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#41
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Information about my experience with Magnetic Loop antenna'son my homepage
On 02/23/2011 10:00 AM, RadioWaves wrote:
Today I have put my homepage online with information about the Magnetic Loop Antenna. http://www.qsl.net/pa7nr/ PA7NR Hmmm. A "magnetic" loop antenna. Must be some other types of loop antennas as well. Maybe there are also "electric" loop antennas. Guess they left something out of all those antenna textbooks I have ;-) Sincerely, and 73s from N4GGO, -- J. B. Wood e-mail: |
#42
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Information about my experience with Magnetic Loop antenna's onmy homepage
On 2 mar, 13:58, "J.B. Wood" wrote:
On 02/23/2011 10:00 AM, RadioWaves wrote: Today I have put my homepage online with information about the Magnetic Loop Antenna. http://www.qsl.net/pa7nr/ PA7NR Hmmm. *A "magnetic" loop antenna. *Must be some other types of loop antennas as well. *Maybe there are also "electric" loop antennas. Guess they left something out of all those antenna textbooks I have ;-) Sincerely, and 73s from N4GGO, -- J. B. Wood * * * * * * * * *e-mail: Hello John, When you cut the loop at two opposite positions, yes, you can make your "electric" loop. It will generate lots of E-field, you may need another coil for matching, and it is probably less efficient then a short straight dipole with massive capacitive disks to get larger I*delta(le) product. Best regards, Wim PA3DJS www.tetech.nl In case of PM, please remove abc first. |
#43
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Information about my experience with Magnetic Loop antenna'son my homepage
On 3/2/2011 8:40 AM, Wimpie wrote:
On 2 mar, 13:58, "J.B. wrote: On 02/23/2011 10:00 AM, RadioWaves wrote: Today I have put my homepage online with information about the Magnetic Loop Antenna. http://www.qsl.net/pa7nr/ PA7NR Hmmm. A "magnetic" loop antenna. Must be some other types of loop antennas as well. Maybe there are also "electric" loop antennas. Guess they left something out of all those antenna textbooks I have ;-) Sincerely, and 73s from N4GGO, -- J. B. Wood e-mail: Hello John, When you cut the loop at two opposite positions, yes, you can make your "electric" loop. It will generate lots of E-field, you may need another coil for matching, and it is probably less efficient then a short straight dipole with massive capacitive disks to get larger I*delta(le) product. Best regards, Wim PA3DJS www.tetech.nl In case of PM, please remove abc first. Hello, and the not-so-subtle point is that there aren't magnetic, electric, or any other such "types" of loop antennas. There are just loop antennas that can further be described as shielded/unshielded, balanced/unbalanced, electrically small or large. Just like we don't transmit (propagate) electric (E) or magnetic (H) fields by themselves. The purpose of an antenna is to radiate and/or intercept an electromagnetic field. By definition energy radiated by a transmitting antenna is not temporarily stored in the antenna's local electric or magnetic field. It's been released into free space subject to interception by a receiving antenna(s) or any other parasitic structures. The receiving antenna transfers part the intercepted energy to the load (receiver and other dissipative losses) and scatters the rest back into free space. By contrast, a transformer, for example, is a "magnetic" device that is intended to transfer energy by a localized means (induction) other than the propagation/interception of electromagnetic radiation. To further confuse the issue, a conductor in the near (reactive) field of a transmitting antenna will have current induced in it by the antenna's local electric and/or magnetic fields. However, that's not the usual purpose for which we design antennas. An exception might be the immoboliser (PATS) system used in late-model motor vehicles that incorporates a ring antenna embedded in the steering column that is closely coupled at RF frequencies to the transponder chip and loop antenna embedded in the vehicle ignition key. So is it a transmit-receive antenna configuration or a primary coil-secondary coil transformer configuration? Given the proximity of the inserted key to the steering column I would guess the latter. Sincerely, -- J. B. Wood e-mail: |
#44
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#45
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Information about my experience with Magnetic Loop antenna'son my homepage
