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Richard Clark wrote:
On Tue, 26 Apr 2005 20:28:01 -0500, Tom Ring wrote: Any references on microphone calibration? Maybe a short tutorial? That is something I have a need to do. Hi Tom, As a second thought, you may not be in the market for the reciprocity technique (it does require that you have a true reference microphone). In that case, you would fall back to a Piston Phone and do a single point calibration. The method is as old as the hills, the math is extremely simple volumetrics, but the implementation (construction of the calibration unit) is not something for the faint of heart. You will need a precision lathe. Again, google using Brüel & Kjær as a jump-off point. Once you do the single point calibration, then you can proceed to a swept frequency analysis. Unfortunately this returns us to the necessity of a reference microphone. However, as relative frequency response is more available (from expensive retail models), you might have a chance. 73's Richard Clark, KB7QHC And thanks for this also. I had a nicely useful, yellow, roughly 11x14, hardcover book that was a handy audio manual that was lost during moving a couple decades ago. It covered beginning through midrange complexity, and had a decent tutorial on bi-amp. Also had a description of Indy Speedway Pit announcement system, high sound pressure level, baseball stadium sound system, and R&R, may have been The Grateful Dead. I think it may have been a husband and wife team that wrote it. Ring any bells? I'd like to order a copy of it. tom K0TAR |
Roy Lewallen wrote:
Correction. Even identical array elements *don't* necessarily extract the same amount of energy. The reason again is mutual coupling. In a four-square receiving array with very low ground loss, one of the elements will actually radiate power. This power comes from power extracted from the wave by the other three. In a Yagi array, the parasitic elements extract no power at all from an impinging wave; only the driven element does. Roy Lewallen, W7EL Another example that can be divorced from ground effects and easily modeled in free space is a 4 by 4 array of yagis - an EME array. The corners are one set, the outside horizontal pairs are another, the outside vertical pairs are another, and the inside 4 are another. I have a simple 4 by 4 by 3 element AO array as an example if anyone would like a copy. tom K0TAR |
On Wed, 27 Apr 2005 20:26:10 -0500, Tom Ring
wrote: Ring any bells? I'd like to order a copy of it. Hi Tom, Sorry, no bells not even decibells. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
On Tue, 26 Apr 2005 21:09:07 -0700, Roy Lewallen wrote: I believe "takeoff angle" is in the same category as "capture area" and "S-unit" -- terms which nobody except amateurs seem to need. Hmmm, Capture area of antennas, 899, 927 of Terman's "Electronic and Radio Engineering. The 899 reference appeals to aperture. The 927 reference gives a value of 1.5 or 0.12 lambda² (also called intercept area or antenna cross section) for a common dipole. . . . I stand corrected. Spurred by your response to do a more exhaustive search, I found "capture area" in the indices of 3 out of the 12 antenna texts and handbooks I have, plus in Terman's _Radio Engineering_. While aperture is more commonly used (judging by this sample), "capture area" is indeed in use outside the amateur community. Think I'm likely to find "S-Unit" if I look hard enough? Roy Lewallen, W7EL |
J. Mc Laughlin wrote:
I too am reluctant to enter this as much resembles a freshman poli-sci student debating a third year law student. However .... please see indented comments below . . . Here I must inject my experience. As part of a topic sentence, "tends to be the same" is OK. However, it is common on real HF paths of over 4 or 5 Mm for the elevation angle at which the strongest signal arrives to be significantly different at the two ends of the path. It is easy on longer paths for the major mode at one end to be using a high virtual-height F2 mode and for the other end to be using a low virtual-height E mode. Allow me to put to rest the notion that optimum elevation angles are necessarily the same at both ends of a longer (multiple hop) HF path! [Reg did not say that optimum elevation angle are necessarily the same.] . . . I'm glad you did get yourself to contribute to the discussion. I'm by no means an expert when it comes to propagation (and quite apparently several other participants aren't either), and I've fallen victim to using oversimplified models where their use isn't appropriate. The little reading I've done on the topic shows there are some really interesting phenomena involved which don't at all fit with the notion of simple reflection or refraction from a layer at a single height. Thanks for reminding us about one of the more important ways in which the simplified models are misleading. As other have said, but not all have heard, the idea is to maximize gain (at both ends of a path) at the elevation angle being used. Even in the 20s, antenna systems were in regular use that attempted to do just this. . . . Even if it were possible to do so, one would not use an antenna that had all of its gain at the predicted optimum elevation angle. One would try to design an antenna (money enters here) that has most of its gain in the expected band of elevation angles expected to be best for a path. Agreed. Roy Lewallen, W7EL |
