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measuring cable loss
On Aug 13, 1:09 pm, Jim Lux wrote:
K7ITM wrote: On Aug 13, 11:50 am, Jim Lux wrote: John Ferrell wrote: On Thu, 9 Aug 2007 08:13:45 -0400, "Jimmie D" wrote: I need to measure the loss of aproximately 200ft of coax @ a freq of 1Ghz. The normal procedure for doing this is to inject a signal at one end and measure the power out at the other. Using available test eqipment this is a real pain to do. I propose to disconnect the cable at the top of the tower terminating it in either a short or open and measure the return loss at the source end. I have done this and measured 6.75 db and I am assuming that 1/2 of this would be the actual loss of the cable. These numbers do fall within the established norms for this cable. Can you think of a reason thiis method would not be valid? Jimmie This is way too complicated for me! My solution would be to build/buy an RF probe and permanently mount it at the top of the tower. Bring a pair of wires (Coax if you want it to look really professional) down to the bottom and measure it whenever or even all the time. Considering he needs sub 1dB accuracy, this is challenging..it would work if you assume your RF probe never needs calibration and is stable over the environmental range of interest. Not a trivial thing to do. A diode and a voltmeter certainly won't do it. (A typical diode detector might vary 1 dB over a 20 degree C range.. judging from the Krytar 700 series data sheet I have sitting here. Granted that's a microwave detector (100MHz to 40 GHz), but I'd expect similar from most other diodes. I've given the link to an Agilent Ap note that describes various detectors in excruciating detail. A diode, voltmeter, and temperature sensor might work, though. useful stuff athttp://rfdesign.com/mag/608RFDF2.pdfhttp://cp.literature.agilent.com/... Seems like modern RF detector ICs offer much better stability than diodes. An AD8302, for example, has a typical +/- 0.25dB variation from -40C to +85C, with a -30dBm signal level. indeed... The temperature variation could be calibrated before installation; if necessary, an especially temperature-stable part could be selected from a batch. Then knowing the ambient within 20C would be sufficient. You'd need to arrange sampling at a low level, which could be a well-constructed 90 degree hybrid. or, even simpler, what about a resistive tap (or a pair of resistive taps separated by a short length of transmission line). If you're sending, say, 100W (+50dBm) up the wire, and you want, say, -30dBm out, you need a 80 dB coupler. Or, something like a 50k resistor into a 50 ohm load will be about 60 dB down, and you could put a 10-20dB pad in before the detector. Calibration would take care of the coupling ratio, although, you might want to be careful about the tempco of the resistor. .... The OP said this is at 1GHz. It's really tough to get a reliable resistive divider at 1GHz, with that sort of ratio. Actually, a capacitive divider probably stands a better chance of working, though getting that really right isn't trivial. (We used to worry about variation in humidity and atmospheric pressure affecting the dielectric constant of air, in using a capacitive sampler...though admittedly that was for work to a level well beyond 1dB accuracy.) I am rather fond of the coupled-line hybrid idea: it can be built in a way that everything stays ratiometric, so the coupling ratio is very nearly constant over temperature, and of course the directionality lets you observe things you can't just from monitoring voltage at a point. It's possible to build one with low coupling without too much trouble; -60dB coupling isn't out of the question, for sure. I'm imagining a design I could make reliably with simple machine tools that would work well for the OP's application: 100 watts at about 1GHz as I recall in the through line, and coupling on the order of -60dB to get to about -10dBm coupled power and have negligible effect on the through line. There's a free fields solver software package that will accurately predict the coupling, and with the right design and normal machine shop tolerances the coupling and impedance should be accurate to a fraction of a dB and better than a percent, respectively. Perhaps I can run some examples to see if I'm off-base on that, but that's what my mental calculations tell me at the moment. Cheers, Tom |
#2
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measuring cable loss
I am rather fond of the coupled-line hybrid idea: it can be built in a way that everything stays ratiometric, so the coupling ratio is very nearly constant over temperature, and of course the directionality lets you observe things you can't just from monitoring voltage at a point. It's possible to build one with low coupling without too much trouble; -60dB coupling isn't out of the question, for sure. I'm imagining a design I could make reliably with simple machine tools that would work well for the OP's application: 100 watts at about 1GHz as I recall in the through line, and coupling on the order of -60dB to get to about -10dBm coupled power and have negligible effect on the through line. There's a free fields solver software package that will accurately predict the coupling, and with the right design and normal machine shop tolerances the coupling and impedance should be accurate to a fraction of a dB and better than a percent, respectively. Perhaps I can run some examples to see if I'm off-base on that, but that's what my mental calculations tell me at the moment. Actually, the exact coupling ratio probably isn't important in this application, because it could be "calibrated out". Stability would be a bigger concern, and it's certainly possible to design a coupler that is very temperature stable by choosing the right dimensions so that things change in the right ratios. |
#3
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measuring cable loss
On Aug 14, 8:45 am, Jim Lux wrote:
