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The prrof of he pudding?
"Wimpie" wrote in message
... The reason that you don't need to take magnetic dipole radiation into account in real mechanical systems is because of the radiated power is very low (low RPM in practical mechanical systems). Friction (bearings/air, eddy current, etc) is orders of magnitude more then the "friction" caused by the EM radiation. Further proof, if any were needed, that a short antenna is a poor radiator. (Frequencies of mechanical rotation representing wavelengthe of several miles, against which the length of the magnet is trivial, and why, in the case of the superconductor example, there is no measurable decrease in rotational speed, in the short term, at least.) |
A short 160M antenna
"Sn!pe" wrote in message
.uk... Wimpie wrote: [...] Jim, I would encourage you to dive into rotating magnetic dipole radiation. For practical electromechanical systems (even in practial vacuo) it is negligible as (c0)^5 is in the denominator and (2*pi*rev/s)^4 is in the numerator, but that doesn't mean it isn't present from a theoretical point of view. Would such radiation *propagate* though? I have a vague recollection from many years ago that there's a difference between a proper *radio* wave and another sort of oscillating field that one also gets close to an antenna. Was it something to do with the phase relationship between the electric and magnetic components of the field perhaps? I may well have imagined this, it was a very long time ago. What you appear to be discussing is the difference between the Near Field and the Far Field |
A short 160M antenna
Wimpie wrote:
snip Jim, I would encourage you to dive into rotating magnetic dipole radiation. You mean like in a pulsar? For practical electromechanical systems (even in practial vacuo) it is negligible as (c0)^5 is in the denominator and (2*pi*rev/s)^4 is in the numerator, but that doesn't mean it isn't present from a theoretical point of view. Again, not talking about any "electromechanical system", just a permanet magnet spining. Also implied is the macro level, i.e. a magnet one can hold in one's hand and velocities well below any relativisitc effects. -- Jim Pennino |
A short 160M antenna
Wimpie wrote:
snip On an astronomical scale things are different. Yep, and the discussion is not about pulsars or astronomical phenomena, it is about magnets one can hold in their hand. -- Jim Pennino |
The prrof of he pudding?
gareth wrote:
snip Further proof, if any were needed, that a short antenna is a poor radiator. All the proof so far having acutal numbers has been that "a short antenna is a poor radiator" is arm waving nonsense, gas bag, and all you have presented so far is arm waving and no numbers. -- Jim Pennino |
The prrof of he pudding?
wrote in message
... All the proof so far having acutal numbers has been that "a short antenna is a poor radiator" is arm waving nonsense, gas bag, and all you have presented so far is arm waving and no numbers. Grow up, child. |
The prrof of he pudding?
gareth wrote:
wrote in message ... All the proof so far having acutal numbers has been that "a short antenna is a poor radiator" is arm waving nonsense, gas bag, and all you have presented so far is arm waving and no numbers. Grow up, child. Show some numbers, gas bag. -- Jim Pennino |
The prrof of he pudding?
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The prrof of he pudding?
"Brian Reay" wrote in message
... On 10/11/14 18:46, wrote: Further proof, if any were needed, that a short antenna is a poor radiator. All the proof so far having acutal numbers has been that "a short antenna is a poor radiator" is arm waving nonsense, gas bag, and all you have presented so far is arm waving and no numbers. It is worse than that, he has dismissed solid theory and tried to replace it with his nonsense. Just as he has done in the rotating magnetic thread. In fact, this is his normal mode of operation, he has a long history of such quackery. The pattern is always the same. Post some nonsense theory, often dressed up as being something that has troubled him or he has been studying. The theories are often rambling nonsense- suggesting any studying has been limited to a 'scan' of a few key terms. When people respond, hand out abuse, ignore anything which clearly looks credible, etc. Hand out more abuse. Change theory, claiming people didn't understand. Hand out more abuse. Claim he was right and others were wrong. Don't be surprised if the same theory is recycled several times. Once again, Brian, you jump in with your childish remarks and chanting your false mantra. Why do you behave like that? Why always shout out your childish interjections? Why not contribute to the technical discussion? FYI, it was jimp who originated the abuse by his pejorative use of, "gas bag", but I doubt that the truth of that will suit your own need to want to shout out your own abuse. I have not dismissed any theory. Out your money where your (big) mouth is, and cite your reference. I do not have a history of quackery, unlike youwith your assertion that reversing the direction od a rotating vector makes it reduce in suze, or more recently, claiming that Maxwell's equations for static fields have no non-zero differential terms, or that Maxwell's Equations refer only to EM propagation and not to the whole of electrical phenomenon. (Perhaps it is no surprise that you do an M6CIR and reort to bluster to mask your own technical ineptitude?) I have never changed any theory; I strongly suspect that your problem is that you yourself are the one who only does a quick scan, jumps to some irrelevant conclusion, and then dives in with your own abuse; abuse that you seek to lay at others' doors. For example, your recent faux pas when I was discussing what leads to a standing wave and you jumping in with what is extant AFTER that standing wave has been set up. Physician, heal thyself. |
The prrof of he pudding?
