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FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote: Tom, W8JI wrote: "I did not claim that effect. Terman certainly did not. (Yuri claims the shield "blocks electric fields" or stops "electrostatic fields".)" I`ll requote Terman from page 38 of his 1955 edition which Tom ignored: "It is possible to shield electrostatic flux without simultaneously affecting the magnetic field by surrounding the space to be shielded wih a conducting cage that is made in such a way as to provide no low-resistance path for the flow of eddy currents while at the same time offering a metallic terminal upon which electrostatic flux lines can terminate." Richard. I know anything Roy Lewallen agrees with, you disagree with. I know anything I say (or even what I don't say) sets Yuri off into a foaming lather. I really wish you guys could put personal hate or dislike aside and look at facts. This is an imporant issue because the myth about shields is imbedded in amateur circles despite many clearly written engineering texts and very simple experiments that prove the concept of time-varying magnetic fields penetating the shield. It's just a fact when the time-varying electric field is taken to zero so is the time-varying magnetic field. Static by definition is not moving or varying. Don't confuse jargon describing a different coupling mode with the mechanics of a loop operating at radio frequencies. When we receive noise or signals, the fields are time-varying. Just as with a piece of coaxial cable, the inner wall of a "shielded loop" is isolated by the skin depth of the conductor from the outside wall. The electric and magnetic coupling effects are what causes a coaxial cable with a dense shield more than a few skin depths thick to ALWAYS have the same current on the inside of the shield as the inner conductor has, and all radiation or common mode current flow over the outside. This isn't something I invented. It has been in nearly every textbook long before I was born. I'm pleased that Yuri credits me for the work, but unfortunately I had little to do with it. It really was people from the 1700's and 1800's that did all the work. You (and Yuri) appear to be confusing how time-varying fields work. I suggest you put Terman aside and actually read some textbooks on fields. It's helpful to actually make a few measurements. A few minutes spent with some very simple test equipment would go a long way to "turning on the light". The loop shield is thus a true Faraday screen, not a Faraday car body or screened room. If you say so. And as one, it also must block any time-varying magnetic field. As K7ITM points out it is the gap in the loop that is actually the feedpoint, and it is the outside of the loop that is the actual antenna. If you do not think a loop behaves this way, you need to get busy doing some real important work. You need to get all the Handbooks to quit talking about common mode currents on shield outsides. You need to get them to quit treating the inside of the shield as a isolated conductor that is independent of the outside. As I and others have suggested it only takes a moment to prove the books are correct. You can prove it with a single sheet of copper and a minimum of test equipment. 73 Tom |
FIGHT? Here is another W8JI myth bone!
wrote in message ps.com... Richard Harrison wrote: Tom, W8JI wrote: "I did not claim that effect. Terman certainly did not. (Yuri claims the shield "blocks electric fields" or stops "electrostatic fields".)" I`ll requote Terman from page 38 of his 1955 edition which Tom ignored: "It is possible to shield electrostatic flux without simultaneously affecting the magnetic field by surrounding the space to be shielded wih a conducting cage that is made in such a way as to provide no low-resistance path for the flow of eddy currents while at the same time offering a metallic terminal upon which electrostatic flux lines can terminate." Richard. I know anything Roy Lewallen agrees with, you disagree with. I know anything I say (or even what I don't say) sets Yuri off into a foaming lather. I really wish you guys could put personal hate or dislike aside and look at facts. This is an imporant issue because the myth about shields is imbedded in amateur circles despite many clearly written engineering texts and very simple experiments that prove the concept of time-varying magnetic fields penetating the shield. It's just a fact when the time-varying electric field is taken to zero so is the time-varying magnetic field. That really nails it! His "technical" response! Perfect picture of a jerk parading as an engineer! Yep, I hate your guts and I made up phony claims on your web site for all to see, so I can "hate you"! Brilliant! Keep it up! Halleluja, now we know that shields are antennas, praise the guru! Bada BUm |
FIGHT? Here is another W8JI myth bone!
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FIGHT? Here is another W8JI myth bone!