I am rather fond of the coupled-line hybrid idea: it can be built in a way that everything stays ratiometric, so the coupling ratio is very nearly constant over temperature, and of course the directionality lets you observe things you can't just from monitoring voltage at a point. It's possible to build one with low coupling without too much trouble; -60dB coupling isn't out of the question, for sure. I'm imagining a design I could make reliably with simple machine tools that would work well for the OP's application: 100 watts at about 1GHz as I recall in the through line, and coupling on the order of -60dB to get to about -10dBm coupled power and have negligible effect on the through line. There's a free fields solver software package that will accurately predict the coupling, and with the right design and normal machine shop tolerances the coupling and impedance should be accurate to a fraction of a dB and better than a percent, respectively. Perhaps I can run some examples to see if I'm off-base on that, but that's what my mental calculations tell me at the moment. Actually, the exact coupling ratio probably isn't important in this application, because it could be "calibrated out". Stability would be a bigger concern, and it's certainly possible to design a coupler that is very temperature stable by choosing the right dimensions so that things change in the right ratios. Bingo. It's that ratiometric thing that is a big plus for stability. In a coupler made of all the same metal, or at least metals that have nearly equal coefficients of expansion, the ratios stay the same, and it's the dimensional ratios that establish the coupling and impedances, not the absolute size. Actually, the change in length does matter, but if you make the assembly a quarter wave long, the d(coupling)/d(length) is zero at that point anyway. In any event, I suppose the thermal coefficient of expansion of metals you'd be most likely to use is small enough that you'd be fine with a shorter coupler. There doesn't need to be anything terribly complex about the geometry of the whole thing, either. It's probably safe to say that changes in the dielectric constant of air due to air pressure and humidity aren't going to be significant in this case. ;-) Cheers, Tom |
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measuring cable loss
On Tue, 14 Aug 2007 09:53:31 -0700, K7ITM wrote:
Bingo. It's that ratiometric thing that is a big plus for stability. In a coupler made of all the same metal, or at least metals that have nearly equal coefficients of expansion, the ratios stay the same, and it's the dimensional ratios that establish the coupling and impedances, not the absolute size. Actually, the change in length does matter, but if you make the assembly a quarter wave long, the d(coupling)/d(length) is zero at that point anyway. In any event, I suppose the thermal coefficient of expansion of metals you'd be most likely to use is small enough that you'd be fine with a shorter coupler. There doesn't need to be anything terribly complex about the geometry of the whole thing, either. It's probably safe to say that changes in the dielectric constant of air due to air pressure and humidity aren't going to be significant in this case. ;-) Cheers, Tom Tom, I thought this thread concerned measurement of attenuation in transmission lines. On the 11th I posted a precedure that involves measuring the line input impedances with the line terminated in both a short circuit and an open circuit, then plugging the measured data into a BASIC program that outputs the attenuation, complex Zo, and electrical length. My thoughts were that this procedure gives results with more accuracy and precision than the procedures discussed before my post appeared. However, I noticed that my post drew zero response. Is my procedure out-of-line, or out dated? Walt, W2DU |
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measuring cable loss
On Tue, 14 Aug 2007 15:01:32 -0400, Walter Maxwell
wrote: My thoughts were that this procedure gives results with more accuracy and precision than the procedures discussed before my post appeared. However, I noticed that my post drew zero response. Is my procedure out-of-line, or out dated? Hi Walt, I provided a posting on how to determine the extent of error that was similarly ignored - don't feel bad. Accuracy isn't all that its cracked up to be. :-) 73's Richard Clark, KB7QHC |
#6
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measuring cable loss
On Aug 14, 12:01 pm, Walter Maxwell wrote:
On Tue, 14 Aug 2007 09:53:31 -0700, K7ITM wrote: Bingo. It's that ratiometric thing that is a big plus for stability. In a coupler made of all the same metal, or at least metals that have nearly equal coefficients of expansion, the ratios stay the same, and it's the dimensional ratios that establish the coupling and impedances, not the absolute size. Actually, the change in length does matter, but if you make the assembly a quarter wave long, the d(coupling)/d(length) is zero at that point anyway. In any event, I suppose the thermal coefficient of expansion of metals you'd be most likely to use is small enough that you'd be fine with a shorter coupler. There doesn't need to be anything terribly complex about the geometry of the whole thing, either. It's probably safe to say that changes in the dielectric constant of air due to air pressure and humidity aren't going to be significant in this case. ;-) Cheers, Tom Tom, I thought this thread concerned measurement of attenuation in transmission lines. On the 11th I posted a precedure that involves measuring the line input impedances with the line terminated in both a short circuit and an open circuit, then plugging the measured data into a BASIC program that outputs the attenuation, complex Zo, and electrical length. My thoughts were that this procedure gives results with more accuracy and precision than the procedures discussed before my post appeared. However, I noticed that my post drew zero response. Is my procedure out-of-line, or out dated? Walt, W2DU Hi Walt, Well, yes, the original posting asked if it was reasonable to check the line attenuation with the other end of the line open and/or shorted. I think that part of it got hashed