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NEC Evaluations
On Mon, 22 Dec 2008 20:15:09 -0500, "J. Mc Laughlin"
wrote: Almost fifty years ago, I led a team who measured field strengths in the 100 to 250 MHz range (FM and TV broadcast transmitters) to verify (qualify) the propagation model. Hi Mac, and season's greetings, Can you relate the specifics of the measurement? At a minimum, what you would deem to be your best accuracy compared to an absolute standard, or to a relative standard (instrumentation, not computational). 73's Richard Clark, KB7QHC |
NEC Evaluations
Dear Richard:
It was almost 50 years ago when the models were rather new..... More background: the terrain was hilly - far from smooth earth - and path profiles were a critical part of the information along with the inherent uncertainties of using "analog" maps and along with the assumption about almost-straight line propagation. (an aside: we found examples of unpredictable propagation along string-like valleys that were aligned with transmitters, but the protected site was in a bowl-like valley.) (I saw one family in a valley using a rhombic antenna to receive TV signals. Their son had been in the Signal Corps.) We were using state-of-the-art Empire measuring systems (run off of a portable gasoline generator) that were calibrated with an impulse generator at each measurement. We selected paths that were similar to the expected paths of interfering transmitters. In other words, the paths were more-or-less normal to ridge lines not along string-like valleys. One more qualification: one path was found to have knife-edge diffraction discovered by the caution of taking measurements spaced a few meters apart at each data point. It was absolutely classic, but that data was not used because the protected site did not have such sharp ridges at its periphery. With those qualifications, my best recollection is that measurements and predicted measurements were within something like 3 or 4 dB. I doubt that repeating those measurements with a GPS receiver, digital topographical map, averaging near straight-line paths, and using a computer to do the arithmetic would be any better. Another note: Because of the expected sensitivity to interference at the site, I would drive over a few hills, erect a dipole in trees, and work my father on HF from the back seat of my car. No cell phones in those days. .... long distance was a big deal too Let us know how your studies are going. Warm regards, Mac N8TT -- J. McLaughlin; Michigan, USA Home: "Richard Clark" wrote in message ... On Mon, 22 Dec 2008 20:15:09 -0500, "J. Mc Laughlin" wrote: Almost fifty years ago, I led a team who measured field strengths in the 100 to 250 MHz range (FM and TV broadcast transmitters) to verify (qualify) the propagation model. Hi Mac, and season's greetings, Can you relate the specifics of the measurement? At a minimum, what you would deem to be your best accuracy compared to an absolute standard, or to a relative standard (instrumentation, not computational). 73's Richard Clark, KB7QHC |
NEC Evaluations
Richard Clark wrote: ...what you would deem to be your best accuracy compared to an absolute standard, or to a relative standard (instrumentation, not computational). ______________ You weren't asking me, but still you may be interested in the link below which leads to a good presentation of this by the NIST. A table on Page 3 there shows a measurement uncertainty at the NIST test facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency range. Field intensity measurements made using uncontrolled path conditions are more a measure of the propagation environment and the pattern/ location of the receive antenna than they are of the absolute performance of the transmitting antenna system. Such measurement errors can be gross, and difficult to quantify. http://ts.nist.gov/MeasurementServic...d/im-34-4b.pdf RF |
NEC Evaluations
Dear Richard Fry:
Thank you for the 1985 reference, which I had not seen before. Too many IEEE groups exist! A closed-loop system much like that shown in Figure 15 was built by me and a student and used by the mid 70s to subject DUTs to up to at least 200 v/m at frequencies up to about 200 MHz. This was for automated evaluation of the EMC of relatively small DUTs and was the prototype of a much larger system implemented by a major manufacturer that allowed the testing of entire cars. This was well before PCs, but after 488 signal sources and wattmeters were available. Confidence to about 1 dB was felt because of the tight correlation with a short voltage probe extending into the TEM cell. Unfortunately, the small effective volume of the TEM cell precluded measurements of antennas. The large room at NBS allowed them to measure antennas and I saw them measuring a large UHF antenna with a near-field probe in the early 1970s. Jumping to HF antennas of 0.5 WL size or mo I am convinced that even with a helicopter being used to measure a pattern, one