Tom, W8JI wrote:
"This is an important issue because the myth about shields is embedded in amateur circles despite many clearly written engineering texts and very simple experiments that prove the concept of time-varying magnetic fields penetrating the shield." Some of that poison reached the 2006 ARRL Handbook on page 13.18. Fig 13.26 says: "Electrostatically-shielded loop for RDF. To prevent shielding of the loop from magnetic fields, leave the shield unconnected at one end." Terman`s RDF loop should have better balance than ARRL`s because Terman`s shield gap is squarely in the center of the loop and not at one end. However, as long as the shield is broken preventing induced current from flowing around the shield, Lenz`s law will be thwarted and magnetic coupling to the coil under the shield will be obtained. Electric field coupling to the coil beneath the shield will be disallowed by the shield`s connection to ground wherever it occurs, though not as elegantly as when care is taken to get the best balance possible. I`ve worked with such Faraday screens in my broadcasting career. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
On Wed, 17 May 2006 10:19:39 -0700, Richard Clark
wrote: It's still the same "1/2 inch copper water tubing (non ferrous material passing the magnetic field)." So, does that wire make the "shield" better, or worse? Hmm, this one must've been experienced exactly as an existential question about the infinite cosmos. Super-extra credit question: If we replaced the non ferrous material (same gap, no link) with (most have probably anticipated this) a ferrous material, does this allow near field region electrical field interference to pass un-impeded? This one must never been experienced either. I've always wondered why perfect academic set-ups like "non ferrous material" (as if it were lossless) always appear in the context of a populist aw-shucks kind of posting. Sorry All, But when such simple questions become imponderables of the century, they merit Cecil's 5 forbidden words woven in. Of course, it makes only the most strained of sense, but there's nothing to compete! ;-) 73's Richard Clark, KB7QHC p.s. as viewed through the bottom of a bottle of Dick's Working Man's Brown Ale (Centralia, Washington) |
FIGHT? Here is another W8JI myth bone!
Sooo, in shielded loop the shield is the antenna according to W8JI and
worshippers. But you take the shield (W8JI antenna) away, now the wires are antenna, some say don't need no stinkin' shield and "antenna" to work as an antenna. I don't know what a W8JI antenna is, except for those I've heard on 160m... :/ But I do know that I've tested various versions of both shielded and unshielded loops, and have never been able to tell a lick of difference as far as close local noise pickup. I spent a whole week testing that very thing. It's not something I just made up, or picked up from W8JI. Amazing how selective in reading and digestion of postings some people are. They tend to ignore the reality and description of it, they pick on selective "proof" of what they were taught and figered out. Only my test results were used to come up with my conclusion. So I guess I taught myself. I've never built a shielded loop yet that was any "quieter" to local noise than any of my good unshielded loops. But my unshielded loops are well balanced. Were yours? So shield works as a electrostatic shield, if you guys like it or not, or refuse to admit. I refuse to admit it, if I can't prove it. And I haven't been able to prove it yet. One thing...How in the heck is a solid shield going to filter one source of RF, and ignore another. In reality, it will shield *all* RF, unless I am missing something here. So the outer shield *must* be the antenna, unless the sky is now green. No RF is going to prevail past the outer skin depth of the solid shield. None. Nada... Sooo, antenna works without shield (not just my assertion), but when you insert it in the shield then shield becomes W8JI antenna. It does? I'm sure if this is the case, it probably tunes 160m.... :/ So his shield, untuned becomes antenna, but my tuned and tunable inside the shield antenna is not the antenna? Makes as much sense as "there is equal current along the loading coil doesn't matter what", riiiiight? If you say so.... Let's stick to some reality in antennas. Thats all I do. I've made a load of loops. I have a diamond loop 44 inches per side right next to me. Almost is as tall as the ceiling... Heck, I even have tried using shielded loops as the coupling loop to unshielded loops. Works pretty well to maintain balance, but mine work just as well with just a simple unshielded coupling loop. Probably cuz my loops are very symmetrical and balanced naturally. The coax feedline itself is the only real issue in my case, and even it's not really very critical. I never saw any indication that using a shielded coupling loop made the loop quieter than not using one. Not once. Myself, I don't really like small loops for receiving on 160m. They are good for cutting the noise when working loud locals, but in my experience they are pretty ho-hum when receiving weak dx. For 160m, I would use the biggest loop I could manage. Probably outside to have enough room... My loops are mainly for MW BC receiving, although the one next to me tunes 500-2300 kc in two stages, by switching cap gangs. I can go LW if I tack on more fixed caps. The real value of small loops are not the "quiet", or the s/n or whatever. It's the nulls... But nulls have much more value in the BC band, than they do on 160m unless maybe you have a noise source in the area you wanna null out. Thats how a loop reduces noise. Using the nulls... :/ I do have to agree with Tom. I think the "shielded loop" theory many hams adhere to is just another batch of wive tailery.. Along with grounds to cure antenna/feedline problems, sticking coax ends in bottles hoping to thwart lightning, etc... And I've never once talked to Tom about small loops. It's all my idea to shun this "shield=quiet" theory, not W8JI's. MK |
FIGHT? Here is another W8JI myth bone!
