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Measuring the fieldstrenght nearby is not reliable.
John Smith wrote:
... Frankly, "re-inventing the wheel" is welcomed here! Regards, JS Yanno? That was a vague response, the above ... What I meant to say is, YES! I worry that "in close" measurements with my field strength meter are/is inaccurate--and, I am ignorant on how to correct this (without going to far-field.) Indeed, it can easily be proven this is the case, EZNEC gives much different results than those plotted with a "close" field strength meter--has anyone found different. If so, I am GREATLY interested, and what am I doing wrong? Sometimes it is just good to ask ... Warm regards, JS |
Measuring the fieldstrenght nearby is not reliable.
Wimpie wrote:
On 7 ene, 04:25, (Richard Harrison) wrote: art wrote: "You did not 'determine" radiated power.....period" Exactly right. Power in the near field is largely reactive. To determine radiated power you measure the in-phase volt-amperes with a wattmeter. The multimeter on the hood maximizes output same as a Bird wattmeter for practical results, but you hanen`t quantified watts out. Best regards, Richard Harrison, KB5WZI Hi Richard, You can determine radiated power and radiation pattern by near field measurements (theoretically). Please search for "poynting theorem" and "Huygens principle", "Huygens Source" or "Fresnel diffraction theory". Most texts require differential vector calculus. This is what is done on a near-field antenna range. For certain scanning geometries, the math is fairly straightforward (e.g. Fourier transforms). For others, it gets complex, especially if the scanning points are not evenly spaced. A further complication is that you need to make 3 axis measurements of both E and H. Jim, W6RMK |
Measuring the fieldstrenght nearby is not reliable.
John Smith wrote:
J. Mc Laughlin wrote: ... To use this technique at, say, 10 MHz with a yagi would be improbable, but not impossible. 73, Mac N8TT It would be interesting to hear ideas of those on "probes", methods, etc., on how to do such, would be attempted--implemented-imployed ... never say never. The challenge is that one needs a probe that measures 3 axis E and H. There are some clever designs out there based on a sphere with 6 half loops sticking out. They've been used to make near field measurements of broadcast stations, among other things. See, for instance, Gassman and Furrer, 1993. Silva, et al., published an interesting fiber optic probe scheme in 97. Driver and Kanda published a optically linked sensor for making Poynting vector measurements in the near field in 1988 (IEEE Trans EMC). In the microwave area, the probe is usually an open ended section of waveguide. There are some clever techniques (see, for instance, the work of Bolomey) where you put an array of small (non resonant) dipoles with switches in the near field, and turn them on and off. You look at the antenna's feedpoint impedance and from that, you can tell what part of the field is affected by that dipole. Once you've got your near field data, you need to post process. A gentleman, A.C. Newell, literally wrote the book(s) on this technique at NIST/NBS. I think it would be interesting to hear on endeavors along these lines, brave hearted individuals may be willing to share their experiences, findings, guesses, etc. ... the bark is often much more dangerous sounding than the the "bite." :-) Frankly, "re-inventing the wheel" is welcomed here! Regards, JS |
Measuring the fieldstrenght nearby is not reliable.
Dear Jim Lux W6RMK:
I was not able to examine the probes that I saw in use at NBS in what was probably the mid-70s. The probes that I did see and use comprised three, orthogonal, very short doublets with attached means for rectifying. The resulting DC was conveyed away through a resistive, plastic transmission-line crafted to be almost transparent to RF. These probes were used to estimate the size of strong EM signals in the vicinity of equipment so as to be able to put better numbers on EMC capabilities. While one can make a reasonable estimate of field strength inside of a TEM cell (a big piece of transmission line) from external measurements, it is desirable to be able to estimate FS at spots inside of the cell. It is important to note that even with care, significant uncertainties remain in the estimates. All measurement comprise a minimum of two numbers: an estimate of the value and an estimate of the uncertainty of that estimate. You, and most on the group, know this, but it needs to be repeated. Safety of life was involved with the testing then done. With the profusion of transmitters in close proximity of safety equipment today, it is a wonder that more lives are not lost with inappropriate actuations just within present cars. A reminder that the need for savvy RF engineers will not diminish, and sending that work off-shore to a low-bidder is dangerous and probably criminally negligent. The major US car makers, to pick an industry, expend a large, expensive effort to see that their cars are safe using resident engineers. Warm regards, Mac N8TT -- J. McLaughlin; Michigan, USA Home: "Jim Lux" wrote in message ... : J. Mc Laughlin wrote: ... To use this technique at, say, 10 MHz with a yagi would be improbable, but not impossible. 73, Mac N8TT The challenge is that one needs a probe that measures 3 axis E and H. There are some clever designs out there based on a sphere with 6 half loops sticking out. They've been used to make near field measurements of broadcast stations, among other things. See, for instance, Gassman and Furrer, 1993. Silva, et al., published an interesting fiber optic probe scheme in 97. Driver and Kanda published a optically linked sensor for making Poynting vector measurements in the near field in 1988 (IEEE Trans EMC). In the microwave area, the probe is usually an open ended section of waveguide. There are some clever techniques (see, for instance, the work of Bolomey) where you put an array of small (non resonant) dipoles with switches in the near field, and turn them on and off. You look at the antenna's feedpoint impedance and from that, you can tell what part of the field is affected by that dipole. Once you've got your near field data, you need to post process. A gentleman, A.C. Newell, literally wrote the book(s) on this technique at NIST/NBS. |
Measuring the fieldstrenght nearby is not reliable.
