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Accuracy of Antenna Testing Ranges
All electrical calibration and testing laboratories issue tables of
claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. ---- Reg, G4FGQ |
Reg Edwards wrote:
All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. There is no simple reply, Reg, but you're very welcome to come down and read three box-files full of references on this subject. It all depends what you're trying to measu simple forward gain or the complete directional radiation pattern; absolute or relative gain; and whether the antenna is a beam or something less directional. The kind of measurement that is subject to the least errors is a comparison of forward gain between two or more directional antennas that are very similar. The more similar the antennas under test (AUTs) are, the better the errors in each individual measurement will match and cancel out. The more directional the AUT is, the less its gain measurement will be affected by unwanted reflections. The largest source of error in this case is probably in the uniformity of field strength and phase across the test space where you will position the AUT. There is no single answer in dB for this: you would have to estimate the error-bars by modeling on a case-by-case basis. Amateur measurements, such as those made by VHF Groups in the USA, typically use a ground reflection range technique that creates a test volume at a height of about 6-10ft above ground, to make it easily accessible by standing on a picnic table and waving the antenna about by hand, but these practical needs will also increase the errors compared with a professional range with remote-controlled positioning and more time to do it properly. However, within their limitations, careful amateur measurements can make valid better/worse comparisons between very similar antennas. Reproducibility of gain measurements on the same yagi is within a few tenths of a dB... and the more similar your AUTs are, the closer you can approach this limit when comparing different antennas. Absolute gain measurement is an additional can of worms. The most common amateur mistake is to attempt to measure gain in dBd by comparing a long yagi against a reference dipole. BIG MISTAKE! A dipole is so non-directional, it makes the so-called "reference" measurement very vulnerable to stray reflections that a sharper beam just doesn't see, so any so-called "standard dipole" is in fact totally worthless. Or even worse than worthless, the "results" can be anything you want, wish or dream of. Amateur antenna literature is full of such examples, all fueled by over-active imagination. The solution is to use a reference antenna that is as directional as the AUTs, and to measure or compute its gain by some other means. For example, there is an IEEE standard gain reference antenna that has been designed to be both directional and reproducible (in the sense that its gain is quite tolerant of construction errors) and the gain of that antenna has been very carefully measured under the best possible lab conditions. For microwaves, the usual reference is a standard horn antenna whose gain can be both measured and computed. What amateur groups like Scott's tend to do is to keep a "gold standard" reference yagi that is used for all their own measurement meets - and above all, to put much more faith in the *relative* gain comparisons than in the claimed absolute gains. For HF antennas, the required physical size of the test range scales up with the wavelength, and all the problems about range reflections and non-directional of AUTs become impossible for professionals and amateurs alike. That means even professionals are thrown back to computer modeling... which amateurs can do equally well. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
"Reg Edwards" wrote in message
... All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. ---- Reg, G4FGQ Interesting topic Reg. I have always been concerned with uncertainties involved in antenna measurements. ATR antennas do not always provide the source for calibration, but assume it could be ANSI/IEEE Std 149-1979. ETS-Lingren quotes, for their conical log spiral, model 3102 antenna factor uncertainty as SAE, ARP 958 1M (With which I am not familiar). The antenna factor uncertainty is specified as +/- 0.8dB from 1 - 10 GHz. More data, including other conical log spirals, biconicals etc. is available on ETS-Lingren's web site at www.ets-lingren.com if anybody is interested. One company I worked for was making measurements in a 3 meter shielded ATR. Their distance measurements from the source were measured from the support pole of a conical log spiral, when it should have been measured from the tip. With the 3102 antenna this introduced a 3 dB error. Radiated spurious limits quoted in V/m were also assumed to be peak, but not specified. Some research indicated that these limits are in fact RMS -- another 3dB error! When used for linear field measurements the conical log spiral gain is 3 dB below that for circular polarization. Conical log spirals are calibrated with a circularly polarized signals. Agilents 11940A has an antenna factor uncertainty of +/- 2dB from 30 MHz to 1 GHz, and is calibrated in the far field, for near field measurements. ETS-Lingren's antennas are all calibrated at 1m irrespective of frequency. When questioned about accuracies of measurements people usually say "This is the way we have always done it". I have very little experience with HF outdoor measurements, but have heard of sites using high wooden towers to minimize ground reflection effects. A helicopter is required to plot the field strength. Very few EMC antenna manufacturers seem concerned with low frequency far-field measurements. Siemens was about the only company I could find for such measurements. Regards, Frank (VE6CB) |
Very interesting post Reg
I see gain figures in Db alone as misleading since one isn't aware if lobe thickness ( and elevation angle) is taken into consideration. As Ian points out, comparison to a dipole is fatal since comparison of max gain inevitably involves different angles of elevation ( I assume that labs take this into consideration but I have no proof of it.) As I have stated in the past, different antenna designs provide different lobe thicknesses, such that comparisons with each other can provide higher antenna gains to the lab's whim if the elevation angle is not taken into consideration. Taken to the extreme, all antennas with the yagi design can be declared equal in gain when measured at the elevation angle where the leading lobes intersect Regards Art "Reg Edwards" wrote in message ... All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. ---- Reg, G4FGQ |
Reg, G4FGQ wrote:
"Naturally, laboratories can differ one from another." A lab may put its stamp of approval on your instrument, but your best assurance may be measurement of known values. The temperature of ice-water or the voltage of new dry cells, for example You usually can try several dry cells for confirmation or averaging. In antennas, one strategy for successful gain determination is comparison with an antenna of known gain. To determine the gain of a SW BC curtain antenna, we hung a 3-wire (to match 600-ohms) folded dipole alongside and at the same height as the curtain. We swiched transmission back and forth every 5 minutes between the dipole and the curtain. We continuously measured and recorded the signal strength for several days in the target area. We averaged strengths of each signal and compared them for periods of the recordings. The HF dBd of the curtain agreed very well with that measured on the model at 400 MHz in the lab before the curtain was built at full scale. Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote in message ... Reg, G4FGQ wrote: "Naturally, laboratories can differ one from another." A lab may put its stamp of approval on your instrument, but your best assurance may be measurement of known values. The temperature of ice-water or the voltage of new dry cells, for example You usually can try several dry cells for confirmation or averaging. In antennas, one strategy for successful gain determination is comparison with an antenna of known gain. Whow, thats a good idea, write it up for QST. They are looking for pearls of wisdom that can be useful for ham radio operators so that we may maintain our perceived leadership of the art of antennas......'Compare with a antenna of known gain'...... Revolutionary! Now why hasn't any Guru on this group thought of this before today? Now we have to decide what we use to measure the gain and more important not to compare or to compare at a single recieving point especially if the receiving depends on skip or propagation. Is it possible that Guru's are unaware that elevation angles can be different when comparing antennas? Another gem for the ARRL and provided solely by the leading gurus of AMATEUR radio operators no less. Ofcourse we need a telephone link with the country that we wish to hear the transmission, some thing on the simple lines of ....."can you hear me now" question as we switch antennas between a dipole and a drape / curtain array every 5 minutes Art To determine the gain of a SW BC curtain antenna, we hung a 3-wire (to match 600-ohms) folded dipole alongside and at the same height as the curtain. We swiched transmission back and forth every 5 minutes between the dipole and the curtain. We continuously measured and recorded the signal strength for several days in the target area. We averaged strengths of each signal and compared them for periods of the recordings. The HF dBd of the curtain agreed very well with that measured on the model at 400 MHz in the lab before the curtain was built at full scale. Best regards, Richard Harrison, KB5WZI |
