affordable. 73's Richard Clark, KB7QHC |
Distance to Fault
wrote in message
... Yeah but in the real world you don't usually need to match a commercial TDR to find a fault. Back in the days of ethernet over RG-58 I was able to find lots of poorly attached connectors and cables crushed by the electricians that installed them (there were no network engineers and installers in those days) with a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50 Ohm line driver. Can you detect where RG-58 was kinked and then straightened out with the damaged area being less than 1/2 inch? Likewise can you detect an abraded shield leaving a 1/4 inch hole in the shielding? An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of how much your line driver sharpens the edges of the pulse, but a good line driver should provide rise and fall times of 5 nanoseconds. The original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters. With a risetime of 5 nS, your resolution will be about 70 times worse (140 cm). Add to this the 100 MHz limitation of the scope (an additional 10 nS), and it becomes far worse. Yes, you can easily detect shorts and open circuits with your setup, but you will not be able to detect the 1 to 2 inches or so where the center conductor has migrated in the foamed dielectric from hanging the cable over a building edge. The accuracy of the measurement for fault finding doesn't need to be much better than a few feet to be able to find the fault visually once you know about where it is and faults at the end are immediately obvious. If the task is to find {gross} faults, a 'scope and simple pulse generator works just fine. If the task is to certify 6 inches of hard line to GHz to some mil-spec, you probably want something more sophisticated. The kind of fault Spamhog originally described, a 75 ohm cable dropping to less than 50 ohms over two inches and going back to 75 ohms, will not be detected by your setup. 73, Barry WA4VZQ |
Distance to Fault
Barry wrote:
wrote in message ... Yeah but in the real world you don't usually need to match a commercial TDR to find a fault. Back in the days of ethernet over RG-58 I was able to find lots of poorly attached connectors and cables crushed by the electricians that installed them (there were no network engineers and installers in those days) with a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50 Ohm line driver. Can you detect where RG-58 was kinked and then straightened out with the damaged area being less than 1/2 inch? Likewise can you detect an abraded shield leaving a 1/4 inch hole in the shielding? I was able to detect where cables were squashed by overly tight bundle lacing and bending over a short radius. It got me close enough that a visual inspection of a few feet found the "bad" place. An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of how much your line driver sharpens the edges of the pulse, but a good line driver should provide rise and fall times of 5 nanoseconds. The original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters. With a risetime of 5 nS, your resolution will be about 70 times worse (140 cm). Add to this the 100 MHz limitation of the scope (an additional 10 nS), and it becomes far worse. Like I said, if you have a pair of eyes, all you have to do is get close. And in any case, the solution for a cable run that is hosed somewhere in the middle is to replace the entire section. Cutting a section out and putting in connectors to splice the cable is just asking for more problems. Yes, you can easily detect shorts and open circuits with your setup, but you will not be able to detect the 1 to 2 inches or so where the center conductor has migrated in the foamed dielectric from hanging the cable over a building edge. Yet I could, so either I'm a lier or you are overestimating how hard it is in the real world. The accuracy of the measurement for fault finding doesn't need to be much better than a few feet to be able to find the fault visually once you know about where it is and faults at the end are immediately obvious. If the task is to find {gross} faults, a 'scope and simple pulse generator works just fine. If the task is to certify 6 inches of hard line to GHz to some mil-spec, you probably want something more sophisticated. The kind of fault Spamhog originally described, a 75 ohm cable dropping to less than 50 ohms over two inches and going back to 75 ohms, will not be detected by your setup. It would detect that there was an impedance lump. It would not tell you it was a 50 Ohm lump. -- Jim Pennino Remove .spam.sux to reply. |
Distance to Fault
On 11/10/2010 11:56 AM, Barry wrote:
wrote in message ... Yeah but in the real world you don't usually need to match a commercial TDR to find a fault. Back in the days of ethernet over RG-58 I was able to find lots of poorly attached connectors and cables crushed by the electricians that installed them (there were no network engineers and installers in those days) with a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50 Ohm line driver. Can you detect where RG-58 was kinked and then straightened out with the damaged area being less than 1/2 inch? Likewise can you detect an abraded shield leaving a 1/4 inch hole in the shielding? An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of how much your line driver sharpens the edges of the pulse, but a good line driver should provide rise and fall times of 5 nanoseconds. The original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters. With a risetime of 5 nS, your resolution will be about 70 times worse (140 cm). Add to this the 100 MHz limitation of the scope (an additional 10 nS), and it becomes far worse. Yes, you can easily detect shorts and open circuits with your setup, but you will not be able to detect the 1 to 2 inches or so where the center conductor has migrated in the foamed dielectric from hanging the cable over a building edge. The accuracy of the measurement for fault finding doesn't need to be much better than a few feet to be able to find the fault visually once you know about where it is and faults at the end are immediately obvious. If the task is to find {gross} faults, a 'scope and simple pulse generator works just fine. If the task is to certify 6 inches of hard line to GHz to some mil-spec, you probably want something more sophisticated. The kind of fault Spamhog originally described, a 75 ohm cable dropping to less than 50 ohms over two inches and going back to 75 ohms, will not be detected by your setup. 73, Barry WA4VZQ Are you telling all the rest of us that may have inferior equipment that we shouldn't even try to measure things? Just asking. And we aren't all dumb enough not to understand what gives us resolution. tom K0TAR |
Distance to Fault
Jim,
I think you need to go back and read Spamhog's original question. He was trying to determine whether the center conductor of a piece of coax had migrated away from center. He knew where this might have happened - 10 feet from the end, and the migration would have occurred over less than two inches. So the question of locating where the problem might be is moot. What is needed is a measurement of the cable impedance in this region. First, let us get an estimate of what the impedance of the damaged section might be. Spamhog was using RG-6 cable with a foamed polyethylene dielectric. Its velocity factor is 0.85 making its relative permittivity 1.384. The center conductor is 1 mm, and the normal diameter of the center insulator is 4.7 mm. The thickness of the insulator is 1.85 mm. We need to know the impedance if the center conductor had migrated 0.925 mm toward the jacket. For a quick estimate, use the formula for off-center coax (http://www.microwaves101.com/encyclo...offcenter.cfm). This gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in the non-distorted coax. A TDR displays the reflection coefficient from -1 (short) to +1 (open). Here the reflection coefficient is -0.06. So the TDR trace will drop from the center line by 6% for 200 picoseconds. If your 100 MHz scope has a typical Gaussian response, its rise time is at least 3.5 nanoseconds. Do you really think that your oscilloscope trace will clearly show the 200 picosecond dip? Even with the wide-screen magnifier that KB7QHC suggested, I think you will have great difficulty seeing this. 73, Barry WA4VZQ |
Distance to Fault
"tom" wrote in message
. net... Are you telling all the rest of us that may have inferior equipment that we shouldn't even try to measure things? No. In fact, playing with a scope and a pulse generator is quite educational. And we aren't all dumb enough not to understand what gives us resolution. tom K0TAR Trying to measure the thickness of a single sheet of paper with a ruler graduated in eighth's of an inch is analogous. About all you can say with certainty is that the paper is much thinner than 1/8 inch. My last post gave a nominal value for the reflection coefficient of coax with a migrated center conductor like N1PR postulated. At 2 GHz, the 2-inch section will be about 0.4 wavelength. Will you get detectible reflections? Yes. 73, Barry WA4VZQ |
Distance to Fault
Barry wrote:
Jim, I think you need to go back and read Spamhog's original question. He was trying to determine whether the center conductor of a piece of coax had migrated away from center. He knew where this might have happened - 10 feet from the end, and the migration would have occurred over less than two inches. So the question of locating where the problem might be is moot. What is needed is a measurement of the cable impedance in this region. First, let us get an estimate of what the impedance of the damaged section might be. Spamhog was using RG-6 cable with a foamed polyethylene dielectric. Its velocity factor is 0.85 making its relative permittivity 1.384. The center conductor is 1 mm, and the normal diameter of the center insulator is 4.7 mm. The thickness of the insulator is 1.85 mm. We need to know the impedance if the center conductor had migrated 0.925 mm toward the jacket. For a quick estimate, use the formula for off-center coax (http://www.microwaves101.com/encyclo...offcenter.cfm). This gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in the non-distorted coax. A TDR displays the reflection coefficient from -1 (short) to +1 (open). Here the reflection coefficient is -0.06. So the TDR trace will drop from the center line by 6% for 200 picoseconds. If your 100 MHz scope has a typical Gaussian response, its rise time is at least 3.5 nanoseconds. Do you really think that your oscilloscope trace will clearly show the 200 picosecond dip? Even with the wide-screen magnifier that KB7QHC suggested, I think you will have great difficulty seeing this. 73, Barry WA4VZQ The bandwidth of the 'scope will make the trace have a ripple intead of the nice, sharp bump you would get from a faster 'scope. Spamhog's original statement was that the cable was crushed to half diameter for about 2 inches as I recall. I was able to see ripples in the display for cables with less crush then that which were on the order of 1/4 wide. The bottom line is the faster the 'scope you use and the faster the rise time of the applied pulse, the better the meaurement. And since this is a hobby and not building man rated space craft, I would say try whatever you can get your hands on for free and see what happens. Or spend eternity arguing whether or not it is possible to do. -- Jim Pennino Remove .spam.sux to reply. |
Distance to Fault
In article ,
"Barry" wrote: "tom" wrote in message . net... Are you telling all the rest of us that may have inferior equipment that we shouldn't even try to measure things? No. In fact, playing with a scope and a pulse generator is quite educational. And we aren't all dumb enough not to understand what gives us resolution. tom K0TAR Trying to measure the thickness of a single sheet of paper with a ruler graduated in eighth's of an inch is analogous. About all you can say with certainty is that the paper is much thinner than 1/8 inch. My last post gave a nominal value for the reflection coefficient of coax with a migrated center conductor like N1PR postulated. At 2 GHz, the 2-inch section will be about 0.4 wavelength. Will you get detectible reflections? Yes. 73, Barry WA4VZQ Well not exactly, IF, one were to take a "Stack" of said paper, that was exactly 1/8 of an Inch thick, then count the number of sheets, and divide the 1/8 inch by the number of sheets, One could get a VERY close approximation of the thickness of a single sheet...... Duh... |
Distance to Fault
"you" wrote in message
... In article , "Barry" wrote: Trying to measure the thickness of a single sheet of paper with a ruler graduated in eighth's of an inch is analogous. About all you can say with certainty is that the paper is much thinner than 1/8 inch. Well not exactly, IF, one were to take a "Stack" of said paper, that was exactly 1/8 of an Inch thick, then count the number of sheets, and divide the 1/8 inch by the number of sheets, One could get a VERY close approximation of the thickness of a single sheet...... Duh... Note that I said the thickness of a _single_ sheet. Your method would give an approximation of the average thickness of the sheets used to make up the stack. But if you have ever worked with a Fourdriner machine that is not controlled well, i.e. the thickness varies with time due to any number of variables, you would need to know the thickness of a number of individual sheets to determine which variable is causing problems. Typical variables that would cause the thickness of a sheet to vary include, but are not limited to, head box level, "wire" speed, pulp "consistency", felt pressure, calendar pressure, the type of wood used, the lignin removal process (Kraft, caustic, or solvent), Crown Control pressure, drying roll steam pressure, and at least a dozen more. For example, sloshing or waves in the head box would cause thickness variations in both the machine and cross direction, while "wire" speed would only cause thickness variations in the machine direction. So there are real reasons for measuring the thickness of a single sheet. |
Distance to Fault
On Thu, 11 Nov 2010 18:17:16 -0500, "Barry" wrote:
But if you have ever worked with a Fourdriner machine that is not controlled well, i.e. the thickness varies with time due to any number of variables, I have. That and a year's worth of studying paper process chemistry for precision (sic) measurement of K and Kappa. When it gets down to what you describe as "not controlled well" that is more the definition of a Paper Mill that is destined for bankruptcy before the end of one week - if not a weekend. I've seen the production floor flood with product when the process encounters a bottle neck. The production pipeline is enormous with a lot of intertia. Measuring a number of sheets achieves sufficient accuracy. 73's Richard Clark, KB7QHC |