can have more confidence in the result of the intelligent use of NEC4 than in any measurements. The measurements made in late 50s (to gain confidence with VHF propagation models) involved cherry-picking the paths to correspond with the goal of understanding propagation of possible interference into the radio-astronomy site. They also involved averaging a series of measurements taken within a few meters of each other. The measurement sites were all very rural and free of significant reflecting surfaces. Warm regards, Mac N8TT -- J. McLaughlin; Michigan, USA Home: "Richard Fry" wrote in message ... Richard Clark wrote: ...what you would deem to be your best accuracy compared to an absolute standard, or to a relative standard (instrumentation, not computational). ______________ You weren't asking me, but still you may be interested in the link below which leads to a good presentation of this by the NIST. A table on Page 3 there shows a measurement uncertainty at the NIST test facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency range. Field intensity measurements made using uncontrolled path conditions are more a measure of the propagation environment and the pattern/ location of the receive antenna than they are of the absolute performance of the transmitting antenna system. Such measurement errors can be gross, and difficult to quantify. http://ts.nist.gov/MeasurementServic...d/im-34-4b.pdf RF |
NEC Evaluations
On Tue, 23 Dec 2008 04:20:26 -0800 (PST), Richard Fry
wrote: A table on Page 3 there shows a measurement uncertainty at the NIST test facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency range. Actually, ±1 dB would be the most likely error for instrumentation error (±¼ dB could never be achieved); matching error would compound that; the antenna would add another ±1 dB; path would scramble that further if not performed in an anechoic chamber or on a calibrated range. Mac's test system (from fig. 15 he reports in other correspondence) would accumulate up to the several dB he reported earlier. It would exhibit a very good relative accuracy, but absolute accuracy would be several dB error as he has already offered in prior correspondence. Path problems would have to be hammered out on their own. 73's Richard Clark, KB7QHC |
NEC Evaluations
Richard Clark wrote:
On Tue, 23 Dec 2008 04:20:26 -0800 (PST), Richard Fry wrote: A table on Page 3 there shows a measurement uncertainty at the NIST test facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency range. Actually, ±1 dB would be the most likely error for instrumentation error (±¼ dB could never be achieved); matching error would compound that; the antenna would add another ±1 dB; path would scramble that further if not performed in an anechoic chamber or on a calibrated range. At HF and VHF, you should be able to do power measurements to a tenth of a dB, with moderate care. (obviously, you'd have to deal with measuring the mismatch, etc.). A run of the mill power meter should give you 5% accuracy (0.2 dB) without too much trouble. A 8902 measuring receiver can do substantially better. Even at microwave frequencies, better than 0.1 dB uncertainty (2 sigma) are possible with free space measurements (e.g. from an orbiting satellite to a ground station), with all the uncertainties stacked up (atmospheric, radome loss, antenna, electronics, etc.), although this is decidedly non-trivial. As mentioned, site effects or chamber uncertainties might contribute more. A typical anechoic chamber might have -20dB worst case reflections from the walls, and -40dB as more typical. A single scattering path will then contribute an uncertainty (worst case) of 1%, or 0.04 dB, although modern measurement technique (using multiple probe positions) can quantify this error and remove it, assuming the UUT and equipment are stable enough over the measurement period. The TEM cell is nice because it gives you a way to create a calibrated field to characterize your probe. Mac's test system (from fig. 15 he reports in other correspondence) would accumulate up to the several dB he reported earlier. It would exhibit a very good relative accuracy, but absolute accuracy would be several dB error as he has already offered in prior correspondence. Path problems would have to be hammered out on their own. 73's Richard Clark, KB7QHC |
NEC Evaluations
On Tue, 23 Dec 2008 10:29:05 -0800, Jim Lux