"Frank" wrote in message news:oNOae.62351$VF5.45329@edtnps89... "Reg Edwards" wrote in message ... All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. ---- Reg, G4FGQ Interesting topic Reg. I have always been concerned with uncertainties involved in antenna measurements. ATR antennas do not always provide the source for calibration, but assume it could be ANSI/IEEE Std 149-1979. ETS-Lingren quotes, for their conical log spiral, model 3102 antenna factor uncertainty as SAE, ARP 958 1M (With which I am not familiar). The antenna factor uncertainty is specified as +/- 0.8dB from 1 - 10 GHz. More data, including other conical log spirals, biconicals etc. is available on ETS-Lingren's web site at www.ets-lingren.com if anybody is interested. One company I worked for was making measurements in a 3 meter shielded ATR. Their distance measurements from the source were measured from the support pole of a conical log spiral, when it should have been measured from the tip. With the 3102 antenna this introduced a 3 dB error. Radiated spurious limits quoted in V/m were also assumed to be peak, but not specified. Some research indicated that these limits are in fact RMS -- another 3dB error! When used for linear field measurements the conical log spiral gain is 3 dB below that for circular polarization. Conical log spirals are calibrated with a circularly polarized signals. A conical log spiral antenna's radiating plane moves along it's axis with frequency. Various models place the support pole at the rear or at the center of the radiating axis. In any case, use this class of antennas was strongly discouraged after 1996 by MIL-STD-461D. Agilents 11940A has an antenna factor uncertainty of +/- 2dB from 30 MHz to 1 GHz, and is calibrated in the far field, for near field measurements. ETS-Lingren's antennas are all calibrated at 1m irrespective of frequency. Your should always calibrate your measurement antenna in accordance with the applicable testing standard. For MIL-STD-461E, this means a 1-meter distance. For commercial emission testing, that means separate calibration tables for 3-meter, 10-meter & 30-meter ranges. And for some conditions, like FCC Part 18 or broadcast station field-strength "footprints", you should obtain a true far-field calibration. Calibration at any distance other than the actual use distance is just not enough. When questioned about accuracies of measurements people usually say "This is the way we have always done it". I have very little experience with HF outdoor measurements, but have heard of sites using high wooden towers to minimize ground reflection effects. A helicopter is required to plot the field strength. Very few EMC antenna manufacturers seem concerned with low frequency far-field measurements. Siemens was about the only company I could find for such measurements. Regards, Frank (VE6CB) Perhaps the lack of interest in "low frequency far-field" measurements is driven by an absence of any "low-frequency, far-field" compliance requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field emissions right down to 10 kHz, but at a 1-meter separation distance, this is decidedly near-field! BTW, calibration of this standard's defined 10 kHz to 30 MHz test antenna (an electrically short 41" monopole standing above a small ground plane) is not done on an antenna range! The calibration technique is all conducted, with a known signal being applied by coax, through a shielded 10 pF capacitor, to the antenna input point of the matching network (a box at the base of the 41" rod). The accuracy of the calibration is dependent only on the test lab's ability to read the RF input & output voltages. -- Ed WB6WSN El Cajon, CA USA |
To determine the gain of a SW BC curtain antenna, we hung a 3-wire
(to match 600-ohms) folded dipole alongside and at the same height as the curtain. We swiched transmission back and forth every 5 minutes between the dipole and the curtain. We continuously measured and recorded the signal strength for several days in the target area. We averaged strengths of each signal and compared them for periods of the recordings. The HF dBd of the curtain agreed very well with that measured on the model at 400 MHz in the lab before the curtain was built at full scale. Best regards, Richard Harrison, KB5WZI ================================== Richard, fine, so that was the measurement procedure. Thanks for the description. Now all we want to know is what was the uncertainty in the measurement. Was it within plus or minus x percent? Or perhaps plus or minus y decibels? Can you remember the uncertainty approximately? Or perhaps it didn't matter what the uncertainty was. In which case it was a waste of time making the measurement. ---- Reg, G4FGQ |
Reg, G4FGQ wrote:
"Or perhaps it didn`t matter what the uncertainty was." Examination of the comparative feild strength data left no doubt that the antenna was working as expected. This was the first of several similar antennas to be constructed. Before proceeding we needed verification of the design and construction.. It worked and we built more. Best regards, Richard Harrison, KB5WZI |
Ed, thanks very much for your most interesting comments.
A conical log spiral antenna's radiating plane moves along it's axis with frequency. Various models place the support pole at the rear or at the center of the radiating axis. In any case, use this class of antennas was strongly discouraged after 1996 by MIL-STD-461D. You raise an interesting point. The fact is, it never occured to me, yet is is obvious when you think about it. This implies that at certain frequencies a radiated spurious emission of a certain polarization could be missed. As with conventional log periodics, at any given freqency, a section of the antenna will be active, so I guess you would not get complete rejection. The ETS-Lingren model 3102, has its support pole at the rear, and the 3101 is about 1/3 from the rear. I was not aware of the discouragement in the use of these class of antennas by MIL-STD-461D. Seems pretty sad, when you consider the company I was working for advertised its ATR capability, with no mention made of the MIL standard. Your should always calibrate your measurement antenna in accordance with the applicable testing standard. For MIL-STD-461E, this means a 1-meter distance. For commercial emission testing, that means separate calibration tables for 3-meter, 10-meter & 30-meter ranges. I have seen cal data for 1, 3, 10, and 30m but all are concerned with radiated EMC, and not antenna field strengths, which always was much more interesting. Still the 30 m calibration would be acceptable for most HF work -- at least above 5 MHz. And for some conditions, like FCC Part 18 or broadcast station field-strength "footprints", you should obtain a true far-field calibration. Calibration at any distance other than the actual use distance is just not enough. Makes sense. Perhaps the lack of interest in "low frequency far-field" measurements is driven by an absence of any "low-frequency, far-field" compliance requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field emissions right down to 10 kHz, but at a 1-meter separation distance, this is decidedly near-field! At 10 kHz it is probably mostly capacative coupling at 1 m. BTW, calibration of this standard's defined 10 kHz to 30 MHz test antenna (an electrically short 41" monopole standing above a small ground plane) is not done on an antenna range! The calibration technique is all conducted, with a known signal being applied by coax, through a shielded 10 pF capacitor, to the antenna input point of the matching network (a box at the base of the 41" rod). The accuracy of the calibration is dependent only on the test lab's ability to read the RF input & output voltages. Sounds like you are talking about a monopole made by EMCO, which had switched frequency ranges. ETS-Lingren (I think they bought out EMCO) now sell model 3301B that has a calibrated antenna factor down to 20 Hz. Must have a very high gain amp, as the antenna factor is only about 25 dB at 20Hz. I have no idea how a cal procedure, using a 10 pF capacitor, can relate the output level to an incident E-field on a 41" monopole. The losses in the matching networks must be very high at the lower frequencies also. Without attempting to analyze such a monopole, the radiation resistance must be in the milli-ohm, to micro-ohm range. -- Ed WB6WSN El Cajon, CA USA Frank VE6CB |
"Frank" wrote in message news:U_Uae.64373$VF5.13953@edtnps89... Ed, thanks very much for your most interesting comments. A conical log spiral antenna's radiating plane moves along it's axis with frequency. Various models place the support pole at the rear or at the center of the radiating axis. In any case, use this class of antennas was strongly discouraged after 1996 by MIL-STD-461D. You raise an interesting point. The fact is, it never occured to me, yet is is obvious when you think about it. This implies that at certain frequencies a radiated spurious emission of a certain polarization could be missed. As with conventional log periodics, at any given freqency, a section of the antenna will be active, so I guess you would not get complete rejection. The ETS-Lingren model 3102, has its support pole at the rear, and the 3101 is about 1/3 from the rear. I was not aware of the discouragement in the use of these class of antennas by MIL-STD-461D. Seems pretty sad, when you consider the company I was working for advertised its ATR capability, with no mention made of the MIL standard. Everbody loves to argue about antennas; their calibration, application & accuracy! In the EMC area (my side of the elephant), we are frequently looking for emissions with a maximum limit so low (imposed by the standard) that we have to be inside a shielded enclosure. Since the cost of a chamber increases as the square (or maybe the cube) of its volume, only extraordinarily well-funded (uhh, governmental) labs can afford really huge chambers. Thus, most EMC testing happens in more modest volumes (my chamber is 36' x 24' x 9'). Because the standard recognizes that a lot of the required test frequency range practically puts the measurements in less than far-field conditions, the standard gets very picky in defining the acceptable antennas and the test setup and methodology. Here's what MIL-STD-461E says about conical logarithmic spiral antennas: "Previous versions of this standard specified conical log spiral antennas. These antennas were convenient since they did not need to be rotated to measure both polarizations of the radiated field. The double ridged horn is considered to be better for standardization for several reasons.At some frequencies, the antenna pattern of the conical log spiral is not centered on the antenna axis. The double ridged horn does not have this problem. The circular polarization of the conical log spiral creates confusion in its proper application. Electric fields from EUTs would rarely be circularly polarized. Therefore, questions are raised concerning the need for 3 dB correction factors to account for linearly polarized signals. The same issue is present when spiral conical