On 3/2/2011 12:14 PM, J.B. Wood wrote:
On 3/2/2011 8:40 AM, Wimpie wrote: To further confuse the issue, a conductor in the near (reactive) field of a transmitting antenna will have current induced in it by the antenna's local electric and/or magnetic fields. However, that's not the usual purpose for which we design antennas. Hello, all. I should also add that in stating the above I was only considering nearby conductors (towers, metal on buildings, etc) and wasn't including the local directors/reflectors that may be incorporated into an antenna to provide the desired radiation pattern characteristics. Sincerely, -- J. B. Wood e-mail: |
#46
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Information about my experience with Magnetic Loop antenna's onmy homepage
Hello John,
On 2 mar, 18:14, "J.B. Wood" wrote: On 3/2/2011 8:40 AM, Wimpie wrote: On 2 mar, 13:58, "J.B. *wrote: On 02/23/2011 10:00 AM, RadioWaves wrote: *Today I have put my homepage online with information about the Magnetic Loop Antenna. http://www.qsl.net/pa7nr/ PA7NR Hmmm. *A "magnetic" loop antenna. *Must be some other types of loop antennas as well. *Maybe there are also "electric" loop antennas. Guess they left something out of all those antenna textbooks I have ;-) Sincerely, and 73s from N4GGO, -- J. B. Wood * * * * * * * * *e-mail: Hello John, When you cut the loop at two opposite positions, yes, you can make your "electric" loop. It will generate lots of E-field, you may need another coil for matching, and it is probably less efficient then a short straight dipole with massive capacitive disks to get larger I*delta(le) product. Best regards, Wim PA3DJS www.tetech.nl In case of PM, please remove abc first. Hello, and the not-so-subtle point is that there aren't magnetic, electric, or any other such "types" of loop antennas. *There are just loop antennas that can further be described as shielded/unshielded, balanced/unbalanced, electrically small or large. *Just like we don't transmit (propagate) electric (E) or magnetic (H) fields by themselves. The purpose of an antenna is to radiate and/or intercept an electromagnetic field. *By definition energy radiated by a transmitting antenna is not temporarily stored in the antenna's local electric or magnetic field. *It's been released into free space subject to interception by a receiving antenna(s) or any other parasitic structures. *The receiving antenna transfers part the intercepted energy to the load (receiver and other dissipative losses) and scatters the rest back into free space. By contrast, a transformer, for example, is a "magnetic" device that is intended to transfer energy by a localized means (induction) other than the propagation/interception of electromagnetic radiation. If in your opinion there do not exist antennas that generate a dominant magnetic or electric field (in the near field), then you are contradicting yourself, as you can't transfer energy with a magnetic field or electric field only. So your transformer also involves electric fields. Maybe you should look into the Poynting theorem. To further confuse the issue, a conductor in the near (reactive) field of a transmitting antenna will have current induced in it by the antenna's local electric and/or magnetic fields. *However, that's not the usual purpose for which we design antennas. *An exception might be the immoboliser (PATS) system used in late-model motor vehicles that incorporates a ring antenna embedded in the steering column that is closely coupled at RF frequencies to the transponder chip and loop antenna embedded in the vehicle ignition key. *So is it a transmit-receive antenna configuration or a primary coil-secondary coil transformer configuration? *Given the proximity of the inserted key to the steering column I would guess the latter. *Sincerely, -- J. B. Wood * * * * * * * * *e-mail: When a noise source is about 5..10m away from an 3.6 MHz antenna, the coupling of that noise source towards a "magnetic" loop antenna may be different from the coupling towards an "electric" antenna, though both antennas may produce the same far field radiation. This is not from a textbook, but from experience (I am also working in power electronics). I fully agree with you on the far field statements, but when you live in an apartment (where significant spurious emission from home equipment are in the near field of your 3.6 MHz antenna), a so-called magnetic loop antenna may behave different (w.r.t. a short "electric" dipole). It can be worse or better. Many radio amateurs know this from experiments, without knowing the EM theory behind it. I have no problems when people talk about a "magnetic loop antenna". It shows me that they are discussing an antenna with a circumference 0.2 lambda. When people talk about a "loop antenna", it can be anything. Best regards, Wim PA3DJS www.tetech.nl |