Distance to Fault
"Richard Clark" wrote in message
... On Thu, 11 Nov 2010 18:17:16 -0500, "Barry" wrote: But if you have ever worked with a Fourdriner machine that is not controlled well, i.e. the thickness varies with time due to any number of variables, I have. That and a year's worth of studying paper process chemistry for precision (sic) measurement of K and Kappa. It has been about 30 years ago, but I remember that while Kappa is related to the amount of lignin remaining in the pulp after cooking, every wet end process had its own peculiar measurement. I worked on the online measurement of Kappa for several solvent pulping processes that were to produce dissolving pulp for Eastman's cellulose ester process. Once the vendor that supplied our pulp got wind that we were researching solvent pulping, they quickly dropped their price and raised their quality enough that we halted work on our own pulping processes. One process we were considering involved the use of supercritical CO2. We never got beyond early pilot plant work on this. I fully understand the {sic} in your statement. When it gets down to what you describe as "not controlled well" that is more the definition of a Paper Mill that is destined for bankruptcy before the end of one week - if not a weekend. I've seen the production floor flood with product when the process encounters a bottle neck. The production pipeline is enormous with a lot of intertia. I have toured quite a few paper mills over the years with both Kraft and caustic pulping. I never really got to see one of the large continuous digesters though. We had a small caustic plant that used hardwoods in town with us. The odors were interesting - mercaptans by them and the occasional butyric acid spill by us. I always wonder how long the cellulose acetate business will last. While cigarette smoking is declining in the US, increasing demand from China seems to more than make up for the loss. Going back to the original TDR discussion, probably the most interesting use I ever put one through was in diagnosing thermocouple problems in our coal gasifier. I saw that the platinum-rhodium thermocouples had water in them. We finally got a metallurgist to do an "autopsy" on one of the thermocouples pulled out during a shutdown. When we broke open the thermocouple, water ran out. We saw a buildup of salts from the Saureisen cement at exactly the place that I predicted. Water was diffusing in through a silicon carbide protection tube and an Inconel sheath. Of course the gasifier operated at high temperature and pressure. A Nastran simulation of the thermowell showed that the temperature at that point was low enough for condensation to occur. Some extra insulation on the external flange solved that problem! 73, Barry WA4VZQ How the heck did we get off topic this far? :-) |
Distance to Fault
On 11/11/2010 8:01 PM, Barry wrote:
"Richard wrote in message snip 73, Barry WA4VZQ How the heck did we get off topic this far? :-) Keep it up. It's interesting. tom K0TAR |
Distance to Fault
On Thu, 11 Nov 2010 20:07:31 -0600, tom wrote:
On 11/11/2010 8:01 PM, Barry wrote: "Richard wrote in message snip 73, Barry WA4VZQ How the heck did we get off topic this far? :-) Keep it up. It's interesting. If that means the lore of paper processing, I was peripherally involved in trying to characterize Black Liquor (http://en.wikipedia.org/wiki/Black_liquor). Gladly I was at the extreme periphery (I am sure my sense of smell was debilitated in those years). However, returning to things RF, I also was tasked with calibrating a wood moisture meter. It used an HF RF source as part of a Z meter were Z was correlated to moisture content. The probe was a fixed ring surrounding point probes much like a Kelvin Bridge. It has always seemed paradoxical that steam is used to dry wood. Now, finding calibrated wood was no easy task. And if we found it, we would have to first validate it (sort of a circular form of Sysphus' task). My best guess at that work set to us was that we gun-decked it. It was some years later that that task came around again when I was measuring K and Kappa as I averred. Here came the requirment for "Bone Dry" paper. Try as you might to dry paper bone dry (absolutely no water content), that as soon as it comes out of the oven it is almost back up to several percent water content (15% to 20% would be the end point). Ironies compound in that I now live in a community where 100 years ago our cedar wood mills produced nearly a Billion shingles in a year. 73's Richard Clark, KB7QHC |