antennas are used for radiated susceptibility testing. If a second spiral conical is used to calibrate the field correctly for a circularly polarized wave, the question arises whether a 3 dB higher field should be used since the EUT will respond more readily to linearly polarized fields of the same magnitude." Perhaps the lack of interest in "low frequency far-field" measurements is driven by an absence of any "low-frequency, far-field" compliance requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field emissions right down to 10 kHz, but at a 1-meter separation distance, this is decidedly near-field! At 10 kHz it is probably mostly capacative coupling at 1 m. BTW, calibration of this standard's defined 10 kHz to 30 MHz test antenna (an electrically short 41" monopole standing above a small ground plane) is not done on an antenna range! The calibration technique is all conducted, with a known signal being applied by coax, through a shielded 10 pF capacitor, to the antenna input point of the matching network (a box at the base of the 41" rod). The accuracy of the calibration is dependent only on the test lab's ability to read the RF input & output voltages. Sounds like you are talking about a monopole made by EMCO, which had switched frequency ranges. ETS-Lingren (I think they bought out EMCO) now sell model 3301B that has a calibrated antenna factor down to 20 Hz. Must have a very high gain amp, as the antenna factor is only about 25 dB at 20Hz. I have no idea how a cal procedure, using a 10 pF capacitor, can relate the output level to an incident E-field on a 41" monopole. The losses in the matching networks must be very high at the lower frequencies also. Without attempting to analyze such a monopole, the radiation resistance must be in the milli-ohm, to micro-ohm range. The 41" (or really, 104 cm, gotta get with the program!) the monopole rod goes way back, to the early 50's. It was originally intended to go down to 150 kHz, and the designs (Stoddart, Empire, Fairchild, Singer, AHS, EMCO) were all variations of a 41" rod atop a box containing manually switched transformers. Later designs incorporated remote switching, but these were still passive antennas, with horrible efficiency and high antenna factors/ A big change happened in the early 70's, when active designs came out. The 41" rod was still there (some designs added a big capactive top-hat for greater pick-up), but it now stood on a switchless box that had a very high input impedance FET. (Don't touch that rod; ESD!) But this design allowed antenna factors to approach 0 dB, and yielded a flat gain across 11 octaves! (That nice for automated acquisition systems.) OTOH, these may not really be antennas any more. They certainly can't be driven with RF power to act as a radiator, so maybe we should be calling them "field probes" instead of antennas. Since you asked about the rod calibration procedure, here's some background on it, again from MIL-STD-461E: "There are two different mounting schemes for baluns of available 104 centimeter rod antennas with respect to the counterpoise. Some are designed to be mounted underneath the counterpoise while others are designed for top mounting. Either technique is acceptable provided the desired 0.5 meter electrical length is achieved with the mounting scheme. The 10 pF capacitor used with the rod antenna in 5.16.3.4.c(3) as part of the system check simulates the capacitance of the rod element to the outside world. With the rod antenna, the electric field present induces a voltage in the rod that is applied to the balun circuitry. One of the functions of the balun is to convert the high impedance input of the antenna element to the 50 ohm impedance of the measurement receiver. The 10 pF capacitor ensures that the correct source impedance is present during the check. Some antennas have a 10 pF capacitor built into the rod balun for calibration purposes and some require that an external capacitor be used. For measurement system checks, establishing the correct voltage at the input to the 10 pF capacitor can be confusing dependent upon the design of the antenna and the associated accessories. Since, the electrical length of the 104 cm rod is 0.5 meters, the conversion factor for the induced voltage at the input to the 10 pF capacitor is 6 dB/m. If the limit at the measurement system check frequency is 34 dBuV/m, the required field level to use for measurement system check is 6 dB less than this value or 28 dBuV/m. The voltage level that must be injected is: 28 dBuV/m – 6 dB/m = 22 dBuV Since the input impedance at the 10 pF capacitor is very high, a signal source must be loaded with 50 ohms (termination load or measurement receiver) to ensure that the correct voltage is applied. A “tee” connection can be used with the signal source connected to the first leg, the 50 ohm load connected to the second leg, and the center conductor of the third leg connected to the 10 pF capacitor (barrel referenced to the balun case). Sometimes a feed-through accessory that acts as a voltage divider is supplied with a rod antenna for the purpose of determining antenna factors. The accessory usually includes the required 10 pF capacitor inside the accessory. If the accessory is used for injecting the measurement system check signal, caution needs to be observed. Since the accessory is intended for only determining antenna factors, the procedures provided with these accessories may not address the actual voltage that appears at the 10 pF capacitor. The design of the accessory needs to be reviewed to determine that the correct voltage is obtained. For a common design, the voltage at the capacitor is 14.6 dB less than the signal source level and 5.0 dB greater than the indication on the measurement receiver." Whew! That's why I'm glad I only use, and not design or calibrate, those things! -- Ed WB6WSN El Cajon, CA USA |
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Richard,
You state that you used a dipole to compare with, which was at the same height !. Which antenna was altered so that the elevation angle of maximum gain was the same for both antennas.such that max gain measurements were truly comparable? Where was the height of the "curtain" measured or referred to so that "same height" could be justified ? ( You also did say it was for SW use which is certainly different to ground wave use) Presumably, the comparison was for the same type of polarization and ignored differences created by the side addition of other types of polarization. Without further information the "Facts" could be seen as correct to plus or minus 100 percent measurement error! An expert in the field of measurements such as Richard could have a field day disecting the test mode as discussed by you and certainly does not reflect the professional antenna analysis aproach which Reg is seeking., which, most certainly, would take into account the elevation angle at which maximum gain occurs as well as many other things Art "Richard Harrison" wrote in message ... Reg, G4FGQ wrote: "Or perhaps it didn`t matter what the uncertainty was." Examination of the comparative feild strength data left no doubt that the antenna was working as expected. This was the first of several similar antennas to be constructed. Before proceeding we needed verification of the design and construction.. It worked and we built more. Best regards, Richard Harrison, KB5WZI |
Art Unwin wrote:
"If Richard really thpoght that comparing to a dipole was unique." Not at all. I was relating an experience which I hoped was accurate and useful. Kraus describes the "comparison method" on page 857 of his 3rd edition of "Antennas". I used the reverse of his example, switching transmitting antennas instead of receiving antennas. Kraus` volume goes into many details of antenna testing. Best regards, Richard Harrison, KB5WZI |
"Ed Price" wrote in message news:H_Vae.2007$pk5.904@fed1read02... Everbody loves to argue about antennas; their calibration, application & accuracy! In the EMC area (my side of the elephant), we are frequently looking for emissions with a maximum limit so low (imposed by the standard) that we have to be inside a shielded enclosure. Since the cost of a chamber increases as the square (or maybe the cube) of its volume, only extraordinarily well-funded (uhh, governmental) labs can afford really huge chambers. Thus, most EMC testing happens in more modest volumes (my chamber is 36' x 24' x 9'). Last place I worked with EMC facilities they only had a 3 m cube chamber. The dimensions you quoted are huge compared to my experience. (I think ETC, in Airdrie Alberta, had a similar chamber to yours; also General Dynamics in Calgary had two similar chambers. Also Nortel has some EMC capabiltiy.) The insides were covered in microwave absorber, and there was some question as to how effective the absorber was at 30 MHz. It must have done something, since before the absorber was installed it was interesting to see the effects on a transmitter keyed inside a shielded enclosure. Because the standard recognizes that a lot of the required test frequency range practically puts the measurements in less than far-field conditions, the standard gets very picky in defining the acceptable antennas and the test setup and methodology. Here's what MIL-STD-461E says about conical logarithmic spiral antennas: "Previous versions of this standard specified conical log spiral antennas. These antennas were convenient since they did not need to be rotated to measure both polarizations of the radiated field. The double ridged horn is considered to be better for standardization for several reasons.At some frequencies, the antenna pattern of the conical log spiral is not centered on the antenna axis. The double ridged horn does not have this problem. The circular polarization of the conical log spiral creates confusion in its proper application. Electric fields from EUTs would rarely be circularly polarized. Therefore, questions are raised concerning the need for 3 dB correction factors to account for linearly polarized signals. The same issue is present when spiral conical antennas are used for radiated susceptibility testing. If a second spiral conical is used to calibrate the field correctly for a circularly polarized wave, the question arises whether a 3 dB higher field should be used since the EUT will respond more readily to linearly polarized fields of the same magnitude." Very interesting Ed, will forward your comments to my last