J. Mc Laughlin wrote:
... A reminder that the need for savvy RF engineers will not diminish, and sending that work off-shore to a low-bidder is dangerous and probably criminally negligent. The major US car makers, to pick an industry, expend a large, expensive effort to see that their cars are safe using resident engineers. Warm regards, Mac N8TT My gawd man, you are paying attention. My question would be, with the constitution guaranteeing us the rights to bear arms against a government which has become insensitive to the majority--when will we finally find it intolerable and correct "them." Or, when the common man has to think of the government as a "separate entity" to the population--it is time for serious correction ... Amen brother, it IS criminal! Warm regards, JS |
Measuring the fieldstrenght nearby is not reliable.
On Jan 9, 5:35 pm, "J. Mc Laughlin" wrote:
Dear Jim Lux W6RMK: I was not able to examine the probes that I saw in use at NBS in what was probably the mid-70s. The probes that I did see and use comprised three, orthogonal, very short doublets with attached means for rectifying. The resulting DC was conveyed away through a resistive, plastic transmission-line crafted to be almost transparent to RF. These probes were used to estimate the size of strong EM signals in the vicinity of equipment so as to be able to put better numbers on EMC capabilities. Yes.. The older works (50s and 60s) used carbon loaded string or thread, but newer stuff uses conductive plastic. If the sheet resistance of the material is 377 ohms/square then it's sort of like lossy freespace. Those three axis probes work fine for measuring the magnitude and direction of the field, but they can't measure the phase, and to do the nearfield to far field conversion you also need the phase. |
Measuring the fieldstrenght nearby is not reliable.
High Mac,
About a year ago I was at an EMC test house. They were experimenting with three axis short dipoles with a detector diode in between. I never saw such a small package for a diode, maybe it was just the chip on top of the substrate. The output goes via a high resistance metallic layer to the interface. The whole interface is fed by fiber optic (data also goes via fiber optic). It wasn't used for antenna design measurements but for EMC high field strength measurements. Their intention was to use it up to 18 GHz. Of course sensitivity (conversion from E-field to DC output) is very poor, but for high incident field that isn't a problem. For indicative measurements I like today's easy to get medium barrier microwave schottky rectifiers (like BAT15 and BAT 62). Together with a CMOS input stage opamp one can make a nice zero bias detector with good temperature stability and that outputs the full available EMF from the rectifying diode. So you can calculate back the incident E- field. Best regards, Wim PA3DJS www.tetech.nl On 10 ene, 02:35, "J. Mc Laughlin" wrote: Dear Jim Lux W6RMK: I was not able to examine the probes that I saw in use at NBS in what was probably the mid-70s. The probes that I did see and use comprised three, orthogonal, very short doublets with attached means for rectifying. The resulting DC was conveyed away through a resistive, plastic transmission-line crafted to be almost transparent to RF. These probes were used to estimate the size of strong EM signals in the vicinity of equipment so as to be able to put better numbers on EMC capabilities. While one can make a reasonable estimate of field strength inside of a TEM cell (a big piece of transmission line) from external measurements, it is desirable to be able to estimate FS at spots inside of the cell. It is important to note that even with care, significant uncertainties remain in the estimates. All measurement comprise a minimum of two numbers: an estimate of the value and an estimate of the uncertainty of that estimate. You, and most on the group, know this, but it needs to be repeated. Safety of life was involved with the testing then done. With the profusion of transmitters in close proximity of safety equipment today, it is a wonder that more lives are not lost with inappropriate actuations just within present cars. A reminder that the need for savvy RF engineers will not diminish, and sending that work off-shore to a low-bidder is dangerous and probably criminally negligent. The major US car makers, to pick an industry, expend a large, expensive effort to see that their cars are safe using resident engineers. Warm regards, Mac N8TT [rest deleted] |
Measuring the fieldstrenght nearby is not reliable.
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Measuring the fieldstrenght nearby is not reliable.
Hello Mac,
A little bit OT, it happens more often that I enter English words with Dutch spelled characters. This sometimes leads to strange constructions... English wasn't my best language when I was at school. Sorry for the mistake. Best regards, Wim PA3DJS www.tetech.nl On 11 ene, 02:55, "J. Mc Laughlin" wrote: Dear Wim: Your observations are appreciated. I am delighted to learn of evidence of improvements in instrumentation. Along with my early observation of the need for capable RF engineers (and that includes antennas), note in the digital age the continuing need for smart analog-electronic engineers, as exhibited by your examples. Western Europe, USA, Canada, ZL, and VK still have such people, but many are reaching retirement. I am please to say that my university still has as one of its objectives to educate engineers who have a good feel for when analog is needed and when digital is needed. They might not have the full analog insight, skill, or experience, but they know when an analog expert is needed. ... and now for a moment with tongue-in-cheek: Too many words in English sound the same, but are spelled differently. Some are spelled the same but have multiple meanings that one must gather from context. This is the consequence of borrowing from many languages - very notably from the north part of your country. "Hi Mac" is a salutation. It should come as no surprise that this Mac has never been "High." The most wonderful thing about English is that because of its inherent redundancies and the way it uses grammar, it is relatively difficult for anyone to loose the meaning - especially in a technical forum. As a contrast, consider the drastic consequences of making an error in an ending of a Slavic language word. The word can become unintelligible. Your contributions are always read with interest. 73, Mac N8TT -- J. McLaughlin; Michigan, USA [rest deleted] |
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