#47
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Information about my experience with Magnetic Loop antenna's on my homepage
On Wed, 2 Mar 2011 12:56:23 -0800 (PST), Wimpie
wrote: When a noise source is about 5..10m away from an 3.6 MHz antenna, the coupling of that noise source towards a "magnetic" loop antenna may be different from the coupling towards an "electric" antenna, though both antennas may produce the same far field radiation. This is not from a textbook, but from experience (I am also working in power electronics). Text books would enlarge that volume to one half to several wavelengths for the "near field." The text books would further clarify this with math (yes, I know, professional and academic discussion in light of this being an amateur forum is anathema) and define the difference with the terms Fresnel diffraction (near-field) and Fraunhofer diffraction (far-field). The operative physical length of the antenna becomes meaningful, but this is getting ahead of what I call the "benchmark" method below. To give the magnetic loop aficionados the benefit of this, all local noise within 100 feet would be susceptible to interfering and it wouldn't be nullable (which is a characteristic only observed in the far-field) except by polarization which is very haphazard in the near-field. I have never seen a magnetic loop mount with the necessary degrees of freedom to employ this method of "nulling." As such, the vaunted characteristic is elusive and thus becomes legendary rather than fulfilled. However, the term "near-field" is rather vague. The more appropriate discussion is found in "reactive near field" and "radiative near field." The discussion of loop coupling to magnetic (while ignoring electric) fields would suggest "reactive near field." In this regard, the 80M volume of reactive interference is still roughly 100 feet in all directions. The "radiative near field" would encompass a volume out to 80 meters (roughly 250 feet). In either case, apartment living finds no panacea in loop antennas. There is another, non-textual (at least to the casual reader), benchmark that such issues are measured by the physical spread of the antenna itself (this usually attends discussion of capture area to many's frustration). Here, I am returning to the allusion above of Fresnel diffraction (near-field) and Fraunhofer diffraction (far-field). The math (non-techs, turn your eyes away) is as simple as: 2·D²/lambda Let's work some examples from the sublime to the ridiculous on 80M. The traditional half-wave dipole antenna that exhibits the traditional usage for distinguishing between near and far: 2·40²/80 = 40 meters a smaller quarter-wave dipole antenna 2·20²/80 = 10 meters a tenth wave dipole antenna 2·8²/80 = 1.6 meters a fortieth wave dipole antenna 2·2²/80 = 10 centimeters Let's see where discussion follows in this regard. 73's Richard Clark, KB7QHC |
#48
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Information about my experience with Magnetic Loop antenna's onmy homepage
Hello Richard,
On 2 mar, 23:10, Richard Clark wrote: On Wed, 2 Mar 2011 12:56:23 -0800 (PST), Wimpie wrote: When a noise source is about 5..10m *away from an 3.6 MHz antenna, the coupling of that noise source towards a "magnetic" loop antenna may be different *from the coupling towards an "electric" antenna, though both antennas may produce the same far field radiation. *This is not from a textbook, but from experience (I am also working in power electronics). Text books would enlarge that volume to one half to several wavelengths for the "near field." *The text books would further clarify this with math (yes, I know, professional and academic discussion in light of this being an amateur forum is anathema) and define the difference with the terms Fresnel diffraction (near-field) and Fraunhofer diffraction (far-field). *The operative physical length of the antenna becomes meaningful, but this is getting ahead of what I call the "benchmark" method below. To give the magnetic loop aficionados the benefit of this, all local noise within 100 feet would be susceptible to interfering and it wouldn't be nullable (which is a characteristic only observed in the far-field) except by