Cecil Moore wrote:
wrote: I know anything Roy Lewallen agrees with, you disagree with. Absolutely false. I'll bet they agree on 99% of technical topics, e.g. ohm's law, Maxwell's equations, etc. Just like arguing whether Coke or Pepsi tastes best. The closer the product, the worse the arguing... - 73 de Mike KB3EIA - |
FIGHT? Here is another W8JI myth bone!
Mark, NM5K wrote:
"I refuse to admit it, if I can`t prove it." A shield is extra work, weight, and cost but despite that, many are in use. As electrons move along a conductor a magnetic field expands from some depth inside the conductor itself. The magnetic lines of force sweep outward from the conductor while inducing an emf in the conductor itself. The self induced emf opposes instantaneous change of current in the inductance of the conductor. This is the basis of Lenz`s law: "In all cases of electromagnetic induction, induced electromotive force and resultant current are in such a direction as to oppose the effect producing them." Skin effect prevents penetration of RF very deep into a good conductor. Skin effect makes RF coil shields impenetrable. Electric hields are shorted to ground by the conductive shield. Magnetic fields induce counter fields from the currents they induce on the surface of the shield. A Faraday screen breaks the current path on the shield preventing the counter fields from being magneticly induced. Result is a shield that is penetrable by the magnetic field but impenetrable by the electric field. The electric field is still shorted to ground by its conductive path. Faraday screens are used because they work. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
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FIGHT? Here is another W8JI myth bone!
"Richard Harrison" wrote in message ... As electrons move along a conductor a magnetic field expands from some depth inside the conductor itself. The magnetic lines of force sweep outward from the conductor while inducing an emf in the conductor itself. The self induced emf opposes instantaneous change of current in the inductance of the conductor. This is the basis of Lenz`s law: "In all cases of electromagnetic induction, induced electromotive force and resultant current are in such a direction as to oppose the effect producing them." Skin effect prevents penetration of RF very deep into a good conductor. Skin effect makes RF coil shields impenetrable. Electric hields are shorted to ground by the conductive shield. Magnetic fields induce counter fields from the currents they induce on the surface of the shield. A Faraday screen breaks the current path on the shield preventing the counter fields from being magneticly induced. Result is a shield that is penetrable by the magnetic field but impenetrable by the electric field. The electric field is still shorted to ground by its conductive path. Faraday screens are used because they work. Best regards, Richard Harrison, KB5WZI I would agree, Richard, but at HF frequencies the current path around the shield isn't really broken by the gap. Due to the skin effect, the RF current flowing on the inside of the loop shield is free to flow around the edge of the shield conductor and onto the outside of the shield at the gap. At very low frequencies, where the skin depth is large, this wouldn't necessarily be true, but at HF as long as there are a few skin depths between the outside and the inside surface of the conductor, then the inside surface of the shield and the outside surface of the shield can be treated as independent conductors. 73, Mike W4EF........................ |
FIGHT? Here is another W8JI myth bone!