wrote: At HF and VHF, you should be able to do power measurements to a tenth of a dB, with moderate care. (obviously, you'd have to deal with measuring the mismatch, etc.). A run of the mill power meter should give you 5% accuracy (0.2 dB) without too much trouble. A 8902 measuring receiver can do substantially better. Nothing astonishes me more than the simple dash-off notes that claim power measurement is a snap. I can well imagine, Jim, that you don't do these measurements with traceability to the limits you suggest. For the other readers: We will specifically start with the 8902 measuring receiver. A premier instrument indeed, but it falls fall FAR short of actually measuring power without a considerable body of necessary instrumentation (well illustrated by Mac's observation found in that fig. 15 already cited). Most claimants peer at one line in a spec sheet and figure that is the end of the discussion. Glances elsewhere begins to build the actual accuracy obtainable through the chain of errors that accumulate. For instance with a 1mW input in the VHF band: Internal power standard: ±1.2% and we have yet to look at the measurement head's error contribution. The so-called "run of the mill power meters" are drawing close, too close to this precision set's expensive quality such that their estimation of 5% is already suspect quality. Scale error demands a full-scale indication to simple keep the error contribution down to 0.1% (a 1/10th scale indication would jump that error to 1%) ±1 digit. Input SWR with the HP 11792 is rated at 1.15 at worst (I've measured with far better matches) to that same source's 1.05 SWR adds 0.4% error. If you are not measuring power at the specific frequency of the internal source, add more error averaging onwards to 2%. Things build up from here for just one instrument and its RF head to a worst case valuation of 5% to 6% error. This further trashes the observation of "run of the mill power meters" vaunted 5% accuracies. Of course, in this computation of error neophytes are tempted to employ the RMS estimation. This clearly reveals those untested in the arts where bench techs who do their best understand that the RSS estimation is what pays their salary. Taking a step above skilled bench work to that of a Calibration lab, you buy all the error at face value (hence the term "worst case" that is used by the professionals employed in this art). THEN we turn our attention to the rest of the bench that holds the remaining components that support the measurement of a power level and accuracy begins to slide drastically. I've been there, and I've been trained to reduce the variables - not an easy task and one that the march of time has NOT improved. Mismatch error climbs like the Himalayas if you don't employ line conditioners (which bring their own mismatch) and isolators (which bring their own mismatch) and so on down the proverbial line towards the source being measured (that antenna every one knows has X amount of power coming from it). For those who are stunned by this bajillion dollar solution giving them 14% best accuracy (and RSS at that) results, confer with: http://www.home.agilent.com/upload/c...EPSG085840.pdf and observe the commentary for slide 36. See if you can cook up a method that doesn't hammer you into the ground. I can anticipate some: 1. Throw a box car of money at the problem; 2. Buy lab time at NIST; 3. Write a report that runs to book length (I've carried most of that water by providing the link above) - or xerox the book that already exists: "Microwave Theory and Applications," Stephen F. Adam; 4. Do it with precision components employing best practices to the best achievable accuracy - you will need further instruction into best practices, however; 5. Ignore reality. Only the last two options are achievable by the ordinary Ham. To claim that "someone else" can do it better and is thus achievable is sophistry serving ego in an argument. 73's Richard Clark, KB7QHC |
NEC Evaluations
On Dec 23, 12:39 pm, Richard Clark wrote:
On Tue, 23 Dec 2008 10:29:05 -0800, Jim Lux wrote: At HF and VHF, you should be able to do power measurements to a tenth of a dB, with moderate care. (obviously, you'd have to deal with measuring the mismatch, etc.). A run of the mill power meter should give you 5% accuracy (0.2 dB) without too much trouble. A 8902 measuring receiver can do substantially better. Nothing astonishes me more than the simple dash-off notes that claim power measurement is a snap. I can well imagine, Jim, that you don't do these measurements with traceability to the limits you suggest. In point of fact, I *do* make measurements like that, and as I said, it requires "moderate care" and good technique and instrumentation. A random diode measured with your $5 Harbor Freight DMM isn't going to hack it. Neither is most of the stuff sold to hams. It is hardly a "snap", but it *is* within the reach of someone at home with a lot of time and care to substitute for expensive gear and calibrations (basically, you have to do your own calibration). For the other readers: We will specifically start with the 8902 measuring receiver. A premier instrument indeed, but it falls fall FAR short of actually measuring power without a considerable body of necessary instrumentation (well illustrated by Mac's observation found in that fig. 15 already cited). Most claimants peer at one line in a spec sheet and figure that is the end of the discussion. Glances elsewhere begins to build the actual accuracy obtainable through the chain of