polarization which is very haphazard in the near-field. *I have never seen a magnetic loop mount with the necessary degrees of freedom to employ this method of "nulling." *As such, the vaunted characteristic is elusive and thus becomes legendary rather than fulfilled. However, the term "near-field" is rather vague. *The more appropriate discussion is found in "reactive near field" and "radiative near field." *The discussion of loop coupling to magnetic (while ignoring electric) fields would suggest "reactive near field." *In this regard, the 80M volume of reactive interference is still roughly 100 feet in all directions. *The "radiative near field" would encompass a volume out to 80 meters (roughly 250 feet). *In either case, apartment living finds no panacea in loop antennas. There is another, non-textual (at least to the casual reader), benchmark that such issues are measured by the physical spread of the antenna itself (this usually attends discussion of capture area to many's frustration). *Here, I am returning to the allusion above of Fresnel diffraction (near-field) and Fraunhofer diffraction (far-field). *The math (non-techs, turn your eyes away) is as simple as: * * * * 2 D /lambda Let's work some examples from the sublime to the ridiculous on 80M. The traditional half-wave dipole antenna that exhibits the traditional usage for distinguishing between near and far: * * * * 2 40 /80 = 40 meters a smaller quarter-wave dipole antenna * * * * 2 20 /80 = 10 meters a tenth wave dipole antenna * * * * 2 8 /80 = 1.6 meters a fortieth wave dipole antenna * * * * 2 2 /80 = 10 centimeters Let's see where discussion follows in this regard. 73's Richard Clark, KB7QHC where is your square? Fraunhofer region starts at (22.5 degrees phase shift): r = 2*D^2/lambda D = largest antenna size (excluding structures that doesn't carry current). Formula is only valid for electrically large structures, so not an electrically small loop or dipole. For electrically small loops, reactive fields are dominant for: r 0.16*lambda Smaller loop size does not result in smaller reactive field zone. The correct formulas you can find everywhere. To make it easy for you: http://www.conformity.com/past/0102reflections.html shows the complete formulas for the electric and magnetic case, and a graph at the end. Best regards, Wim PA3DJS www.tetech.nl |
#49
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Information about my experience with Magnetic Loop antenna's on my homepage
On Wed, 2 Mar 2011 17:40:15 +0000, ka7niq
wrote: OK, IF you just wanted to buy a magnetic loop antenna vs build one, what are the ones to look at that are for sale, and why ? AEA once made one when I left Tampa and lived out west in Seattle. I live in a small, non deed restricted house in Tampa with a flat membrane roof. I could get a loop on my roof I suppose, if the installation looked clean. Hi OM, The specifications of the major vendors that I have seen are usually reliable. You still need to answer what bands do you want to transmit on? If it is 40M and up, you are pretty sure to find a lot of useful designs. 73's Richard Clark, KB7QHC |
#50
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Information about my experience with Magnetic Loop antenna's on my homepage
On Wed, 2 Mar 2011 15:29:55 -0800 (PST), Wimpie
wrote: Formula is only valid for electrically large structures, so not an electrically small loop or dipole. "Large" or "small" are not quantities. For electrically small loops, reactive fields are dominant for: and how small (quantifiable) is small (qualifiable)? r 0.16*lambda given that I have already demonstrated that, and more, what importance do you attach to this that hasn't already been shown? Smaller loop size does not result in smaller reactive field zone. What a curious defense for magnetic antennas's noise immunity. However, the magnetic antenna is not immune from the reactive fields of noise emitters that are very much larger than any loop discussed here. It is the field of the emitter that is important. I thought I would wait and see if anyone cottoned on to that aspect of the discussion. If we proceed with the assumption (repeated here): Smaller loop size does not result in smaller reactive field zone. then the magnetic antenna is doomed to noise in the same sense as an electric antenna is. Offhand I would speculate that in an apartment situation, a magnetic antenna on the balcony is saturated with reactive noise fields. 73's Richard Clark, KB7QHC |
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