Mike, W4EF wrote:
"I would agree, Richard, but at HF frequencies, the current path around the shield isn`t real;ly broken by the gap." To best describe what broken means, a picture helps. There is a picture on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which says: "To prevent shielding of the loop from magnetic fields, leave the shield unconnected at one end." I think the handbook has it right. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
"Richard Harrison" wrote in message ... Mike, W4EF wrote: "I would agree, Richard, but at HF frequencies, the current path around the shield isn`t real;ly broken by the gap." To best describe what broken means, a picture helps. There is a picture on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which says: "To prevent shielding of the loop from magnetic fields, leave the shield unconnected at one end." I am a bit behind on ARRL Handbooks, Richard, but from what you describe, this is the same figure that appears in my 1992 edition (chapter 38, figure 2). In any case, what is shown in the figure agrees with my understanding of "broken", although admittedly when I made my previous post, I was thinking of the case where the shield is broken on the side of the loop opposite the feedpoint. For the purposes of this discussion, however, it doesn't matter whether the break is at the top (opposite the feed) or at the bottom (adjacent to the feed). In either case, current induced on the inside of the shield by current flowing on the center conductor loop has a continuous back to ground via the outside surface of the shield. IOW, the gap doesn't suppress the eddy current, rather it forces it to flow on the outside surface of the shield, thereby causing the loop to radiate. I think the handbook has it right. Yes, I agree it does. If you connect the shield at both ends, the loop can't radiate because the eddy current caused by current flowing on the inner conductor loop will confined to the inside of the shield. Likewise, eddy currents induced on the outside of the shield by EM waves passing the antenna will be confined to the outside of the shield if there is no gap (reciprocity holds - the antenna won't receive with no gap). 73, Mike W4EF.............................................. ... Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Michael Tope wrote: I am a bit behind on ARRL Handbooks, Richard, but from what you describe, this is the same figure that appears in my 1992 edition (chapter 38, figure 2). In any case, what is shown in the figure agrees with my understanding of "broken", although admittedly when I made my previous post, I was thinking of the case where the shield is broken on the side of the loop opposite the feedpoint. For the purposes of this discussion, however, it doesn't matter whether the break is at the top (opposite the feed) or at the bottom (adjacent to the feed). In either case, current induced on the inside of the shield by current flowing on the center conductor loop has a continuous back to ground via the outside surface of the shield. IOW, the gap doesn't suppress the eddy current, rather it forces it to flow on the outside surface of the shield, thereby causing the loop to radiate. Absolutely nothing, neither electic nor magnetic, couplesthrough the wall of a conductor more than several skin depths thick. This isn't anything that can be debated, it is simply how it works. It is very easy to demonstrate, it takes only a few minutes and a minimum of test equipment. It is something very basic in physics and underlies how coaxial cables and things with shields of all types work. The gap is the feedpoint no matter where the gap is placed. The radiation and coupling of any time-varying field, magnetic or electric, occurs on a frequency where the shield is more than a few skin depths thick comes by the gap. This is such a very basic thing it is important everyone understand it. I think the handbook has it right. Yes, I agree it does. If you connect the shield at both ends, the loop can't radiate because the eddy current caused by current flowing on the inner conductor loop will confined to the inside of the shield. Absolutely. When the gap is closed there is no potential difference across the gap the outside of the shield is not connected to the inside of the shield via the potential developed across the gap. The outer wall is not coupled to the inner wall, the feedpoint is shorted. When the gap is opened, the outside of the shield IS the antenna. Not the inside or anything inside the inside. Likewise, eddy currents induced on the outside of the shield by EM waves passing the antenna will be confined to the outside of the shield if there is no gap (reciprocity holds - the antenna won't receive with no gap). Again true. This is a very basic thing we must understand if we are to understand how shields, walls, or conductors of any kind or form work with HF currents, voltages, or fields of any type. There isn't any way to change this effect. 73 Tom |
FIGHT? Here is another W8JI myth bone!