Richard Harrison wrote:
My original comment was in support of Arnold B. Bailey who said something about increasing antenna gain by 3 dB every time you double its size. Precisely, that`s not true, but I gave an example from Kraus where he did much the same thing. +3dB is a valid generalization, based on sound physics - but it is only a generalization. At the time those Grand Old Men were writing their textbooks, such generalizations were the best that anybody could manage. But they had no way of checking their accuracy - or more important, why and when they start to become INaccurate. 50 years on, we do have a way, and we now know much more than they did. That makes it very dangerous to quote those Grand Old Generalizations as accurate and universal truths. Richard was quite correct to describe the "+3dB rule" as "naive" - because, at today's level of knowledge, it is. But we still need to know that the +3dB generalization exists; and understand the fundamental reasons for it. That fundamental understanding is what protects us against stupid mistakes. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Thu, 28 Apr 2005 00:11:44 -0700, Roy Lewallen
wrote: Think I'm likely to find "S-Unit" if I look hard enough? Hi Roy, One could see stars in the middle of the day if they squint their eyes hard enough. Seems to me Collins literature covered S Units adequately and they were certainly the Cadillac of the pro's. Hammerlund is another probable source. National another. RCA as early as 1941. On the other hand, I certainly wouldn't waste my eyes scanning the shelves of the engineering library. 73's Richard Clark, KB7QHC |
"Ian White GM3SEK" wrote
Richard Harrison wrote: My original comment was in support of Arnold B. Bailey who said something about increasing antenna gain by 3 dB every time you double its size. Precisely, that`s not true, but I gave an example from Kraus where he did much the same thing. +3dB is a valid generalization, based on sound physics - but it is only a generalization. At the time those Grand Old Men were writing their textbooks, such generalizations were the best that anybody could manage. _______________ No doubt the 'GOM' knew the exact gain changes from successive doublings of an antenna aperture, or could calculate them if they wished to. The difference between the two isn't very important except when it is part of the equation to arrive at some legally required ERP, such as in commercial broadcasting. Below are the gains of a series of commercial FM broadcast transmit arrays to illustrate the point. The elements (bays) in these arrays all are one wavelength apart, and driven with equal power and phase. # Elements C-pol Gain (dBd) 1 -3.55 2 -0.21 4 3.09 8 6.34 Starting with the gain of the 1-bay and adding exactly 3 dB per doubled aperture in this example would result in 5.45 dBd gain for the 8-bay, meaning that FM ERP when using this approach would be more than 18% below its licensed value (illegal). RF |
On Thu, 28 Apr 2005 00:40:11 -0700, Richard Clark
wrote: On Thu, 28 Apr 2005 00:11:44 -0700, Roy Lewallen wrote: Think I'm likely to find "S-Unit" if I look hard enough? Yep. Hi Roy, One could see stars in the middle of the day if they squint their eyes hard enough. Seems to me Collins literature covered S Units adequately and they were certainly the Cadillac of the pro's. Hammerlund is another probable source. National another. RCA as early as 1941. On the other hand, I certainly wouldn't waste my eyes scanning the shelves of the engineering library. Scanning my library I spy: "Fundamentals of Single Side Band", 2nd Edition, Sept 1959, Collins Radio Company, Cedar Rapids, Iowa. On page 13-7, in describing the circuitry of the 75S-1 it says in part: "The S-meter is calibrated in S-units and dB. The S-unit scale is standard up to midscale (S-9). The db scale reads relative signal strength above the avc threshold which is approximately 1 microvolt. Thus 40 dB on the meter is 100 microvolts of signal (which also corresponds to S-9 on the S-unit scale). To read dB over S-9 on the S-unit scale, subtract 40 from the corresponding dB reading. For instance, a 60-dB