errors that accumulate. For instance with a 1mW input in the VHF band: Internal power standard: ±1.2% and we have yet to look at the measurement head's error contribution. The so-called "run of the mill power meters" are drawing close, too close to this precision set's expensive quality such that their estimation of 5% is already suspect quality. Standard power measuring head on a Agilent power meter is better than 5% at HF, probably in the range of 1% for one head in comparison measurements over a short time. The 8902 is sort of a special case, but can do very accurate relative measurements. FWIW, the 8902 calibrates out the measurement head effects. Scale error demands a full-scale indication to simple keep the error contribution down to 0.1% (a 1/10th scale indication would jump that error to 1%) ±1 digit. This oversimplifies a bit. Typically, you'll have some uncertainty that is proportional to the signal measured (e.g. mismatch will affect the signal the same way regardless of level) and some that is absolute, independent of the signal level (e.g. the analog noise in the voltmeter). As you say, bigger signals are easier to measure precisely.. the real limiting factor is the accuracy with which you know the attenuation of the attenuators you're using to get the steps. With regard to mismatch, if you're interested in tenth dB accuracies, you're going to have to measure the mismatch and account for it. It's not that hard, just tedious. The typical power meter head doesn't change it's Z very much, so once you've measured YOUR head and keep the data around, you're good to go for the future. (and do your tests at the same temperature, don't use the head for a door stop, etc.) As far as calculating uncertainties.. you bet.. it's not just stacking em up. But that's true of ANY precision measurement, so if one is quoting better than half dB numbers (i.e. if you give any digits to the right of the decimal point) one should be able to back it up with the uncertainty analysis (which is all described on the NIST website and in the tech notes). This isn't hard, it's just tedious. But the whole thing about high quality amateur measurements is you're trading off your time to do tedious extra measurements and analysis in exchange for not sending a cal-lab a check. The how to do it is all out there. What was "state of the art" for a national laboratory in 1970 is fairly straightforward garage work these days, and, a heck of a lot easier because you've got inexpensive automation for making the measurements and inexpensive computer power for doing the calibration calculations and uncertainty analysis. The slide 36 discussion refers to measuring a signal at -110dBm, which I would venture to say is well below the levels that most hams will be interested in measuring. And, they are talking about where the source Z is unconstrained. In a typical ham situation, these things probably aren't the case. If you were interested in measuring, for instance, the loss of a piece of coax or the output of a 0dBm buffer amplifier to a tenth of a dB, that's a whole lot easier than a -110dBm signal from some probe into a 8902. The context of this discussion was making measurements of antennas, and for that, one can normally arrange to have decent signal levels, etc. OR, one is interested in relative measurements, rather than absolute calibration. It's a whole lot easier to measure a 0.1 dB difference between two signals. You suggested 5 alternatives: ee if you can cook up a method that doesn't hammer you into the ground. I can anticipate some: 1. Throw a box car of money at the problem; Or, throw some time at the problem.. this is the classic ham tradeoff... "I don't have money, but I do have time" It's no different than grinding your own telescope mirrors, building your own Yagi or wire antenna, etc. 2. Buy lab time at NIST; That's the money thing (and it doesn't require boxcar loads.. perhaps a kilobuck or two.. and for some folks, it's worth it.. although I can't see any amateur radio need. I can see doing it as part of a hobby involving precision, like the folks on time-nuts who operate multiple Cs clocks and build very high performance quartz oscillators for the thrill of getting to 1E-14 or 1E-16 Allan deviation.. Folks who do home nuclear fusion also might avail themselves of pro cal services for their neutron detectors, because there isn't a convenient way of doing home cals, unlike for RF power, where it's at least possible) 3. Write a report that runs to book length (I've carried most of that water by providing the link above) - or xerox the book that already exists: "Microwave Theory and Applications," Stephen F. Adam; Or any of a variety of sources. One doesn't need a book for this, but one does need some care in technique and some background knowledge. It's like reading John Strong's book on building scientific instruments (back in the 40s, one built one's physics experimental gear and calibrated it yourself) 4. Do it with precision components employing best practices to the best achievable accuracy - you will need further instruction into best practices, however; 5. Ignore reality. --- |