wrote in message ups.com... Michael Tope wrote: I am a bit behind on ARRL Handbooks, Richard, but from what you describe, this is the same figure that appears in my 1992 edition (chapter 38, figure 2). In any case, what is shown in the figure agrees with my understanding of "broken", although admittedly when I made my previous post, I was thinking of the case where the shield is broken on the side of the loop opposite the feedpoint. For the purposes of this discussion, however, it doesn't matter whether the break is at the top (opposite the feed) or at the bottom (adjacent to the feed). In either case, current induced on the inside of the shield by current flowing on the center conductor loop has a continuous back to ground via the outside surface of the shield. IOW, the gap doesn't suppress the eddy current, rather it forces it to flow on the outside surface of the shield, thereby causing the loop to radiate. Absolutely nothing, neither electic nor magnetic, couplesthrough the wall of a conductor more than several skin depths thick. This isn't anything that can be debated, it is simply how it works. It is very easy to demonstrate, it takes only a few minutes and a minimum of test equipment. I don't think we disagree on that point, Tom. Perhaps I should have chosen my words more carefully. I didn't mean to imply that gap somehow forces the current on the inside of the shield to pass through shield. When I said that the gap forces the current to flow on the outside surface of the shield, I meant that in the sense that the eddy current flows on the inside of the shield until it reaches the break in the shield at which point the current flow wraps around the edge of the shield and onto the outside surface (thereby reversing direction relative to the direction of the eddy current on the inside of the shield). The skin effect in effect separates the shield into two distinct conductors, the inner surface being one conductor and the outer surface of the shield being the other. The gap is the circuit node where these two independent conductors are connected. The eddy current flows out of one conductor (the inner surface of the shield ) and into the other conductor (the outer surface of the shield). 73, Mike W4EF.............................................. ........... |
FIGHT? Here is another W8JI myth bone!
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FIGHT? Here is another W8JI myth bone!
Tom, W8JI wrote:
"Absolutely nothing, neither electric nor magnetic, couples through the wall of a conductor several skin depths thick." That`s wrong for a "Faraday screen". Terman is right. At the bottom of page 38 of his 1955 edition he writes: "It is possible to shield electrostatic flux without simultaneously affecting the magnetic field by surrounding the space to be shielded with a conducting cage that is made in such a way as to provide no low-resistance path for the flow of eddy currents, while at the same time offering a metallic terminal upon which electrostatic flux lines can terminate." An example exists in the AM broadcast stations I`ve worked in. Every tower was coupled to its transmission line through a 1:1 air-core traansformer. Two identical single-layer solenoids sharing the same axis. Between the coils was a metal picket fence. One end of the pickets was firmly grounded to the coupling cabinet. The other end of all pickets was an open circuit. Electric lines of force were intercepted by the pickets and directly shorted to ground. However, the fences had no effect on the magnetic coupling between them because the open circuit at the ends of the pickets prevented circulating currents which would have opposed magnetic coupling according to Lenz`s law. Voila! Magnetic coupling but no electrostatic coupling between coils of a transformer. It`s time for W8JI to turn-off his misinformation machine. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Richard,
"this can't be" because "gurus" know otherwise. Why do you hate Tom? You don't like anything he says on his "myth overturning" web pages. He describes in a such detail and explains that "shield is an antenna" - why don't you get it? :-)))) According to Tom, RF gets induced on the outside "wire" of the shield, then it crolls to the "inside" wire of the shield around the edge of the tubing and sees another wire and jumps over, and then to coax. If tubing or shield was the antenna, then it would receive DX and near field signals the same way. The fact that shield is shielding the near field signals should make any guru wonder. There was ZS1 on TopBand reflector reporting that he used shielded loop and other loop antennas, and shielded loop was the only one that suppressed the local TV birdies. Tom "explained" to him "how things work" and he apologized that he did not mean to have this as an example of what I was saying. There are other examples where shield "doesn't shield" - like link coupling made of coax with end shield open and center conductor soldered to the shield. As I mentioned I have magnetothermia machine that produces about 200W from single shielded