reading would be 20 dB over S-9 (100 uv) or 10,000 uv of signal at the antenna. This reading is then 60 dB over the dB scale reference of one microvolt and 20 dB over the S-9 reference of 100 uv" Whew. Next to it I find: "Amateur Single Sideband", 1st Edition, 1962, Collins Radio Company, Cedar Rapids, Iowa. Its only reference to S-meters is on page 111 where, in a section on distortion tests with a receiver, it states in part: "Variations and nonlinearities in S-meter calibration can introduce considerable error in the measurement of signal levels by means of a receiver. The commonly used figure of 6 dB per S-unit is appreciably higher than the actual response of many amateur receivers. The meter in the S-Line receivers, for instance, is calibrated to read approximately S-9 with and input signal of 100 microvolts. The agc threshold is set nominally to 1.5 microvolts, and varies slightly from band to band. Therefore, the total range from S-zero to S-9 under normal conditions can represent 30 to 40 dB or from approximately 3.3 to slightly over 4 dB per S-unit." |
On Thu, 28 Apr 2005 00:40:11 -0700, Richard Clark
wrote: On Thu, 28 Apr 2005 00:11:44 -0700, Roy Lewallen wrote: Think I'm likely to find "S-Unit" if I look hard enough? Yep. Hi Roy, One could see stars in the middle of the day if they squint their eyes hard enough. Seems to me Collins literature covered S Units adequately and they were certainly the Cadillac of the pro's. Hammerlund is another probable source. National another. RCA as early as 1941. On the other hand, I certainly wouldn't waste my eyes scanning the shelves of the engineering library. Scanning my library I spy: "Fundamentals of Single Side Band", 2nd Edition, Sept 1959, Collins Radio Company, Cedar Rapids, Iowa. On page 13-7, in describing the circuitry of the 75S-1 it says in part: "The S-meter is calibrated in S-units and dB. The S-unit scale is standard up to midscale (S-9). The db scale reads relative signal strength above the avc threshold which is approximately 1 microvolt. Thus 40 dB on the meter is 100 microvolts of signal (which also corresponds to S-9 on the S-unit scale). To read dB over S-9 on the S-unit scale, subtract 40 from the corresponding dB reading. For instance, a 60-dB reading would be 20 dB over S-9 (100 uv) or 10,000 uv of signal at the antenna. This reading is then 60 dB over the dB scale reference of one microvolt and 20 dB over the S-9 reference of 100 uv" Whew. Next to it I find: "Amateur Single Sideband", 1st Edition, 1962, Collins Radio Company, Cedar Rapids, Iowa. Its only reference to S-meters is on page 111 where, in a section on distortion tests with a receiver, it states in part: "Variations and nonlinearities in S-meter calibration can introduce considerable error in the measurement of signal levels by means of a receiver. The commonly used figure of 6 dB per S-unit is appreciably higher than the actual response of many amateur receivers. The meter in the S-Line receivers, for instance, is calibrated to read approximately S-9 with and input signal of 100 microvolts. The agc threshold is set nominally to 1.5 microvolts, and varies slightly from band to band. Therefore, the total range from S-zero to S-9 under normal conditions can represent 30 to 40 dB or from approximately 3.3 to slightly over 4 dB per S-unit." |
Richard Fry wrote:
"The elements (bays) in these arrays all are one wavelength apart, and driven with equal power and phase. # Elements C-pol Gain (dBd) 1 -3.55 2 -0.21 4 3.09 8 6.34" Bailey`s Table 10-I, which Richard Clark referred to as "naive", appears on page 484 of "TV and Other Receiving Antennas". The heading is Array Gain (approximate rule). Nunber( of Half-Wave Rods (N) and Numeric PowerRatio Gain (dB) 1 0 2 3 4 6 8 9 First difference from Richard Fry`s table is the loss of 3.55 dB as the result of circular polarization (mostly) as half of the power which a linearly polarized reference dipole would use is cross-polarized. The steps between doubling the number of elements in Richard Fry`s table are all nearly 3 dB. Bailey says "approximate rule". He is vindicated. Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote
First difference from Richard Fry`s table is the loss of 3.55 dB as the result of circular polarization (mostly) as half of the power which a linearly polarized reference dipole would use is cross-polarized. That, and the fact that the radiation pattern from each element is not the pure cosine function assumed for a 1/2-wave dipole. It has slightly less gain peak gain. The steps between doubling the number of elements in Richard Fry`s table are all nearly 3 dB. "Nearly" is right, but the difference is not uniform for successive doubling of apertures. A small variation in the bay-bay spacing (departing from 1 wavelength) is needed as a function of the number of bays, to maximize the peak gain from this type of an array. The arrays in my table all have exactly 1-wavelength element spacing, and the peak gain from arrays of them is lower than expected for lower numbers of elements, and higher than expected for higher numbers of elements -- which stretches/compresses that nominal 3 dB delta. Fine points, to be sure. RF |