NEC Evaluations
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NEC Evaluations
On Jan 2, 4:59 pm, Richard Clark wrote:
On Wed, 31 Dec 2008 08:50:28 -0800 (PST), wrote: On Dec 23, 12:39 pm, Richard Clark wrote: Nothing astonishes me more than the simple dash-off notes that claim power measurement is a snap. I can well imagine, Jim, that you don't do these measurements with traceability to the limits you suggest. In point of fact, I *do* make measurements like that, I note you slip loose from the constraint of "traceability." Doing measurements "like that" is vastly different in outcome and holds accuracy claims like a sieve holds water. Traceability only applies when you need absolute measurements, and there you will need something as a standard, since most RF power measurements are really more of "transfer" measurements. Since power is basically an energy measurement, then all manner of calorimetric approaches will work (or radiometric, if you're working in microwaves).. then it comes down to how accurately you can measure/ hold temperature. It comes back to what's a reasonable thing for a ham to have that can serve as a local standard. Time/frequency are clearly the easiest to get to high precision (1E-10 is straightforward these days with a GPS disciplined oscillator), voltage is a bit tougher.. 1ppm would be very, very good for hams, 1e-4 seems plausible with decent high quality voltage references that cost $100. Temperature to 0.1 degree should be doable, so that gives you a part in 3000, roughly. A ham seriously interested in 0.1 dB measurements will probably be able to scrounge up something to use as a transfer standard and scrounge up a way to get it calibrated. For instance, Noise/Com used to offer a discounted calibration service for sources based on their noise diodes. Once you've got a standard, if you take care of it, then you can use it for lots of things. The original question had to do with accuracy of measurements vs NEC, and those would be relative, and, I maintain, not too tough to do to 0.1 dB, because you're making comparison measurements with the same sensor, at pretty much the same level, etc. moving on to some very telling points offered in rebuttal to obtaining 0.1dB accurate power determinations:... it *is* within the reach of someone at home with a lot of time and care to substitute for expensive gear and calibrations (basically, you have to do your own calibration). And again with: (back in the 40s, one built one's physics experimental gear and calibrated it yourself) where both reveal a disastrously circular logic of what could only be called "self determination" with a very tenuous grasp to accuracy. All calibrations, whether in a cal lab or your garage start somewhere. It's how much trouble are you going to go to for that first standard. Do you do it calorimetrically and use water triple point and boiling point as references? Do you trust a good calibrated DVM? 0.1dB means 2% in power.. not exactly gnats eyelash precision (e.g. measuring temperature to 1 degree C out of a change of 100 degrees is 1%) Standard power measuring head on a Agilent power meter is better than 5% at HF, probably in the range of 1% for one head in comparison measurements over a short time. The 8902 is sort of a special case, but can do very accurate relative measurements. FWIW, the 8902 calibrates out the measurement head effects. I have already cited accuracies and errors that conflict with your supposition. You are taking characteristics in isolation and citing them as being representative of the whole scope of determination of power to a stated accuracy. The single example of your stating: probably in the range of 1% for one head in comparison measurements over a short time. Sure.. if you are concerned about 0.1dB, then you're going to need to calculate for yourself, and not take an offhand assertion. That said, I still think that 0.1dB is reasonable after you take into account all the uncertainties (and eliminate things that add to the error.. mate/ demate, temperature changes, equipment changes, etc.). I am interested in tenth dB accuracy, aren't you? Let's recall where this began: On Tue, 23 Dec 2008 10:29:05 -0800, Jim Lux wrote: At HF and VHF, you should be able to do power measurements to a tenth of a dB, with moderate care. This statement has now been dismissed by your inclusion (supporting my observation) of mismatch error - which you subsequently diminish: you're going to have to measure the mismatch and account for it. It's not that hard, just tedious. Accounting for mismatch does not correct it. The error it contributes remains. This is not an actuarial gimmick of Enron bookkeeping. The off-hand hard/tedious baggage appears to be another objection without substance. No.. if you KNOW the mismatch, it's