loop, according to Tom, it should be frying the coax in the gap, with all that RF power trying to make the corner :-) There is more nonsense on his web site. 73 Yuri, K3BU "Richard Harrison" wrote in message ... Tom, W8JI wrote: "Absolutely nothing, neither electric nor magnetic, couples through the wall of a conductor several skin depths thick." That`s wrong for a "Faraday screen". Terman is right. At the bottom of page 38 of his 1955 edition he writes: "It is possible to shield electrostatic flux without simultaneously affecting the magnetic field by surrounding the space to be shielded with a conducting cage that is made in such a way as to provide no low-resistance path for the flow of eddy currents, while at the same time offering a metallic terminal upon which electrostatic flux lines can terminate." An example exists in the AM broadcast stations I`ve worked in. Every tower was coupled to its transmission line through a 1:1 air-core traansformer. Two identical single-layer solenoids sharing the same axis. Between the coils was a metal picket fence. One end of the pickets was firmly grounded to the coupling cabinet. The other end of all pickets was an open circuit. Electric lines of force were intercepted by the pickets and directly shorted to ground. However, the fences had no effect on the magnetic coupling between them because the open circuit at the ends of the pickets prevented circulating currents which would have opposed magnetic coupling according to Lenz`s law. Voila! Magnetic coupling but no electrostatic coupling between coils of a transformer. It`s time for W8JI to turn-off his misinformation machine. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Richard,
Think again about what you wrote. The "Faraday screen" is full of openings between the wires of the picket fence. There is no evidence that anything magnetic or electric penetrates the walls of the conductors beyond a very shallow layer. Terman certainly did not deny the existence of skin effect that keeps the fields out of the interior of conductors. 73, Gene W4SZ Richard Harrison wrote: Tom, W8JI wrote: "Absolutely nothing, neither electric nor magnetic, couples through the wall of a conductor several skin depths thick." That`s wrong for a "Faraday screen". Terman is right. At the bottom of page 38 of his 1955 edition he writes: "It is possible to shield electrostatic flux without simultaneously affecting the magnetic field by surrounding the space to be shielded with a conducting cage that is made in such a way as to provide no low-resistance path for the flow of eddy currents, while at the same time offering a metallic terminal upon which electrostatic flux lines can terminate." An example exists in the AM broadcast stations I`ve worked in. Every tower was coupled to its transmission line through a 1:1 air-core traansformer. Two identical single-layer solenoids sharing the same axis. Between the coils was a metal picket fence. One end of the pickets was firmly grounded to the coupling cabinet. The other end of all pickets was an open circuit. Electric lines of force were intercepted by the pickets and directly shorted to ground. However, the fences had no effect on the magnetic coupling between them because the open circuit at the ends of the pickets prevented circulating currents which would have opposed magnetic coupling according to Lenz`s law. Voila! Magnetic coupling but no electrostatic coupling between coils of a transformer. It`s time for W8JI to turn-off his misinformation machine. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
"Yuri Blanarovich" wrote in message ... There are other examples where shield "doesn't shield" - like link coupling made of coax with end shield open and center conductor soldered to the shield. As I mentioned I have magnetothermia machine that produces about 200W from single shielded loop, according to Tom, it should be frying the coax in the gap, with all that RF power trying to make the corner :-) Yuri, think about how the "link coupling" magnetic loop you describe above works. When the loop is energized where does the RF current leaving the center conductor go? It has to flow onto the outside of the shield. Where else could it go? RF current "makes the corner" around to the outside surface of the shield in coax all the time. If it didn't we wouldn't need choke balun's. 73, Mike W4EF.............................................. ................... |
FIGHT? Here is another W8JI myth bone!
Gene Fuller wrote:
Richard, Think again about what you wrote. The "Faraday screen" is full of openings between the wires of the picket fence. There is no evidence that anything magnetic or electric penetrates the walls of the conductors beyond a very shallow layer. Terman certainly did not deny the existence of skin effect that keeps the fields out of the interior of conductors. Gene, You might have to find a book that quotes the description of a screen with parallel wires and large air gaps as compared to a wall or cylinder several skin depths thick. :-) 73 Tom |
FIGHT? Here is another W8JI myth bone!