company. Doubt they will do anything tho, as they never want to spend any money. Assume the recomended type of antenna is a linearly polarized log periodic. The 41" (or really, 104 cm, gotta get with the program!) the monopole rod goes way back, to the early 50's. It was originally intended to go down to 150 kHz, and the designs (Stoddart, Empire, Fairchild, Singer, AHS, EMCO) were all variations of a 41" rod atop a box containing manually switched transformers. Later designs incorporated remote switching, but these were still passive antennas, with horrible efficiency and high antenna factors/ I remember the Singer (Was it Singer-Metrics), and using it to measure radiated spurious in a cow pasture at 50 m from a 1kW TMC linear (Canadian Marconi, Montreal). The test monopole had a cylindrical base with a rotary switch. A big change happened in the early 70's, when active designs came out. The 41" rod was still there (some designs added a big capactive top-hat for greater pick-up), but it now stood on a switchless box that had a very high input impedance FET. (Don't touch that rod; ESD!) But this design allowed antenna factors to approach 0 dB, and yielded a flat gain across 11 octaves! (That nice for automated acquisition systems.) OTOH, these may not really be antennas any more. They certainly can't be driven with RF power to act as a radiator, so maybe we should be calling them "field probes" instead of antennas. Since you asked about the rod calibration procedure, here's some background on it, again from MIL-STD-461E: "There are two different mounting schemes for baluns of available 104 centimeter rod antennas with respect to the counterpoise. Some are designed to be mounted underneath the counterpoise while others are designed for top mounting. Either technique is acceptable provided the desired 0.5 meter electrical length is achieved with the mounting scheme. The 10 pF capacitor used with the rod antenna in 5.16.3.4.c(3) as part of the system check simulates the capacitance of the rod element to the outside world. With the rod antenna, the electric field present induces a voltage in the rod that is applied to the balun circuitry. One of the functions of the balun is to convert the high impedance input of the antenna element to the 50 ohm impedance of the measurement receiver. The 10 pF capacitor ensures that the correct source impedance is present during the check. Some antennas have a 10 pF capacitor built into the rod balun for calibration purposes and some require that an external capacitor be used. For measurement system checks, establishing the correct voltage at the input to the 10 pF capacitor can be confusing dependent upon the design of the antenna and the associated accessories. Since, the electrical length of the 104 cm rod is 0.5 meters, the conversion factor for the induced voltage at the input to the 10 pF capacitor is 6 dB/m. If the limit at the measurement system check frequency is 34 dBuV/m, the required field level to use for measurement system check is 6 dB less than this value or 28 dBuV/m. The voltage level that must be injected is: 28 dBuV/m - 6 dB/m = 22 dBuV Since the input impedance at the 10 pF capacitor is very high, a signal source must be loaded with 50 ohms (termination load or measurement receiver) to ensure that the correct voltage is applied. A "tee" connection can be used with the signal source connected to the first leg, the 50 ohm load connected to the second leg, and the center conductor of the third leg connected to the 10 pF capacitor (barrel referenced to the balun case). Sometimes a feed-through accessory that acts as a voltage divider is supplied with a rod antenna for the purpose of determining antenna factors. The accessory usually includes the required 10 pF capacitor inside the accessory. If the accessory is used for injecting the measurement system check signal, caution needs to be observed. Since the accessory is intended for only determining antenna factors, the procedures provided with these accessories may not address the actual voltage that appears at the 10 pF capacitor. The design of the accessory needs to be reviewed to determine that the correct voltage is obtained. For a common design, the voltage at the capacitor is 14.6 dB less than the signal source level and 5.0 dB greater than the indication on the measurement receiver." Whew! That's why I'm glad I only use, and not design or calibrate, those things! It does seem a bit confusing. I have never seen this procedure before, and do not understand how a physical length of 1.04 m can have an electrical length of 0.5m. I guess the 10pf capacitance of the rod is its capacitance with a defined ground plane size. I don't think I would be 100% convinced as to the procedures accuracy unless I could verify it with a known E field. At least, in principal, I understand what is being done. -- Ed WB6WSN El Cajon, CA USA Frank VE6CB |
On Sun, 24 Apr 2005 19:38:10 GMT, "
wrote: | |"Richard Harrison" wrote in message ... | Reg, G4FGQ wrote: | "Naturally, laboratories can differ one from another." | | A lab may put its stamp of approval on your instrument, but your best | assurance may be measurement of known values. The temperature of | ice-water or the voltage of new dry cells, for example You usually can | try several dry cells for confirmation or averaging. | | In antennas, one strategy for successful gain determination is | comparison with an antenna of known gain. | |Whow, thats a good idea, write it up for QST. They are looking for pearls of |wisdom |that can be useful for ham radio operators so that we may maintain our |perceived |leadership of the art of antennas......'Compare with a antenna of known |gain'...... Revolutionary! |Now why hasn't any Guru on this group thought of this before today? Perhaps because it's so commonplace that it doesn't bear mentioning. |Now we have to decide what we use to measure the gain and more important |not to compare or to compare at a single recieving point especially if the |receiving depends | on skip or propagation. Is it possible that Guru's are unaware that |elevation angles |can be different when comparing antennas? Another gem for the ARRL and |provided |solely by the leading gurus of AMATEUR radio operators no less. Ofcourse we |need |a telephone link with the country that we wish to hear the transmission, |some thing on the simple lines of |...."can you hear me now" | question as we switch antennas |between a dipole and a drape / curtain array every 5 minutes If you believe that precision antenna gain measurements are made under ionospheric propagation conditions, you are clearly delusional. But I repeat myself. |
"Wes Stewart" *n7ws*@ yahoo.com wrote in message ... On Sun, 24 Apr 2005 19:38:10 GMT, " wrote: | |"Richard Harrison" wrote in message ... | Reg, G4FGQ wrote: | "Naturally, laboratories can differ one from another." | | A lab may put its stamp of approval on your instrument, but your best | assurance may be measurement of known values. The temperature of | ice-water or the voltage of new dry cells, for example You usually can | try several dry cells for confirmation or averaging. | | In antennas, one strategy for successful gain determination is | comparison with an antenna of known gain. | |Whow, thats a good idea, write it up for QST. They are looking for pearls of |wisdom |that can be useful for ham radio operators so that we may maintain our |perceived |leadership of the art of antennas......'Compare with a antenna of known |gain'...... Revolutionary! |Now why hasn't any Guru on this group thought of this before today? Perhaps because it's so commonplace that it doesn't bear mentioning. That's what I thought. So why did Richard say it unless he felt that Reg's education in antennas was a bit lacking. Reg's question was specific and of high caliber Richard's answer tried to bring it down to a level for dummies which did not begin to reflect on the question posed |Now we have to decide what we use to measure the gain and more important |not to compare or to compare at a single recieving point especially if the |receiving depends | on skip or propagation. Is it possible that Guru's are unaware that |elevation angles |can be different when comparing antennas? Another gem for the ARRL and |provided |solely by the leading gurus of AMATEUR radio operators no less. Ofcourse we |need |a telephone link with the country that we wish to hear the transmission, |some thing on the simple lines of |...."can you hear me now" | question as we switch antennas |between a dipole and a drape / curtain array every 5 minutes If Yes ,,,a big "IF" isn't it? But you could supply the info Reg was looking for since you perceive yourself as a GURU . It would be much more rewarding to the group as a whole than picking out somebody to demean.My point is that a gain figure alone is meaningless unless the elevation angle differences or perhaps a 3 dB window comparison are also supplied. If you think otherwise I would welcome a technical response rather than something lead by emotion Art you believe that precision antenna gain measurements are made under ionospheric propagation conditions, you are clearly delusional. But I repeat myself. |
Art Unwin wrote:
"My point is that again figure alone is meaningless unless the elevation angle differences or perhaps a 3 dB window comparison are also supplied." Reg knows very well that a quantity is determined by comparing it with a known standard. The power gain of a resonant dipole in free-space is given by Terman on page 871 of his 1955 edition as 1.64. Kraus agrees on page 54 of his 1950 edition and converts Terman`s power gain of 1.64 to 2.14 dB (referenced to an isotropic). The values given by Terman and Kraus are accepted. Horizontal antennas at the same heights tend to have similar elevation angles, but even if they didn`t, comparison of the signals our two antennas laid on the target represented our interest in the matter. What we confirmed was that the new curtain antenna had a gain comparable with our rhombics but over a wider beamwidth which meant listeners on the edges of our coverage got a better signal with the new curtain antenna. The bandwidth was less than a phombic so the curtain meant more work for the operators, but the broadcasts were for the listeners` benefit. Signal strengths were measured at many locations around the target area to define the coverage of the antenna pattern. Best regards, Richard Harrison, KB5WZI |