Distance to Fault [now chemistry]
"Richard Clark" wrote in message
... If that means the lore of paper processing, I was peripherally involved in trying to characterize Black Liquor (http://en.wikipedia.org/wiki/Black_liquor). Gladly I was at the extreme periphery (I am sure my sense of smell was debilitated in those years). I once had to check on an instrument I had designed that was used in our cellulose acetate butyrate esterification area. (Cellulose Acetate Butyrate is used to make Xelite and Craftsman tool handles.) Acetic anhydride and butyric acid were used for esterification. For those not familiar with the odor, think of a mixture of rancid butter, vomit, and the contents of a cat's week old litter box. Readers who watched the last season of Whale Wars on TV saw it being used against the Japanese whaling ship and its cargo. Since I was unfamiliar with the building, one of my technicians who had installed the instrument went with me. We stepped off the elevator and walked into the processing area. What had been an extremely offensive odor in the elevator suddenly hit me like a ton of bricks. I couldn't see as my eyes were tearing badly, and it hurt to breathe. My tech told me to stand still for a moment or two and things would get better. Within thirty seconds to a minute, my eyes cleared up, my sinuses opened up, and my breathing became normal again. I could no longer smell the mixed acids at all. We checked the calibration of the instrument and talked to the operators about their experiences in using it. We then left the building and walked the half mile back to the research labs. Eastman was still using dimethyl-terephthalate in their polyester production at that time, and as I waked past the DMT distillation area, I could easily smell the DMT and the Dowtherm heat transfer fluid. My technician explained that as soon as we returned to the research complex, we should drop by our group leader's office to let him know where we had been. We had just opened his door when he said that he could smell where we had been and to get the heck out of his office! Of course, we couldn't smell our strong odor at all. Fortunately my technician had warned me to wear old clothes, old shoes, and an old belt for when I came home that evening, my wife made me strip in the foyer and go stright to the shower. She washed the clothes, but the shoes and belt had to hang outside for a week. It seems that leather is particularly bad about absorbing the odor. However, returning to things RF, I also was tasked with calibrating a wood moisture meter. It used an HF RF source as part of a Z meter were Z was correlated to moisture content. The probe was a fixed ring surrounding point probes much like a Kelvin Bridge. It has always seemed paradoxical that steam is used to dry wood. We used microwave absorption to measure moisture in cellulose acetate filter tow (cigarette filters are made from this). Operators would cut off a length of tow and stuff a weighed amount in a short X-band waveguide. Attenuation was proportional to moisture. Now, finding calibrated wood was no easy task. And if we found it, we would have to first validate it (sort of a circular form of Sysphus' task). My best guess at that work set to us was that we gun-decked it. Calibration of the microwave quipment was checked by weekly sending samples to be tested by nuclear magnetic resonance. We also used samples kept in jars over certain salt solutions that maintaned a constant relative humidity in the jar. See http://www.conservationphysics.org/satslt/satsalt.php for some typical solutions. To bring a little electronics into this, the same principle is used in electrolytic capacitors where glycols, sorbitol, and various salts are used in the electrolyte to insure that it remains moist. Electrolytic capacitors are about the only electronic components that are harmed by an exceptionally dry environment. It was some years later that that task came around again when I was measuring K and Kappa as I averred. Here came the requirment for "Bone Dry" paper. Try as you might to dry paper bone dry (absolutely no water content), that as soon as it comes out of the oven it is almost back up to several percent water content (15% to 20% would be the end point). You can get close to "bone dry" in a sealed container containing zeolite molecular sieves baked at 600 C. But as soon as the container is opened, moisture rushes in from the surround air. We would use glove boxes pressurized slightly with nitrogen obtained from a liquid nitrogen source when we needed a really low humidity environment. Ironies compound in that I now live in a community where 100 years ago our cedar wood mills produced nearly a Billion shingles in a year. 73's Richard Clark, KB7QHC I always enjoyed visiting the University of Washington where Eastman was on the industrial advisory board of the Center for Process Analytical Chemistry. I never order salmon from any restaurant in the southeast after eating the wonderful, fresh-caught salmon you have there! 73, Barry WA4VZQ P.S. You mentioned black liquor from the Kraft process. I once visited the Glidden-Durkee plant in Jacksonville, FL. They originally produced turpentine from pine stumps, but now they start with tall oil extracted from black liquor. It is hard to believe that essential oils, perfume stock, and flavorings are all derived from turpentine. |