I don't think that's a valid excuse. The 3 dB rule applies to phased
arrays only when mutual coupling is ignored or in a few special cases. Mutual coupling had to have been known at least at the time of the invention of the Yagi-Uda antenna in 1926, and probably long before that. It was being calculated for geometrically simple antennas at least as early as 1943 (cf. R. King, Proc. IRE). Work proceeded rapidly through the '40s, with papers describing increasingly accurate techniques with antennas of increasing complexity. We now have the means to calculate mutual coupling much more easily than before, and for geometries which were impossible to deal with before we had computers to do the work, but I don't think we've modified our understanding of the phenomenon for many decades (some notable antenna charlatans notwithstanding). Anyone measuring the gain of a short Yagi, the gain of which routinely exceeds 3 dB per doubling of elements by a considerable margin, must have become aware of the shortcoming of the 3 dB rule. I suspect that if we were to read the cited quotations very carefully, we'd see qualifications that explain neglecting mutual coupling. Roy Lewallen, W7EL Ian White GM3SEK wrote: +3dB is a valid generalization, based on sound physics - but it is only a generalization. At the time those Grand Old Men were writing their textbooks, such generalizations were the best that anybody could manage. But they had no way of checking their accuracy - or more important, why and when they start to become INaccurate. 50 years on, we do have a way, and we now know much more than they did. That makes it very dangerous to quote those Grand Old Generalizations as accurate and universal truths. Richard was quite correct to describe the "+3dB rule" as "naive" - because, at today's level of knowledge, it is. But we still need to know that the +3dB generalization exists; and understand the fundamental reasons for it. That fundamental understanding is what protects us against stupid mistakes. |
Roy Lewallen, W7EL wrote:
"I don`t think that`s a valid excuse." That old authors were satisfied with approximations may have less to do with ignorance than with not having computers and programs to make analysis fast and easy. The computer gurus have done well. A seconndary effect of a paucity of computer power is a requirement for more measurements. As the title of this thread is:"Accuracy of Antenna Testing Ranges". measurement is still a concern. As one who was doing plenty of tests and measurements 50 years ago, I`d like to testify that if I could get 1-dB accuracy, I was satisfied. Bailey may not have thought that was good enough accuracy, but I think it was realistic for the period in the field. I`m sure the NBS did better. But for ordinary purposes. 1 dB is probably good enough for graphs and tables to be comparable in accuracy to the measurements you can make. Of course, everyone wants complete accuracy. Richard Fry`s and Arnold Bailey`s tables were within 1-dB. I think it`s satisfactory. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
1 dB is probably good enough for graphs and tables to be comparable in accuracy to the measurements you can make. Of course, everyone wants complete accuracy. I remember asking my college prof back in the '50's: How can we trust a graph where none of the measured values actually fall *on* the graph line? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
"Frank" wrote in message news:VPqbe.902$0X6.797@edtnps90... Thanks again Ed. From everyone of your posts I learn something new. The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250 MHz. I have 24" tall pyramidal foam, and that meets the requirement. As frequency decreases, the foam essentially disappears. By 10 MHz, it has almost no effect. I think we were using 12" pyramdal foam, even on the floor, with inverted foam to provide a walking area. The pyramidal foam is expensive, about $50 / sq ft. If you want more return loss, you need taller pyramids; those mythical governmental labs have had foam up to 72" tall (and the wall absorbers tend to droop a bit g). With a 3m chamber, anything greater than 12" is not really practical. Well, that's kind of what I was trying to say. The right way to do the work is to start with the size of the device you need to test, then consider the test standard that you need to apply, and that will tell you how