On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote: All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. As stated by Ian, there's no simple answer. The bane of antenna testing is reflections reflections reflections. It may come as a surprise to our correspondent who likes to disparage "gurus" that "standard-gain" antennas are widely used as reference standards. To head off the question of how the standard gain is determined, that is done by testing three "identical" antennas in pairs; each one against the other two, with one the source and the other the receiver. A bit of algebra and you have the gain of each one individually. http://www.mi-technologies.com/literature/a00-044.pdf The foregoing paper might help answer Reg's question about achievable accuracy. While not addressing hf and vhf measurements, some of the following might be of interest. Indoor measurements are usually conducted in anechoic chambers where the shape is often tapered to control reflections and the walls are covered in absorber material. A chamber will have a "quiet zone" where the reflections are specified to be X db down. Very often the antennas under test are being characterized for side lobe levels or in the case of monopulse radar, the null depth of the difference pattern(s). If you're trying to measure a 60 dB null, it doesn't pay to have a quiet zone of -40 dB. These measurements also require an amplitude and phase front that mimics a source at infinite distance. This used to require huge chambers, often hundreds of feet long. A new way to accomplish this is to "fold" the range by using specially shaped reflectors to flatten the amplitude/phase across the test aperature. This has the added benefit of shorter cables between sources, DUT and measurement receiver. At X and K band, cable loss can be a killer. Likewise moving cables around and even temperature changes can affect the measurments. I have used such a range to measure antennas from L to Ka band. Outdoor ranges often "feature" the ground reflection, since it is difficult to eliminate it physically. This is particularly true at hf/vhf. I have used a technique that utilized the time-domain capability of a modern network analyzer (HP-8510) to identify the reflection and then place absorber material to attenuate it. Similarly, a frequency-domain measurement, that includes ground reflection, can be transformed to the time domain where the reflection is gated out and then transformed back to the frequency domain for "reflection free" analysis. See also: http://www.lehman-inc.com/pdf/mag2.pdf |
Wes,
What you have posted is very interesting and is not spewing out alot of stuff regarding isentropic gain etc that is really not relevent to an actual testing range. Rather than deflect away from Reg's needs may I go back to the "compared to a dipole" statement which Richard keeps brushing off. If the gains are different then the angle for max radiation is different and if you do not take this into account by searching for the individual point of maximum gain position then the the measurements are in total error. To put antennas at the same height and then measuring at the same stationary point for receive, switching back and forth is not a true comparison because of the different elevation angles. If one was to compare a long yagi to a dipole ando make it a true comparison measurement one must surely take into account the two degree or so difference when positioning the listening posts and not relying on a single listening position which to me appears to be a NO No . Richards response to the "error" question totally ignored TOA saying they are usually the same . He also ignored what he considered as an "equal" height for the curtain, i.e the top,bottom or the center line of the curtain array which alone would introduce error with respect to comparible measurement. If Richard was pointing out that his was a typical professional method of measurement then I would view his statement in complete disbelief. Your posting, thankyou, confirms my thinking in that the use of a dipole only confirms the reliability of the set up used and that is the end of it with respect to measurement of a competing antenna where I suspect a pro lab would identify the particular resulting elevation measurement. If the last sentence is in error I would apreciate a correction Regards Art "Wes Stewart" wrote in message ... On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards" wrote: All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. As stated by Ian, there's no simple answer. The bane of antenna testing is reflections reflections reflections. It may come as a surprise to our correspondent who likes to disparage "gurus" that "standard-gain" antennas are widely used as reference standards. To head off the question of how the standard gain is determined, that is done by testing three "identical" antennas in pairs; each one against the other two, with one the source and the other the receiver. A bit of algebra and you have the gain of each one individually. http://www.mi-technologies.com/literature/a00-044.pdf The foregoing paper might help answer Reg's question about achievable accuracy. While not addressing hf and vhf measurements, some of the following might be of interest. Indoor measurements are usually conducted in anechoic chambers where the shape is often tapered to control reflections and the walls are covered in absorber material. A chamber will have a "quiet zone" where the reflections are specified to be X db down. Very often the antennas under test are being characterized for side lobe levels or in the case of monopulse radar, the null depth of the difference pattern(s). If you're trying to measure a 60 dB null, it doesn't pay to have a quiet zone of -40 dB. These measurements also require an amplitude and phase front that mimics a source at infinite distance. This used to require huge chambers, often hundreds of feet long. A new way to accomplish this is to "fold" the range by using specially shaped reflectors to flatten the amplitude/phase across the test aperature. This has the added benefit of shorter cables between sources, DUT and measurement receiver. At X and K band, cable loss can be a killer. Likewise moving cables around and even temperature changes can affect the measurments. I have used such a range to measure antennas from L to Ka band. Outdoor ranges often "feature" the ground reflection, since it is difficult to eliminate it physically. This is particularly true at hf/vhf. I have used a technique that utilized the time-domain capability of a modern network analyzer (HP-8510) to identify the reflection and then place absorber material to attenuate it. Similarly, a frequency-domain measurement, that includes ground reflection, can be transformed to the time domain where the reflection is gated out and then transformed back to the frequency domain for "reflection free" analysis. See also: http://www.lehman-inc.com/pdf/mag2.pdf |
" wrote in message news:u9_ae.18115$NU4.14900@attbi_s22... Richard, You state that you used a dipole to compare with, which was at the same height !. Which antenna was altered so that the elevation angle of maximum gain was the same for both antennas.such that max gain measurements were truly comparable? Where was the height of the "curtain" measured or referred to so that "same height" could be justified ? ( You also did say it was for SW use which is certainly different to ground wave use) Presumably, the comparison was for the same type of polarization and ignored differences created by the side addition of other types of polarization. Without further information the "Facts" could be seen as correct to plus or minus 100 percent measurement error! And that sums up most antenna testing rather well! -- Ed WB6WSN El Cajon, CA USA |
Art Unwin wrote:
"Richard`s response to the "error" question totally ignored TOA saying they are usually the same." Propagation dictates the take off angle that the signal actually follows regardless of what your antennas do. We made meadurements on different days so that propagation may have been different on different days. We were checking over nearly the actual paths under what might be typical conditions. Did the curtain produce louder signals? You bet! Even though the curtain antenna had sharper vertical directivity as well as sharper horizontal directivity than the lone dipole, these were the goals of the design. Produce more signal on target to try to overcome the myriad of jammers that were trying to drown us out. During our tests, the paths between transmitter and the receivers were the same in most cases. The width of a curtain was only about one wavelength and the dipole was immediately adjacent to the curtain. The curtain was two dipoles high, two dipoles wide and two dipoles deep as I recall. Those dipoles in front were all driven in phase. Those behind were tuned parasitic reflectors. It wasn`t unique at all. I`ve seen many since then which look very much like our curtains. They were well behaved and brought in lots of fan mail. They obviously radiated ok. The reflectors seemed to shield the villiage behind them from being drowned in radio frequency energy. Whatever differences there may have been between the conditions imposed on the dipole and curtain, they were tuned and loaded for the same transmitted power. Received signal differences were likely due to gain in the curtain versus gain in the dipole. Averiging a large number of samples likely straightened out inevitable minor differences. I would wager our results were good enough. My employer was satisfied and all the contractors got paid. Best regards, Richard Harrison, KB5WZI |