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The prrof of he pudding?
In rec.radio.amateur.antenna gareth wrote:
snip Why not contribute to the technical discussion? You mean like showing some actual numbers, which you have so far failed to do? -- Jim Pennino |
A short 160M antenna
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A short 160M antenna
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A short 160M antenna
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A short 160M antenna
"Wimpie" wrote in message
... El 10-11-14 19:42, escribió: wrote: On an astronomical scale things are different. Yep, and the discussion is not about pulsars or astronomical phenomena, it is about magnets one can hold in their hand. Someone said that a rotating magnet produces an EM radiation field. You said it is nonsense without arguments, a link to a useful reference, or query. And, of course, with the complete lack of the numbers for which he is issuing challenges. |
A short 160M antenna
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A short 160M antenna
Wimpie wrote:
El 10-11-14 19:42, escribió: wrote: snip On an astronomical scale things are different. Yep, and the discussion is not about pulsars or astronomical phenomena, it is about magnets one can hold in their hand. Someone said that a rotating magnet produces an EM radiation field. You said it is nonsense without arguments, a link to a useful reference, or query. There is nothing wrong with a jump to another branch of science (for example astrophysics) to find out whether the statement is right or wrong. You mean other than the fact that the context is immediately obvious? -- Jim Pennino |
A short 160M antenna
gareth wrote:
"Wimpie" wrote in message ... El 10-11-14 19:42, escribi?: wrote: On an astronomical scale things are different. Yep, and the discussion is not about pulsars or astronomical phenomena, it is about magnets one can hold in their hand. Someone said that a rotating magnet produces an EM radiation field. You said it is nonsense without arguments, a link to a useful reference, or query. And, of course, with the complete lack of the numbers for which he is issuing challenges. How short is a "short antenna"? What is the metric for "poor performance"? -- Jim Pennino |
A short 160M antenna
On 11/10/2014 6:00 PM, Wimpie wrote:
El 10-11-14 19:39, escribió: wrote: snip Jim, I would encourage you to dive into rotating magnetic dipole radiation. You mean like in a pulsar? To be more precise, I mean the radiation with same frequency as pulsar rotation, of course only present when there is a net magnetic dipole moment. I am not pointing to the pulsed RF radiation. I want to be clear on this. You are saying that a rotating magnetic dipole *does* create EM waves just the same as any antenna? The only difference between a Pulsar and a handheld magnet is one of scale? For practical electromechanical systems (even in practial vacuo) it is negligible as (c0)^5 is in the denominator and (2*pi*rev/s)^4 is in the numerator, but that doesn't mean it isn't present from a theoretical point of view. Again, not talking about any "electromechanical system", just a permanet magnet spining. That just spinning magnet produces dB/dt, hence an E-field (not conservative). Superposition of two quadrature magnetic oscillating dipoles (small loop antnenas) gives a rotating magnetic dipole field. Such a quadrature setup can be exchanged by a rotating permanent magnet. Also implied is the macro level, i.e. a magnet one can hold in one's hand and velocities well below any relativisitc effects. I considered non-relativistic velocities only. I'm not interested in Jim's hand waving. Either a magnet can or can't generate EM waves. If a big one does it, then a little one does it too. -- Rick |
A short 160M antenna
Wimpie wrote in :
There is nothing wrong with a jump to another branch of science (for example astrophysics) to find out whether the statement is right or wrong. I agree with that. It looks like a question of scale, not an absolute. I don't know enough to say much, so I haven't, but if this is like relativistic effects in that it is real, but extremely insignificant on the scales presented for discussion, then arguing about it is surely proportional in its significance. |
A short 160M antenna
El 09-11-14 19:40, Lostgallifreyan escribió:
wrote in : What Maxwell's Equations say is a moving magnet produces a moving magnetic field. A moving magnetic field is not an electromagnetic field. Never mind Maxwell, why is no-one asking "why is an electromagnet not used as an antenna when driven by AC?" That should be a basic reality check because I've neve heard of such a thing working, and if it did it would be widely known. You may know that an AC current through a loop does generate near fields, and far (EM radiation) fields. This follows from Maxwell's equations. The solution for radiated power from a loop carrying AC current is: Prad = 3.85*10^-30*(A*I)^2*f^4 [W] The product of A (Loop surface area) and I (rms current through loop) is the magnetic moment (m). m is used in formulas frequently. The formula contains f^4, when you halve the frequency, the radiation drops to 6.25%. This is the reason that at low frequency the EM radiation is negligible in most cases. This behavior is also the reason that you need large antennas to produce low frequency EM radiation fields with useful efficiency. If you take two vertically oriented loops that are perpendicular to eachoter, and feed them 90 degrees out of phase, you create a rotating/spinning magnetic field around the Z-axis. Because of the orientation of the two coils, they don't interact with eachother. So by using two oscillating magnetic dipoles, you can generate a rotating magnetic dipole. The summation of the two fields (our rotating field), generate a vertically polarized EM radiation field in the XY-plane. This is like in a normal loop antenna, but now the pattern is omnidirectional (because of two loops instead of one). In positive and negative Z direction there is a pure circular field. How you generate the rotating magnetic field doesn't matter. Instead of using two out-of-phase fed loops, I can generate exactly the same rotating field by using a rotating permanent magnet with its N-S direction in the horizontal plane. I can also use a rotating coil fed with DC current. Due to mechanical limitations, you can't generate high rev/s (that is the frequency). Because of the f^4 behavior EM radiation is negligible in practical mechanical systems involving permanent magnets. In the "AC fed two coils case" energy is delivered by the sources providing the current, in the "rotating permanent magnets case" the energy is provided by the drive mechanism. -- Wim PA3DJS Please remove abc first in case of PM |
A short 160M antenna
El 11-11-14 12:43, Lostgallifreyan escribió:
wrote in : There is nothing wrong with a jump to another branch of science (for example astrophysics) to find out whether the statement is right or wrong. I agree with that. It looks like a question of scale, not an absolute. I don't know enough to say much, so I haven't, but if this is like relativistic effects in that it is real, but extremely insignificant on the scales presented for discussion, then arguing about it is surely proportional in its significance. You are right, it is a matter of scale and especially rotation frequency, but you don't need relativistic velocities to make it happen. I posted an explanation based on two loops in the same topic. -- Wim PA3DJS Please remove abc first in case of PM |
A short 160M antenna
On 11/11/2014 7:00 AM, Wimpie wrote:
El 09-11-14 19:40, Lostgallifreyan escribió: wrote in : What Maxwell's Equations say is a moving magnet produces a moving magnetic field. A moving magnetic field is not an electromagnetic field. Never mind Maxwell, why is no-one asking "why is an electromagnet not used as an antenna when driven by AC?" That should be a basic reality check because I've neve heard of such a thing working, and if it did it would be widely known. You may know that an AC current through a loop does generate near fields, and far (EM radiation) fields. This follows from Maxwell's equations. The solution for radiated power from a loop carrying AC current is: Prad = 3.85*10^-30*(A*I)^2*f^4 [W] The product of A (Loop surface area) and I (rms current through loop) is the magnetic moment (m). m is used in formulas frequently. The formula contains f^4, when you halve the frequency, the radiation drops to 6.25%. This is the reason that at low frequency the EM radiation is negligible in most cases. This behavior is also the reason that you need large antennas to produce low frequency EM radiation fields with useful efficiency. If you take two vertically oriented loops that are perpendicular to eachoter, and feed them 90 degrees out of phase, you create a rotating/spinning magnetic field around the Z-axis. Because of the orientation of the two coils, they don't interact with eachother. So by using two oscillating magnetic dipoles, you can generate a rotating magnetic dipole. The summation of the two fields (our rotating field), generate a vertically polarized EM radiation field in the XY-plane. This is like in a normal loop antenna, but now the pattern is omnidirectional (because of two loops instead of one). In positive and negative Z direction there is a pure circular field. How you generate the rotating magnetic field doesn't matter. Instead of using two out-of-phase fed loops, I can generate exactly the same rotating field by using a rotating permanent magnet with its N-S direction in the horizontal plane. I can also use a rotating coil fed with DC current. Due to mechanical limitations, you can't generate high rev/s (that is the frequency). Because of the f^4 behavior EM radiation is negligible in practical mechanical systems involving permanent magnets. In the "AC fed two coils case" energy is delivered by the sources providing the current, in the "rotating permanent magnets case" the energy is provided by the drive mechanism. Excellent explanation, Wim. Many thanks for that. I learn a lot from you. |
More commentary on short antennae
"Wimpie" wrote in message