much "working volume" you need inside the chamber. Then, you can decide on anechoic treatments, and that then defines the size of the shielded chamber. This "working outward" approach is the right way, but if you find that you have now specified a 30' high by 50' wide by 100' long chamber, likely you can't afford that much "goodness." g Most people find themselves in a situation where they have a chamber of some kind, and then they are challenged to do good work on a product inside that volume. Sometimes you can do "good enough" work, with a lot of effort and some known limitations. Sometimes what you do is pretty decent, and good enough for "pre-compliance" requirements. If your product line is rather consistent (size, peripherals, external cabling), you can often use data from a fancy, fully capable lab and compare that with data generated in your own limited facility. When you find the deviations, you can use those as future "correction factors." -- Ed WB6WSN El Cajon, CA USA |
"Frank" wrote in message news:VPqbe.902$0X6.797@edtnps90... Thanks again Ed. From everyone of your posts I learn something new. The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250 MHz. Assume you would test the chamber return loss with a tuned dipole having free space return loss 10dB. Again, the 250 MHz verification of return loss is measured with a horn antenna, typically a double-ridged model like the ARA 2020 or the EMCO 3106. A newer technique is to use ferrite tiles, especially on the floor. They are less than a half-inch thick, and perform much better at low frequencies. And the cost is about $100 / sq ft. I like to think of my walls and ceiling as covered with $5 bills, and the floor carpeted with $10's. Your anechoic chamber is never really perfect; however, it becomes "good enough" when you run out of money. I have heard of the ferrite floor tiles, and are probably a much better solution than inverted pyamids fitted into the floor mounted pyramids. Before I installed the ferrite floor tiles, I had considerable problems with resonances, starting around 7 MHz and continuing through about 150 MHz, associated with the chamber XYZ dimensions. After the ferrite installation, the resonances have nearly disappeared. I did a lot of analysis to figure out what was required, but never got to finish it, on account of being laid-off! Nobody ever seems to want to spend the money to get it right. You can write that on your chamber wall (but management will be ****ed). OK, just for trivia's sake. If the antenna base was cylindrical, painted grey crinkle, had a 6-position range switch and a brown bakelite top insulator, it was an Empire VA-105. Describes it perfectly But, if it was almost a cube, painted battleship grey, had a black front panel and an 8-position range switch, it was a Stoddart 92138-1 (that number is a hazy memory). Both were passive antennas. The Empire was used with the NF-105 receiver, That was the one I used, now you mention it I remember the model number as the NF-105 So you're older than dirt too? g -- Ed WB6WSN El Cajon, CA USA |
"Reg Edwards" wrote in message ... Reg propped up this tar baby and everyone's taken a punch at it. Perhaps it is time to check in and see if you have your answer yet Reg. ========================================== Wes, Not everybody has yet taken a punch at it. There are several regular names who are missing. All I want is a number, eg., of decibels, preferably from a standards lab. But it has only been been demonstrated "Measurements" is not a "Science" - it is an "Art". Perhaps I can clarify my question. Suppose a customer, perhaps an antenna manufacturer, walks into the lab wheeling behind him a weird contraption (we've heard of them) and asks for the forward and reverse gains to be determined and for a calibration certificate to be issued. For present purposes actual forward and reverse figures don't matter. But for the two figures to be of value the uncertainties in the determination should be stated on the certificate (a legal document). What are TYPICAL uncertainties, in dB, which appear above the Head of the Laboratory's signature. Reg: I'm primarily doing the US MIL-STD-461E protocol, on widely varying test specimens (hand-held, man-worn, 2-meter tall racks, 2-meter diameter parabolic tracking antennas), so I am getting my antennas calibrated for 1 & 3 meter separation distances. The lab I use has an outdoor range, and I get a Certificate of Conformance for each antenna which includes the following statements: "Test and Measurement Equipment used for performance verification is calibrated with traceability to the U.S. National Institute of Standards and Technology. Inspection records, test data, and other evidence of conformance are on file at the sellers facility are are available