"Frank" wrote in message news:1V8be.56318$yV3.14588@clgrps12... "Ed Price" wrote in message news:H_Vae.2007$pk5.904@fed1read02... Everbody loves to argue about antennas; their calibration, application & accuracy! In the EMC area (my side of the elephant), we are frequently looking for emissions with a maximum limit so low (imposed by the standard) that we have to be inside a shielded enclosure. Since the cost of a chamber increases as the square (or maybe the cube) of its volume, only extraordinarily well-funded (uhh, governmental) labs can afford really huge chambers. Thus, most EMC testing happens in more modest volumes (my chamber is 36' x 24' x 9'). Last place I worked with EMC facilities they only had a 3 m cube chamber. The dimensions you quoted are huge compared to my experience. (I think ETC, in Airdrie Alberta, had a similar chamber to yours; also General Dynamics in Calgary had two similar chambers. Also Nortel has some EMC capabiltiy.) The insides were covered in microwave absorber, and there was some question as to how effective the absorber was at 30 MHz. It must have done something, since before the absorber was installed it was interesting to see the effects on a transmitter keyed inside a shielded enclosure. 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. 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). 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. With the dark blue pyramids and black tiles, a chamber looks like a bat cave. One vendor decided that the new millenia needed white paint on the foam; another vendor touts pyramids that have a 90-degree axial rotation part way up the taper, and yet another truncates the pointy tips, telling us that works better. It's just like the antenna game. Here's what MIL-STD-461E says about conical logarithmic spiral antennas: "Previous versions of this standard specified conical log spiral antennas. These antennas were convenient since they did not need to be rotated to measure both polarizations of the radiated field. The double ridged horn is considered to be better for standardization for several reasons. Very interesting Ed, will forward your comments to my last company. Doubt they will do anything tho, as they never want to spend any money. Assume the recomended type of antenna is a linearly polarized log periodic. No, 461 doesn't like log periodics either, saying: "Other linearly polarized antennas such as log periodic antennas are not to be used. It is recognized that these types of antennas have sometimes been used in the past; however, they will not necessarily produce the same results as the double ridged horn because of field variations across the antenna apertures and far field/near field issues. Uniform use of the double ridge horn is required for standardization purposes to obtain consistent results among different test facilities." The MIL-STD defines a 104 cm rod from 10 kHz to 30 MHz, then a biconical from 30 MHz to 200 MHz, and finally, horns above there. Since pyramidal horns are only good for about an octave, a smart Navy guy added exponentially flared ridges to the horns, and came up with multi-octave horns. A typical horn for 200 MHz to 1 GHz has an aperture of about 1 meter, then another horn tries to go from 1 GHz to 18 GHz. That's a bit too far for me, as the antenna factor really climbs above about 14 GHz, so I switch to a common, non-ridged horn for 12 GHz to 18 GHz. For 18 GHz to 26 GHz and 26 GHz to 40 GHz, I use standard-gain flared horns. With a pre-selected spectrum analyzer, really good coax, and a couple of low-noise pre-amps, that lets me get comfortably below the most stringent RE102 limits. I remember the Singer (Was it Singer-Metrics), and using it to measure radiated spurious in a cow pasture at 50 m from a 1kW TMC linear (Canadian Marconi, Montreal). The test monopole had a cylindrical base with a rotary switch. 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. 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, while the Stoddart antenna was associated with the NM-22A (that's why the range switches were different, to match the ranges on their associated receivers). -- Ed WB6WSN El Cajon, CA USA |
Richard, it is now quite clear that you were not undertaking a test
referenced to a dipole. All you were doing is confirming a target area under average conditions to ensure the language used was compatable to the target area.....Period More important to me is your statement that : " Propagation dictates the take off angle that the signal actually follows regardless of what your antennas do" This statement seems to echo a conclusion arrived at by a regular poster ( I should call him a guru) on this group tho leaving me unconvinced. Would you kindly point out to me what book you are extracting this statement from so I may examine the boundaries under which that statement is deemed correct? Thanking you in advance Art "Richard Harrison" wrote in message ... Art Unwin wrote: "Richard`s response to the "error" question totally ignored TOA saying they are usually the same." Propagation dictates the take off angle that the signal actually follows regardless of what your antennas do. We made meadurements on different days so that propagation may have been different on different days. We were checking over nearly the actual paths under what might be typical conditions. Did the curtain produce louder signals? You bet! Even though the curtain antenna had sharper vertical directivity as well as sharper horizontal directivity than the lone dipole, these were the goals of the design. Produce more signal on target to try to overcome the myriad of jammers that were trying to drown us out. During our tests, the paths between transmitter and the receivers were the same in most cases. The width of a curtain was only about one wavelength and the dipole was immediately adjacent to the curtain. The curtain was two dipoles high, two dipoles wide and two dipoles deep as I recall. Those dipoles in front were all driven in phase. Those behind were tuned parasitic reflectors. It wasn`t unique at all. I`ve seen many since then which look very much like our curtains. They were well behaved and brought in lots of fan mail. They obviously radiated ok. The reflectors seemed to shield the villiage behind them from being drowned in radio frequency energy. Whatever differences there may have been between the conditions imposed on the dipole and curtain, they were tuned and loaded for the same transmitted power. Received signal differences were likely due to gain in the curtain versus gain in the dipole. Averiging a large number of samples likely straightened out inevitable minor differences. I would wager our results were good enough. My employer was satisfied and all the contractors got paid. Best regards, Richard Harrison, KB5WZI |
On Mon, 25 Apr 2005 14:48:38 -0700, Wes Stewart
wrote: It may come as a surprise to our correspondent who likes to disparage "gurus" that "standard-gain" antennas are widely used as reference standards. To head off the question of how the standard gain is determined, that is done by testing three "identical" antennas in pairs; each one against the other two, with one the source and the other the receiver. A bit of algebra and you have the gain of each one individually. http://www.mi-technologies.com/literature/a00-044.pdf Hi All, The method described by the paper offered above is a commonplace of Metrology called "Reciprocity." I have calibrated precision microphones against this method, and the error math offered is consistent with my experience (much less the actual values offered as examples). As an aside, this method is also as old as the pyramids - literally. The Egyptians planned their blocks of granite to have nearly flat faces to within 10s of microinches using three blocks, by abrading one against the other and then rotating their positions. Accuracy is far more a matter of protocol or technique than it is about a ruler (or other scale). 73's Richard Clark, KB7QHC |
wrote:
Rather than deflect away from Reg's needs may I go back to the "compared to a dipole" statement which Richard keeps brushing off. If the gains are different then the angle for max radiation is different and if you do not take this into account by searching for the individual point of maximum gain position then the the measurements are in total error. To put antennas at the same height and then measuring at the same stationary point for receive, switching back and forth is not a true comparison because of the different elevation angles. I don't think Richard is attempting to deny that. His tests were not intended to measure the gain of the antenna. They were intended to answer a much more practical question: "How much stronger is the signal from the curtain array, as delivered into the BC target area, compared with using a dipole?" That's what the station owners wanted to know, and they specifically wanted that answer to include all the variables of antenna patterns and ionospheric propagation. As you have correctly pointed out, in any environment except free space, that number is not the same as the antenna gain in dBd. Anybody who has thought about it is aware of the problem, and that clearly includes Richard. Everybody agrees with you, so you can stop banging on that open door. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
" wrote about Richard Harrison's post:
Richard, it is now quite clear that you were not undertaking a test referenced to a dipole. All you were doing is confirming a target area under average conditions to ensure the language used was compatable to the target area.....Period More important to me is your statement that : " Propagation dictates the take off angle that the signal actually follows regardless of what your antennas do" _________ Your arguments arise from trying to compare two different test goals, e.g., accurately measuring the free space az/el radiation patterns of an antenna itself, versus how those radiation patterns may perform in a particular application (height above ground, ground characteristics, ionospheric propagation characteristics, reflection sources, target coverage zone, etc). Classic antenna test ranges are designed to measure the az/el radiation patterns of antennas themselves, independent of their environment. What that radiation will provide in terms of a desired "coverage" result is another matter, and is the responsibility of the RF system designer -- not the antenna test range. RF Visit http://rfry.org for FM transmission system papers. |