not an uncertainty anymore. There is an uncertainty in the amount of mismatch, but in a relative measurement (e.g. received power from a probe on an antenna range.. the original question) the mismatch doesn't change from measurement to measurement, so it doesn't contribute uncertainty to the measurement. Likewise, if you actually measure the reflected power (e.g. in a VNA) then you don't have to use the "power uncertainty due to mismatch" equation which assumes that the reflection coefficient is of unknown angle. Yes, the reflected power measurement will have an uncertainty, but that is a smaller contributor to the overall uncertainty than the "unknown phase of reflection" uncertainty. The typical power meter head doesn't change it's Z very much, And yet it still is NOT the Z you would like it to be, except by some margin of error. Change is not the issue, absolute value is. This appears to be yet another manufactured objection that points out error only to dismiss it with a cavalier diminution of "very much." Metrology doesn't accept adjectives in place of measurables. For a relative measurement the source and load Zs are constant, and whatever mismatch there is will be the same for all measurements (e.g. in an IF substitution measurement, it's the Z looking into the measurement system's attenuator or amplifier). If you're trying to measure the output of a source with varying Z, then, of course, the mismatch will affect the net amount of power crossing the reference plane into the sensor, and you'll need to do that. But there are lots and lots of cases where a ham might make measurements to 0.1dB where the source Z is constant. (say, making noise temperature measurements of the sun with an antenna, or measuring the received signal from a distant transmitter.. same antenna, same physical location, same receiving system.. the measurement system isn't changing) Let's return to the claim, however. In fact, a typical power meter head DOES change its Z, and it is by this very physical reality that it performs power determination. You may be relying on a specific and rather atypical head to support your argument. As you offer nothing in the way of your typical head's design to support another off-hand comment, we will have to wait for that coverage. The datasheet for the head gives this. If you're looking at the thermistor/thermocouple mount style head, the Z looking into the head is basically that of the load resistor, which, if held at constant temperature (constant = within a few degrees), I doubt it changes more than a fraction of a percent. A diode head (like the 8481,8487, etc. for HP/Agilent meters) is also going to be pretty good. Agillent claims the increase in uncertainty for ALL causes from an extended temperature (0-50) over the specified 25 +/- 5 is something like 0.9%. Obviously, if want to dot is and cross ts, then you can actually measure it, but you'd only need to characterize it once (that's the moderate care thing.. a lot of good metrology is just record keeping). There's also a change in Z with frequency (an issue if your transfer cal standard is at a different frequency than your measurement frequency; not the case in a relative measurement on an antenna range), but again, you can either take the worst case in the data sheet, or measure it yourself. That's sort of the difference beween modern VNAs and old style measurements. The modern VNA uses a set of cal standards that has properties determined by its mechanical construction (e.g. short, open, thru) and does the arithmetic for you. Even the $1000 N2PK and TAPR VNAs do this. As long as you're at the same temperature, it should be good. so once you've measured YOUR head and keep the data around, you're good to go for the future. (and do your tests at the same temperature, don't use the head for a door stop, etc.) No, you do not make traceable measurements. Your statement of futurity is an illusion only. Within the context of HP's fine craftsmanship, it is a fairly safe illusion, but not into the unlimited future. The calibration cycle for an RF head AND its reference source (two sources of error) is 3 to 6 months where that calibration data would be amended and changed to follow the natural variation in characteristics. A skilled bench tech might trust the instrument out for several years for relative loss measurements, but absolute power determinations will have long lost their credentials. I think that reasonable folks could differ on the concept of "required calibration cycle" and "aging life"... It's not like the thermocouple or load resistor in a power meter head has an aging process that causes it to suddenly go out of cal after 6 months. More likely, the cycle is a good blend of economics and the expected variation from a typical rough and tumble bench environment. Most of the time, the calibration cycle is more to make sure that the device hasn't "broken". If your device is using something like a crystal oscillator, then