... The formula contains f^4, when you halve the frequency, the radiation drops to 6.25%. This is the reason that at low frequency the EM radiation is negligible in most cases. This behavior is also the reason that you need large antennas to produce low frequency EM radiation fields with useful efficiency. Watch out for the redneck troll lurking beneath the rickety-rackety bridge! |
A short 160M antenna
Wimpie wrote in :
Never mind Maxwell, why is no-one asking "why is an electromagnet not used as an antenna when driven by AC?" That should be a basic reality check because I've neve heard of such a thing working, and if it did it would be widely known. You may know that an AC current through a loop does generate near fields, and far (EM radiation) fields. This follows from Maxwell's equations. The solution for radiated power from a loop carrying AC current is: I did know, but I didn't think it through. :) What I had in mind was the kind of electromagnet usually used to simulate a magnet, one with a big lump magnetic material to guid the flux and concentrate it, etc. I imagine the moment you do that, you get closer to a normal magnet, but it probably makes the coil unusable as an antenna at the same time. The maths is beyond my easy grasp, but I do have one thought... Is this one reason an Oudin coil is using a high frequency? (High-ish... many tens of KHz, compared to usual mains AC frequency.) They're often used to start erratic ion laser tubes, to avoid damage to glass and seals by arcs. I'm wondering if the combination of extreme proximity and higher frequency than mains-AC coils might be using EM radiation to help penetrate the tube safely in ways an arc could not, without damage, and in ways a low frequency coild could not at all, with or without the magnetic material as a core. If this is the case, then an Oudin coil might be a rare practical case for EM from something far laser to a magnet than an antenna. I'm just guessing here, but interested enough to ask. I like the quadrature pair of loops idea. I should have thought of that at least. I have explored quadrature generation and circular distributions in audio, and forgot to make the connection somehow. |
A short 160M antenna
Lostgallifreyan wrote in
: something far laser to a magnet than an antenna "...far CLOSER to a magnet..." Sorry, I saw the typo in the main post before sending, then 'corrected' something even more wrong... Can't see, been busy for 6 hours elsewhere, very bad eyestrain today. |
A short 160M antenna
On 11/11/2014 2:42 PM, Lostgallifreyan wrote:
Wimpie wrote in : Never mind Maxwell, why is no-one asking "why is an electromagnet not used as an antenna when driven by AC?" That should be a basic reality check because I've neve heard of such a thing working, and if it did it would be widely known. You may know that an AC current through a loop does generate near fields, and far (EM radiation) fields. This follows from Maxwell's equations. The solution for radiated power from a loop carrying AC current is: I did know, but I didn't think it through. :) What I had in mind was the kind of electromagnet usually used to simulate a magnet, one with a big lump magnetic material to guid the flux and concentrate it, etc. I imagine the moment you do that, you get closer to a normal magnet, but it probably makes the coil unusable as an antenna at the same time. I think there are a lot of things you don't think through, lol. Ever hear of a ferrite core antenna? Not much different. Why would an iron core make a poor antenna? -- Rick |
A short 160M antenna
rickman wrote in :
I think there are a lot of things you don't think through, lol. Ever hear of a ferrite core antenna? Not much different. Why would an iron core make a poor antenna? Fair enough. I'll plead diminished responsibility though. :) I repaired an amp during 5 hours at a house yesterday where the guy smokes. I don't, but passive smoking does my head in sometimes. I fixed the amp just fine, but by the time I got back here my brain was mush. |
A short 160M antenna
On 11/12/2014 5:43 AM, Lostgallifreyan wrote:
rickman wrote in : I think there are a lot of things you don't think through, lol. Ever hear of a ferrite core antenna? Not much different. Why would an iron core make a poor antenna? Fair enough. I'll plead diminished responsibility though. :) I repaired an amp during 5 hours at a house yesterday where the guy smokes. I don't, but passive smoking does my head in sometimes. I fixed the amp just fine, but by the time I got back here my brain was mush. Most of the posts in this thread are jim and Gareth arguing so I'm going to killfile it. If you have anything else to say I won't see it unless you start a new thread. -- Rick |
A short 160M antenna
"rickman" wrote in message
... Most of the posts in this thread are jim and Gareth arguing Incorrect. The onslaught of abuse comes only from Jim, whereas I play the role of the patient schoolteacher dealing with the unruly disruptive infants that are Jim (and you, and Lostie). In your own case, see above. |
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