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. i.e. some physically realizable antenna with a return loss of 40 dB at the test frequency. I suppose, with an inductivly loaded dipole, you could test the return loss of a 3 m chamber down to 30 MHz. There were some questions raised about possible reflections in the 3 m chamber due to imperfections in the installation of the pyramidal foam. I tried sweeping from 1 to 10 GHz with the log spiral antenna, coupling to a non-standard antenna, and performing an inverse FFT on the network analyzer data to generate a time domain plot. I had very little success in actually seeing reflections. For best resolution the ideal would have been to sweep from 30 MHz to 20 GHz with two wide band antennas, but the company did not want to spend the money for any new antennas. What I am thinking is that careful return loss measurements may have shown if any reflections were present. 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. 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. With the dark blue pyramids and black tiles, a chamber looks like a bat cave. One vendor decided that the new millenia needed white paint on the foam; another vendor touts pyramids that have a 90-degree axial rotation part way up the taper, and yet another truncates the pointy tips, telling us that works better. It's just like the antenna game. I have heard of the ferrite floor tiles, and are probably a much better solution than inverted pyamids fitted into the floor mounted pyramids. No, 461 doesn't like log periodics either, saying: "Other linearly polarized antennas such as log periodic antennas are not to be used. It is recognized that these types of antennas have sometimes been used in the past; however, they will not necessarily produce the same results as the double ridged horn because of field variations across the antenna apertures and far field/near field issues. Uniform use of the double ridge horn is required for standardization purposes to obtain consistent results among different test facilities." The MIL-STD defines a 104 cm rod from 10 kHz to 30 MHz, then a biconical from 30 MHz to 200 MHz, and finally, horns above there. Since pyramidal horns are only good for about an octave, a smart Navy guy added exponentially flared ridges to the horns, and came up with multi-octave horns. A typical horn for 200 MHz to 1 GHz has an aperture of about 1 meter, then another horn tries to go from 1 GHz to 18 GHz. That's a bit too far for me, as the antenna factor really climbs above about 14 GHz, so I switch to a common, non-ridged horn for 12 GHz to 18 GHz. For 18 GHz to 26 GHz and 26 GHz to 40 GHz, I use standard-gain flared horns. With a pre-selected spectrum analyzer, really good coax, and a couple of low-noise pre-amps, that lets me get comfortably below the most stringent RE102 limits. I think they were considering horns and low noise amps to get above 10 GHz. 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. 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 while the Stoddart antenna was associated with the NM-22A (that's why the range switches were different, to match the ranges on their associated receivers). -- Ed WB6WSN El Cajon, CA USA 73, Frank |
"Ian White GM3SEK" wrote in message ... wrote: Rather than deflect away from Reg's needs may I go back to the "compared to a dipole" statement which Richard keeps brushing off. If the gains are different then the angle for max radiation is different and if you do not take this into account by searching for the individual point of maximum gain position then the the measurements are in total error. To put antennas at the same height and then measuring at the same stationary point for receive, switching back and forth is not a true comparison because of the different elevation angles. I don't think Richard is attempting to deny that. The question is about lab techniques and error measurement and Richards post was in answer to that. Another person insinuated that a person who tests against a dipole and measures after a skip has taken place is in Lu Lu land because he assumed he was literally describing a normal lab test of comparing to a dipole! Remember, Richard was replying to the initial post which was very specific in nature regarding lab testing and degree of error ! Assumption has no part in a real laboratory. His tests were not intended to measure the gain of the antenna. They were intended to answer a much more practical question: "How much stronger is the signal from the curtain array, as delivered into the BC target area, compared with using a dipole?" That's what the station owners wanted to know, and they specifically wanted that answer to include all the variables of antenna patterns and ionospheric propagation. I could not agree more and stated so in my last post As you have correctly pointed out, in any environment except free space, that number is not the same as the antenna gain in dBd. Anybody who has thought about it is aware of the problem, and that clearly includes Richard. Then why is he introducing dbi into the subject using Kraus as a backup? Why does he state that TOA are "usually" the same when the opposite is true especially when comparing a curtain to a dipole ? I don't believe that to be correct In the absence of denial by a guru must I assume he is correct or he meant something else and everybody knows what he said is true? So you agree with the poster who stated that if a person thought that a dipole comparison test consistes of comparing after skip took place, is in Lu Lu land? Everybody agrees with you, so you can stop banging on that open door. They do ....???? And the question regarding propagation and antenna function can we assume he is correct on that also ? I don't like to "assume" that he meant something different and let the newbies as well as I to be lead astray. Must I assume he is correct in that last sentence he made where I am asking for a corroborating technical written statement ? Richards last statment was /is an echo of a similar posting made a few months ago and the Gurus said nothing to confirm or deny it's voracity?. What are we meant to assume , that if a guru doesn't question it it must be correct? I personally would rather see corrobaration in a accepted technical writing than set up the beginnings of an old wives tale The old saying is still true , don't rely on one gurus answer, ask another and then another and ensure that context is correct. I await Richards response with interest so that I may read an authoritive statement for myself without the need for "assumption" as to what he "really" meant to say but didn't. If you know what he "meant" to say on that last remaining subject why not supply a helping hand ? Art -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Art Unwin wrote:
"---may I go back to the "compared to a dipole" statement which Richard keeps brushing off." I accept a resonant dipole reference as a given. It is true that the antenna under test and the reference dipole have different radiation patterns. Our goal was to compare received signal strengths at locations of interest. The assumption was that on average, the propaqgation was nearly the same for the signals received from both transmitting antennas. Good or bad propagation, the difference between the signals depended on gain in the direction of the receiver as the transmitted power was the same to both antennas no matter where it landed. Kraus says on page 535 of his 3rd edition of "antennas": "Suppose that we express the gain with respect to a single lambda/2 element as the reference antenna. Let the same power P be supplied to this antenna. Then assuming no heat losses, the current Io is the sq rt of the power divided by the resistance of the reference antenna. In general, the gain in field intensity of an array over a reference antenna is given by the ratio of the field intensity from the array to the field intensity from the reference antenna when both are supplied with the same power P." Kraus` example was our intended case. Our expectations were met and our contractors were paid. Best regards, Richard Harrison, KB5WZI |
On Tue, 26 Apr 2005 00:04:02 -0700, Richard Clark
wrote: On Mon, 25 Apr 2005 14:48:38 -0700, Wes Stewart wrote: It may come as a surprise to our correspondent who likes to disparage "gurus" that "standard-gain" antennas are widely used as reference standards. To head off the question of how the standard gain is determined, that is done by testing three "identical" antennas in pairs; each one against the other two, with one the source and the other the receiver. A bit of algebra and you have the gain of each one individually. http://www.mi-technologies.com/literature/a00-044.pdf Hi All, The method described by the paper offered above is a commonplace of Metrology called "Reciprocity." I have calibrated precision microphones against this method, and the error math offered is consistent with my experience (much less the actual values offered as examples). It is also a method used for determining the phase noise of low noise oscillators. As an aside, this method is also as old as the pyramids - literally. The Egyptians planned their blocks of granite to have nearly flat faces to within 10s of microinches using three blocks, by abrading one against the other and then rotating their positions. Accuracy is far more a matter of protocol or technique than it is about a ruler (or other scale). 73's Richard Clark, KB7QHC |
On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote: All electrical calibration and testing laboratories issue tables of claimed accuracies of measurements. Measurement uncertainties stated on calibration certificates are legally binding. All stated measurement results must be traceable to International Standards or a laboratory or testing station loses its status. Consequently there is no incentive for a laboratory to overstate its capabilities in its sales literature. Indeed, it is dangerous, illegal even! Naturally, laboratories can differ widely, one from another. It would be interesting to compare laboratory uncertainties with performance figures claimed by antenna manufacturers. Or anyone else. Does anyone have typical examples of measurement uncertainties claimed by antenna testing stations? Answers in decibels please. A reply from a testing station, at HF or VHF, would be specially appreciated. 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. |
"Richard Harrison" wrote
The assumption was that on average, the propaqgation was nearly the same for the signals received from both transmitting antennas. Good or bad propagation, the difference between the signals depended on gain in the direction of the receiver as the transmitted power was the same to both antennas no matter where it landed. "Propagation" has to include ALL means by which EM energy radiated from a wire antenna finally arrives at a receiving location. That necessarily includes the radiation effects of reflecting/obstructing objects and surfaces, each of which may be illuminated by varying ERP from the wire antenna -- depending on the radiation envelope of the wire antenna itself, its installation detail, and site topology. The ERP directed toward a particular receiving site depends on more than the free space gain of the tx antenna along a single launch angle (which I believe is Art's point). RF |
