| Home |
| Search |
| Today's Posts |
|
#11
April 25th 05, 12:30 AM
|
|||
|
|||
"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 |
|
#12
April 25th 05, 02:32 AM
|
|||
|
|||
|
|
|
#13
April 25th 05, 04:48 AM
|
|||
|
|||
|
|
|
#14
April 25th 05, 05:15 AM
|
|||
|
|||
|
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 |
|
#16
April 25th 05, 03:35 PM
|
|||
|
|||
|
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 |
|
#17
April 25th 05, 05:28 PM
|
|||
|
|||
|
"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 |
|
#18
April 25th 05, 07:04 PM
|
|||
|
|||
|
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. |
|
#19
April 25th 05, 07:38 PM
|
|||
|
|||
|
"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. |
|
#20
April 25th 05, 10:34 PM
|
|||
|
|||
|
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 |
| Reply |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Imax ground plane question | CB | |||
| Testing for gain/loss in an antenna | Antenna | |||
| Questions -?- Considering a 'small' Shortwave Listener's (SWLs) Antenna | Shortwave | |||
| The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF} | Shortwave | |||
| EH Antenna Revisited | Antenna | |||
Linear Mode