"Michael Tope" wrote in message . .. "Yuri Blanarovich" wrote in message ... There are other examples where shield "doesn't shield" - like link coupling made of coax with end shield open and center conductor soldered to the shield. As I mentioned I have magnetothermia machine that produces about 200W from single shielded loop, according to Tom, it should be frying the coax in the gap, with all that RF power trying to make the corner :-) Yuri, think about how the "link coupling" magnetic loop you describe above works. When the loop is energized where does the RF current leaving the center conductor go? It has to flow onto the outside of the shield. Where else could it go? RF current "makes the corner" around to the outside surface of the shield in coax all the time. If it didn't we wouldn't need choke balun's. We need RF chokes and baluns to supress curents induced on the shield from the unbalance at the antenna feedpoint. Sooo, according to W8JI "teachings", RF current gets induced onto the outside surface of tubing, then crolls around the edges and goes inside the tubing? Sooo, we should cork the elements, or the current will get confused inside of dark tubing elements, Eh? Any formulas to calculate the resonance of such "antenna"?? 73, Mike W4EF.............................................. ................... -- Yuri Blanarovich, K3BU, VE3BMV |
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Gene Fuller wrote:
"Terman certainly did not deny the existence of skin effect that keeps fields out of the interior of conductors.' true. The point is, shielding from magnetic fields is different from electric fields. On page 35 of his 1955 edition, Terman writes: "Magnetic flux in attempting to pass through a shield (copper or aluminum) induces voltage in the shield which gives rise to eddy currents. These eddy currents oppose the action of the flux, and in large measure prevent its penetration through the shield." On page 38, Terman writes: "Electrostatic shielding is obtained by enclosing free space to be shielded by a conducting surface." On page 45, is problem 2-45 which contains an illustration of a grid of open-circuit wires which "will provide electrostatic shielding without magnetic shielding---." This works just like the picket fences used in broadcast stations to inhibit harmonic transmission. Terman did not make this stuff up. It was already in wide use at the time. Best regards, Richard Harrison, KB5WZI |
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On Mon, 22 May 2006 10:42:40 -0400, "Yuri Blanarovich"
wrote: "Michael Tope" wrote in message ... "Yuri Blanarovich" wrote in message ... There are other examples where shield "doesn't shield" - like link coupling made of coax with end shield open and center conductor soldered to the shield. As I mentioned I have magnetothermia machine that produces about 200W from single shielded loop, according to Tom, it should be frying the coax in the gap, with all that RF power trying to make the corner :-) Yuri, think about how the "link coupling" magnetic loop you describe above works. When the loop is energized where does the RF current leaving the center conductor go? It has to flow onto the outside of the shield. Where else could it go? RF current "makes the corner" around to the outside surface of the shield in coax all the time. If it didn't we wouldn't need choke balun's. We need RF chokes and baluns to supress curents induced on the shield from the unbalance at the antenna feedpoint. Sooo, according to W8JI "teachings", RF current gets induced onto the outside surface of tubing, then crolls around the edges and goes inside the tubing? Sooo, we should cork the elements, or the current will get confused inside of dark tubing elements, Eh? Any formulas to calculate the resonance of such "antenna"?? 73, Mike W4EF.............................................. ................... Yuri, It is true that current will not flow on the inside of a tube from current on the outside. The "waveguide beyond cutoff" effect keeps it from doing so. The currents quickly cancel a short distance inside the tube. However, if you put a conductor inside that tube (wire) now it acts like a coax cable and the energy on the center conductor couples to the inside wall of the tube. At the end of the tube the current is free to wrap around to the outside. 73 Gary K4FMX |
FIGHT? Here is another W8JI myth bone!