Distance to Fault
On Nov 10, 7:44*pm, "Barry" wrote:
Jim, I think you need to go back and read Spamhog's original question. *He was trying to determine whether the center conductor of a piece of coax had migrated away from center. *He knew where this might have happened - 10 feet from the end, and the migration would have occurred over less than two inches. *So the question of locating where the problem might be is moot. *What is needed is a measurement of the cable impedance in this region. First, let us get an estimate of what the impedance of the damaged section might be. *Spamhog was using RG-6 cable with a foamed polyethylene dielectric. *Its velocity factor is 0.85 making its relative permittivity 1.384. *The center conductor is 1 mm, and the normal diameter of the center insulator is 4.7 mm. *The thickness of the insulator is 1.85 mm. *We need to know the impedance if the center conductor had migrated 0.925 mm toward the jacket. For a quick estimate, use the formula for off-center coax (http://www.microwaves101.com/encyclo...offcenter.cfm). *This gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in the non-distorted coax. *A TDR displays the reflection coefficient from -1 (short) to +1 (open). *Here the reflection coefficient is -0.06.. So the TDR trace will drop from the center line by 6% for 200 picoseconds. If your 100 MHz scope has a typical Gaussian response, its rise time is at least 3.5 nanoseconds. *Do you really think that your oscilloscope trace will clearly show the 200 picosecond dip? *Even with the wide-screen magnifier that KB7QHC suggested, I think you will have great difficulty seeing this. * * 73, Barry *WA4VZQ It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. In retirement, I no longer have access to test equipment that would support my point. |
Distance to Fault
On Sun, 14 Nov 2010 18:22:54 -0800 (PST), "Sal M. Onella"
wrote: It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. The simple correlation between risetime and bandwidth is roughly: BW = 1/(3·t) Unfortunately, peaking bandwidth can degrade risetime, and vice-versa. O'scopes have a lot of conflicting adjustments within them. The TEK545 had something like an 8 to 12 hour tune-up procedure for the average bench tech (a calibration specialist could do it in 3 to 4 hours). 73's Richard Clark, KB7QHC |
Distance to Fault
"Sal M. Onella" wrote in message
... It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. In retirement, I no longer have access to test equipment that would support my point. See the following article: http://www.eetimes.com/design/microw...ight-Bandwidth There are two paragraphs in the article of importance he "All oscilloscopes exhibit a low-pass frequency response that rolls-off at higher frequencies, as shown in Figure 1. Most scopes with bandwidth specifications of 1GHz and below typically have what is called a Gaussian response, which exhibits a slow roll-off characteristic beginning at approximately one-third the -3dB frequency. Oscilloscopes with bandwidth specifications greater than 1GHz typically have a maximally-flat frequency response, as shown in Figure 2. This type of response usually exhibits a flatter in-band response with a sharper roll-off characteristic near the -3dB frequency. "Closely related to an oscilloscope's bandwidth specification is its rise time specification. Scopes with a Gaussian-type response will have an approximate rise time of 0.35/f(sub)BW based on a 10- to 90-percent criterion. Scopes with a maximally-flat response typically have rise time specifications in the range of 0.4/f(sub)BW depending on the sharpness of the frequency roll-off characteristic. But it is important to remember that a scope's rise time is not the fastest edge speed that the oscilloscope can accurately measure. It is the fastest edge speed the scope can possibly produce if the input signal has a theoretical infinitely fast rise time (0 ps). Although this theoretical specification is impossible to test (since pulse generators don't have infinitely fast edges) from a practical perspective, you can test your oscilloscope's rise time by inputting a pulse that has edge speeds that are 3 to 5 times faster than the scope's rise time specification." 73, Dr. Barry L. Ornitz WA4VZQ |
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