there IS an aging thing to worry about. You have raised an interesting question, though, so I'll have to go ask the folks at the cal lab to see if we have any long term data on, say, a 848x head to see what sort of aging or changes there are. This repetition of the hard/tedious mantra has the odd appearance of an objection diminishing the importance of established procedures of power determination. It reduces the profession of precision electronics to the repetitive motions of a trained monkey. No, I think you misunderstand "hard" in this context. It doesn't require any special new thinking to do accurate power measurements. The methodology is well known, as are the error sources, and the evaluation of uncertainty. The "hard" part is reducing the uncertainties (i.e. the equipment design) in the first place or in choosing a measurement method that tends to cancel errors (e.g. Dicke switch radiometers use the same sensor for both reference and unknown measurement, eliminating sensor/sensor uncertainty, at the expense of the uncertainty due to the switch). The tedious part is in being careful, doing repeated measurements, controlling the environment, and then grinding the math. that crude metric of adjectives. Throwing more gear and procedure that is freely available (rather than as costly necessary) at the problem, compounds error outrageously. I'd say "may" compound error. This, again, is an appeal to substituting just-plain-hard-work for accuracy. You fail to show any correlation to standards or their necessity. The effort you describe may well pay off in superlative precision; again, investing in resolution without paying for the cost of accuracy. Or, perhaps, getting your accuracy from standards you DO have handy, rather than relying on the instrument's internal transfer standard. An example might be using thermal hot/cold loads to calibrate a radiometer rather than a calibrated diode noise source (where the diode was calibrated somewhere else against a thermal standard). If you don't have the diode (or the wherewithal to send it to NIST for calibration on their radiometer), then perhaps hard work and time can get you a comparison against something you CAN measure accurately (boiling LN2, boiling water, etc.). Z is unconstrained. In a typical ham situation, these things probably aren't the case. This appears to be yet another objection: "probably aren't the case." The original post had to do with comparing measured antenna patterns against NEC models. That IS the case there. This was your choice of instrumentation; this was your choice of power determination. I have provided an incontrovertible example of how your off-hand assertion failed from your own choices. It doesn't get remarkably better however you decide to amend the conditions and those amendments certainly won't come close to your original off-hand observation of power determination of 0.1dB or less error. I note that a later page in the same presentation shows absolute power measurement worst case uncertainty at 30 MHz of 0.02dB over a power range of -10 to -70 dBm (slide 47).. I just picked the 8902 arbitrarily as an example of something that I know can do relative measurements to this sort of accuracy (as opposed to, say, a Bird 43 watt meter, which cannot) to refute your original statement (paraphrased) that measurements to better than 0.5 dB were impractical. Some may appeal that their Relative Accuracy has the merit of Absolute Accuracy - until you ask them to measure the actual, absolute value of their ad-hoc standard. I have built primary standards and measured them to 7 places. Over several years they migrated in value by 2 of those least significant digits, but only in comparison to standards that had "aged" and been calibrated at a national primary standards laboratory. By themselves, I could have fooled myself (and perhaps others less sophisticated) that they were absolutely accurate to the extent of the number of digits my instruments could resolve. -- So, you measured to 1E-7, and over several years, they changed by 1E-5? That's a whole heck of a lot better than 1E-2 (which is what 0.1dB implies). Those 3 orders of magnitude are why I think it's reasonable and plausible for hams to make measurements to 0.1dB. -- http://hdl.handle.net/2014/18497 describes one measurement system I designed, built and calibrated. That paper was aimed at a more general audience and doesn't give much of the uncertainty analysis, but it can be found in the several dissertations based on the calibration station. There's nothing special in this system that couldn't be duplicated by a ham for HF or VHF use. Certainly, the NIST Type IV power meter (used in the system to measure the level of the reference source used to calibrate the receiver chain) is something eminently doable for ham use (See Larsen's paper from 1975) and mostly depends on a "really good" DVM for its accuracy. Jim |
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