Art Unwin wrote:
"Remember, Richard was replying to the initial post which was very specific in nature regarding lab testing and degree of error." Antenna test facilities involve far fields. Kraus says on page 831 of his 3rd edition of "Antennas": "---it is obvious that measurement usually takes place in the far field." This can be far indeed with highly directive antennas. My initial response included: "A lab may put its stamp of approval on your instrument, but your best assurance may be measurement of known values. The termperature of ice-water or the voltage of new dry cells, for example. You usually can try several dry cells for confirmation or averaging. In antennas, one strategy for successful gain determination is comparison with an antenna of known gain." My posting was imperfect. There`s nothing that can`t be improved, but were I re-writing my posting, I can`t think how I might improve it. I don`t think my example of checking gain of an array using skywaves was amiss. We build shortwave antennas to use skywaves. We give antenna gains in free-space because it makes sense. I said we built a small-scale model first because we can measure the model`s characteristics without a helicopter. The full-scale antenna performed exactly like the model. Computer modeling has eliminated the small-scale model step in new designs. We checked only the first off of the new design, with the full confidence that subsequent antennas of the same design would perform the same. Of the first antenna, we measured everything including the currents along each element. We used an R-F ammeter in a loop suspended from the element and towed along with a string. We read it using a telescope. The antenna was a scientific success as well as a practical success. This differs from some of the oil wells I was to drill later, though some of those succeeded too. I can only post what I know and it will never satisfy what everybody wants to read. Sometimes my postings are more responsive than others. That`s part of the fun. Best regards, Richard Harrison, KB5WZI |
Richard,
You are at it again, avoiding the supply of corroberation to what you say is true. Stick to the basic statement that you made, which from their silence, the gurus concur with. Your statement was that: propagation is what determines TOA and I ask for confirmation of the correctness of that statement from you in the nature of some written text. The gurus obviously accept your statement as fact, but I do not. Usually you refer to a text to back up your statement ,but this time you haven't, winging it and relying solely on the fact that the gurus agree with you. Surely you or some guru can come up with a written text that states that propagation is what determine TOA.! That is what this group is all about where gurus debunk the untruths and supply the real truths and not to let old wives tale dominate. You also stated that you made the ":assumption" presumably based on the "facts" stated above that the Curtain could be considered as similar to the dipole since propagation determines that they are the same. This is total junk ,in its entirety, unless you or the gurus can come up with a written text that confirmes their positions. Art "Richard Harrison" wrote in message ... Art Unwin wrote: "---may I go back to the "compared to a dipole" statement which Richard keeps brushing off." I accept a resonant dipole reference as a given. It is true that the antenna under test and the reference dipole have different radiation patterns. Our goal was to compare received signal strengths at locations of interest. The assumption was that on average, the propaqgation was nearly the same for the signals received from both transmitting antennas. Good or bad propagation, the difference between the signals depended on gain in the direction of the receiver as the transmitted power was the same to both antennas no matter where it landed. Kraus says on page 535 of his 3rd edition of "antennas": "Suppose that we express the gain with respect to a single lambda/2 element as the reference antenna. Let the same power P be supplied to this antenna. Then assuming no heat losses, the current Io is the sq rt of the power divided by the resistance of the reference antenna. In general, the gain in field intensity of an array over a reference antenna is given by the ratio of the field intensity from the array to the field intensity from the reference antenna when both are supplied with the same power P." Kraus` example was our intended case. Our expectations were met and our contractors were paid. Best regards, Richard Harrison, KB5WZI |
"Takeoff angle" can have two meanings. The first, and really a misuse of
the term, is the one used by antenna modeling programs such as EZNEC. It means the elevation angle at which an antenna's radiation is maximum. This is a property of the antenna and its local environment (particularly the height above ground for horizontal antennas, and local ground quality for vertical antennas). The second meaning is the elevation angle at which propagation occurs. This is dictated mainly by the propagation path -- the distance and the effective height of the ionosphere. The antenna pattern can play a role only when more than one path is possible, for example single and double hop, by modifying the amount which propagates by each path. The "takeoff angle" of the first meaning (angle at which the radiaion is maximum) isn't a particularly useful measure of and antenna's performance, and it certainly doesn't determine the real "takeoff angle" of the second meaning (angle at which propagation occurs). Art has used "takeoff angle" of the first meaning liberally in his writings, often with the added and incorrect implication that all the radiation from an antenna occurs at its "takeoff angle", with none at other elevation angles. So his confusion about Richard's statement (which correctly used "takeoff angle" in the second sense) is understandable. Roy Lewallen, W7EL wrote: Richard, You are at it again, avoiding the supply of corroberation to what you say is true. Stick to the basic statement that you made, which from their silence, the gurus concur with. Your statement was that: propagation is what determines TOA and I ask for confirmation of the correctness of that statement from you in the nature of some written text. The gurus obviously accept your statement as fact, but I do not. Usually you refer to a text to back up your statement ,but this time you haven't, winging it and relying solely on the fact that the gurus agree with you. Surely you or some guru can come up with a written text that states that propagation is what determine TOA.! That is what this group is all about where gurus debunk the untruths and supply the real truths and not to let old wives tale dominate. You also stated that you made the ":assumption" presumably based on the "facts" stated above that the Curtain could be considered as similar to the dipole since propagation determines that they are the same. This is total junk ,in its entirety, unless you or the gurus can come up with a written text that confirmes their positions. Art |
Art Unwin wrote:
"Surely you or some guru can come up with written text that states that propagation is what determines TOA." I don`t find TOA in any index. I find "elevation angle", which I suppose is a synonym, in my 19th edition of The ARRL Antenna Book. On page 2-9 it says: "The elevation angle is referenced to the horizon at the earth`s surface , where the elevation angle is 0-degrees." On page 3-5, the same book says: "Now look at Fig. 4A, which compares the computed vertical-angle response for two half-wave dipoles at 14 MHz." The Antenna Book is not very definitive. "Transmission Lines, Antennas, and Wave Guides" on page 314 says: In order to escape from the earth without excessive ground attenuation, a sky wave must leave the earth at an angle of at least 3-degrees above the horizon.---At 3-degrees elevation, the distance per hop is about 3,500 km (2,100 miles). Longer distances are automatically broken up into units not exceeding 3.500 knm." It`s the medium breaking up the hops, not the antenna. Best regards, Richard Harrison, KB5WZI |
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. A laboratory or ex-member should be able to put me in the right ballpark even if it is only for one typical case. For TRUE antenna performance measurements the best source of information is from a standards lab. There is no incentive to overstate performance. If discovered, exaggeration of a laboratory's capabilities results in loss of reputation. In the UK, Standards Laboratories were regularly monitored for performance by the National Physical Laboratory (NPL), in effect Government controlled. I have been out of touch for 20 years with what happens these days. In the 1970's I was a Government Approved Head of Laboratory. I personally set up the lab from scratch begining with a 30 x 40 feet empty room. All our own standards were traceable directly to the National Measurement Standards at the NPL. An offshoot of the lab, also under my control, was a central calibration service for instruments used nationally by field engineers for investigation of radio interference complaints by the general public and other parties. Many of the instruments were of Eddystone manufacture whose factory was in Birmingham a few miles from the Standards Lab. In between Eddystone's works and the lab lay B'ham University from which the very first 3000 Mhz magnetron appeared during the WW2 air raids on the city. Just in time to defeat the U-Boats which were sinking a 10,000 ton cargo ship every day in the horrible Battle of the North Atlantic. More than 100,000 merchant seamen and suicidal iron-cross submarine crews still lie sleeping in Davy Jones' vast locker. That's quite enough variation for one paragraph. To return to normal - Although we had a small screened room to calibrate RFI instruments, the laboratory's capabilities did not include measurement of antenna gains and losses. Hence my modern enquiry about uncertainties. Note: Uncertainties are best considered because they arise from a multplicity of sources. Therefore they accumulate arithmetically - whereas accuracies do not and are more inconvenient! ---- Reg, G4FGQ. Alias Brer Rabbit or Punchinello. |
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