out pretty well early on, before your posting. If I'm not mistaken the OP has a VNA he can use to do the measurement. By sweeping over a narrow frequency range (about 200 feet of line; he's interested in the loss at about 1GHz), he can easily and very quickly see the line impedance and the return loss. If he's worried about his VNA calibration, I suggested he get a couple calibrated attenuators that bracket the return loss of his line, which he has to check occasionally. We just don't ever see much change in attenuators from reliable vendors, from one check to the next. Beyond that, we got into some "basenote drift" along the lines of "how can you provide reasonably cheaply a way to continuously monitor the performance?" That's where the stuff about putting something up the tower to pick off an RF sample came in. Since your posting appears as a response to one of mine where I was writing about the top-end monitoring, that may be an additional reason it didn't generate any responses. On the top-end monitoring, I claim that it's not all that difficult to make a stable coupled-line hybrid with very low coupling, and combine that with one of the modern RF power monitoring chips (esp. the AD8302 which has good temperature stability, and can tell you the phase relationship and amplitudes of two signals) to look at either just incident nom. 100 watts of power, or both incident and reflected. With something like that in place, you'd have added peace of mind on a continuous basis that everything was behaving as it's supposed to. It's reasonable to ask if there's much benefit beyond just monitoring the forward power at both ends of the line, but it seems like a small incremental effort to add a reflected measurement if you're doing a forward one. Of course, even continuous monitoring of the forward power at the top end may be well beyond what the OP has in mind. Cheers, Tom |
#7
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measuring cable loss
"K7ITM" wrote in message oups.com... On Aug 14, 12:01 pm, Walter Maxwell wrote: On Tue, 14 Aug 2007 09:53:31 -0700, K7ITM wrote: Bingo. It's that ratiometric thing that is a big plus for stability. In a coupler made of all the same metal, or at least metals that have nearly equal coefficients of expansion, the ratios stay the same, and it's the dimensional ratios that establish the coupling and impedances, not the absolute size. Actually, the change in length does matter, but if you make the assembly a quarter wave long, the d(coupling)/d(length) is zero at that point anyway. In any event, I suppose the thermal coefficient of expansion of metals you'd be most likely to use is small enough that you'd be fine with a shorter coupler. There doesn't need to be anything terribly complex about the geometry of the whole thing, either. It's probably safe to say that changes in the dielectric constant of air due to air pressure and humidity aren't going to be significant in this case. ;-) Cheers, Tom Tom, I thought this thread concerned measurement of attenuation in transmission lines. On the 11th I posted a precedure that involves measuring the line input impedances with the line terminated in both a short circuit and an open circuit, then plugging the measured data into a BASIC program that outputs the attenuation, complex Zo, and electrical length. My thoughts were that this procedure gives results with more accuracy and precision than the procedures discussed before my post appeared. However, I noticed that my post drew zero response. Is my procedure out-of-line, or out dated? Walt, W2DU Hi Walt, Well, yes, the original posting asked if it was reasonable to check the line attenuation with the other end of the line open and/or shorted. I think that part of it got hashed out pretty well early on, before your posting. If I'm not mistaken the OP has a VNA he can use to do the measurement. By sweeping over a narrow frequency range (about 200 feet of line; he's interested in the loss at about 1GHz), he can easily and very quickly see the line impedance and the return loss. If he's worried about his VNA calibration, I suggested he get a couple calibrated attenuators that bracket the return loss of his line, which he has to check occasionally. We just don't ever see much change in attenuators from reliable vendors, from one check to the next. Beyond that, we got into some "basenote drift" along the lines of "how can you provide reasonably cheaply a way to continuously monitor the performance?" That's where the stuff about putting something up the tower to pick off an RF sample came in. Since your posting appears as a response to one of mine where I was writing about the top-end monitoring, that may be an additional reason it didn't generate any responses. On the top-end monitoring, I claim that it's not all that difficult to make a stable coupled-line hybrid with very low coupling, and combine that with one of the modern RF power monitoring chips (esp. the AD8302 which has good temperature stability, and can tell you the phase relationship and amplitudes of two signals) to look at either just incident nom. 100 watts of power, or both incident and reflected. With something like that in place, you'd have added peace of mind on a continuous basis that everything was behaving as it's supposed to. It's reasonable to ask if there's much benefit beyond just monitoring the forward power at both ends of the line, but it seems like a small incremental effort to add a reflected measurement if you're doing a forward one. Of course, even continuous monitoring of the forward power at the top end may be well beyond what the OP has in mind. Cheers, Tom Having a power sensor at the top of the tower would mean that it would have to be calibrated at least annually. This would be deafeating the purpose of the investigation. The purpose being finding an easier method with acceptable accuracy of measuring the loss of the cable. Initial test taken by the enginnering staff show that this may be feasble and possibly even moe accurate than using the current appoved method. Jimmie |
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