for inspection on request." They further state a "Calibration Uncertainty of (2 sigma) (+/- 1 dB)" and reference "SAE ARP-958". I then get a listing of the specific instruments used in the generation of my antenna data, along with their calibration certificate numbers and the date of each calibration period expiration. As for the data, I get a tabulation and a plot. For a typical antenna (an EMCO 3115 double-ridged horn covering 1 GHz to 18 GHz), I get a tabular array of antenna correction factor, dB return loss, SWR, numeric gain and dBi, in 250 MHz step increments, across the range. I also get a continuous swept plot of return loss, and from this, I infer that the lab uses my unknown antenna as the transmitting element on their range. This data and the C of C is enough to keep my internal Metrology guy happy, and it gives me a sufficiently warm feeling. I have been using the same antenna lab for almost 10 years, and I informally track the calibration data from year to year on each of my antennas. So far, the data never has looked "copied", fudged, or otherwise egregious. However, that may just mean I'm easily fooled. -- Ed WB6WSN El Cajon, CA USA |
Reg:
I'm primarily doing the US MIL-STD-461E protocol, on widely varying test specimens (hand-held, man-worn, 2-meter tall racks, 2-meter diameter parabolic tracking antennas), so I am getting my antennas calibrated for 1 & 3 meter separation distances. The lab I use has an outdoor range, and I get a Certificate of Conformance for each antenna which includes the following statements: "Test and Measurement Equipment used for performance verification is calibrated with traceability to the U.S. National Institute of Standards and Technology. Inspection records, test data, and other evidence of conformance are on file at the sellers facility are are available for inspection on request." They further state a "Calibration Uncertainty of (2 sigma) (+/- 1 dB)" and reference "SAE ARP-958". I then get a listing of the specific instruments used in the generation of my antenna data, along with their calibration certificate numbers and the date of each calibration period expiration. As for the data, I get a tabulation and a plot. For a typical antenna (an EMCO 3115 double-ridged horn covering 1 GHz to 18 GHz), I get a tabular array of antenna correction factor, dB return loss, SWR, numeric gain and dBi, in 250 MHz step increments, across the range. I also get a continuous swept plot of return loss, and from this, I infer that the lab uses my unknown antenna as the transmitting element on their range. This data and the C of C is enough to keep my internal Metrology guy happy, and it gives me a sufficiently warm feeling. I have been using the same antenna lab for almost 10 years, and I informally track the calibration data from year to year on each of my antennas. So far, the data never has looked "copied", fudged, or otherwise egregious. However, that may just mean I'm easily fooled. -- Ed WB6WSN El Cajon, CA USA ============================================ Much obliged to you Ed for interesting infomation from a reliable source. Including 2-Sigma uncertainty limits of +/- 1 dB. Thanks. Reg, G4FGQ |
On Fri, 29 Apr 2005 20:27:51 -0700, "Ed Price"
wrote: I had considerable problems with resonances, starting around 7 MHz and continuing through about 150 MHz, associated with the chamber XYZ dimensions. Hi Ed, When I was in the Navy, my buddy had, in his former life, been a pipe organ technician (the old fashion type, not the Hammond home organ type). He had worked in the really large Pizza 'n' Pipes types of concessions up and down the Pacific coast, and in the classic theaters of the 30s vintage (Paramounts, Pantages, Orpheums, etc.). He related how those venues made sure that when constructed, no two walls met at 90° nor were parallel so as to break up resonances. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Fri, 29 Apr 2005 20:27:51 -0700, "Ed Price" wrote: I had considerable problems with resonances, starting around 7 MHz and continuing through about 150 MHz, associated with the chamber XYZ dimensions. Hi Ed, When I was in the Navy, my buddy had, in his former life, been a pipe organ technician (the old fashion type, not the Hammond home organ type). He had worked in the really large Pizza 'n' Pipes types of concessions up and down the Pacific coast, and in the classic theaters of the 30s vintage (Paramounts, Pantages, Orpheums, etc.). He related how those venues made sure that when constructed, no two walls met at 90° nor were parallel so as to break up resonances. 73's Richard Clark, KB7QHC Umm, yes. Unfortunately, the suppliers of modular shielded enclosures are rigorously orthogonal guys! g -- Ed WB6WSN El Cajon, CA USA |
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