"Yuri Blanarovich" wrote in message ... RF current "makes the corner" around to the outside surface of the shield in coax all the time. If it didn't we wouldn't need choke balun's. We need RF chokes and baluns to supress curents induced on the shield from the unbalance at the antenna feedpoint. Actually what oftentimes happens with a coax feed is that the RF current leaving the inside of the feedline shield can flow in two directions. It can flow down the antenna element half connected to the shield (desired path), or it can flow down the outside of the shield (undesired path). The electrons are dumb, all they are looking for is the path of least resistance. They can't tell that the metal surface on the outside of the coax isn't supposed to be part of the antenna. The only way to keep current from flowing down the shield is make the antenna element-half connected to the shield look like a lower impedance than the outside of the shield. If you place ferrite beads around the outside of the shield, this will raise the impedance of the shield path, thereby diverting the bulk of the RF current into the element-half and off of the shield's outside surface. Sooo, according to W8JI "teachings", RF current gets induced onto the outside surface of tubing, then crolls around the edges and goes inside the tubing? As per K4FMX's comments, this can only happen if there is a center conductor inside the tubing, or if the tubing diameter is greater than ~1/2 wavelength in diameter, otherwise the inside of the tubing looks like a circular waveguide beyond cutoff. This is why coax of a given diameter becomes useless above a certain upper frequency limit. Once the I.D. of the coax becomes a significant fraction of a wavelength in diameter, the coax will start to support propagation of waveguide modes (e.g. non-TEM modes). At HF frequencies, even large diameter tubing is well beyond waveguide cutoff, so there is no concern about "corking" open tubing with no center conductor (it corks itself). 73, Mike W4EF.............................................. ....... Sooo, we should cork the elements, or the current will get confused inside of dark tubing elements, Eh? Any formulas to calculate the resonance of such "antenna"?? |
FIGHT? Here is another W8JI myth bone!
Michael Tope wrote:
The electrons are dumb, all they are looking for is the path of least resistance. Hmmmm, electrons that know ohm's law sound pretty smart to me. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote:
Gene Fuller wrote: "Terman certainly did not deny the existence of skin effect that keeps fields out of the interior of conductors.' true. The point is, shielding from magnetic fields is different from electric fields. On page 35 of his 1955 edition, Terman writes: "Magnetic flux in attempting to pass through a shield (copper or aluminum) induces voltage in the shield which gives rise to eddy currents. These eddy currents oppose the action of the flux, and in large measure prevent its penetration through the shield." . . . Am I mistaken, but is this not a clear statement that a copper or aluminum shield will block magnetic flux, along with an explanation of why it happens? Roy Lewallen, W7EL |
FIGHT? Here is another W8JI myth bone!
Seems pretty clear to me, Roy. The effectiveness of a copper strap
around a mains-frequency power transformer at reducing the exterior magnetic field is well known and often used. It's all very clear from Faraday's law of magnetic induction: the net magnetic flux through an area enclosed by a perfect conductor may not change, so time-varying magnetic fields are perfectly blocked by perfect conductors. Copper's a reasonable approximation of a perfect conductor in the case of RF shields. Cheers, Tom |
FIGHT? Here is another W8JI myth bone!
Roy Lewallen wrote: Richard Harrison wrote: Gene Fuller wrote: "Terman certainly did not deny the existence of skin effect that keeps fields out of the interior of conductors.' true. The point is, shielding from magnetic fields is different from electric fields. On page 35 of his 1955 edition, Terman writes: "Magnetic flux in attempting to pass through a shield (copper or aluminum) induces voltage in the shield which gives rise to eddy currents. These eddy currents oppose the action of the flux, and in large measure prevent its penetration through the shield." . . . Am I mistaken, but is this not a clear statement that a copper or aluminum shield will block magnetic flux, along with an explanation of why it happens? Roy Lewallen, W7EL It would only be a clear statement to those who understand what was quoted from Terman. If a person is confused by or somehow DOESN'T understand what Terman is saying, he or she might take it to mean magnetic fields can travel unimpeded through a shield. It sure is difficult to drive a stake through the heart of myths like the loop shield "shielding the electric field and not the magnetic field" when clearly written text in dozens of engineering textbooks is misunderstood. 73 Tom |
FIGHT? Here is another W8JI myth bone!
On 23 May 2006 10:17:31 -0700, "K7ITM" wrote:
Seems pretty clear to me, Roy. The effectiveness of a copper strap around a mains-frequency power transformer at reducing the exterior magnetic field is well known and often used. It's all very clear from Faraday's law of magnetic induction: the net magnetic flux through an area enclosed by a perfect conductor may not change, so time-varying magnetic fields are perfectly blocked by perfect conductors. Copper's a reasonable approximation of a perfect conductor in the case of RF shields. Hi Tom, However, Richard's explanation is the analogue of the effectiveness of a copper strap (with a non-contacting overlap so as to not be a shorted turn) between windings of a mains-frequency power transformer, and grounded to provide electrostatic separation of the two circuits. 73's Richard Clark, KB7QHC |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote:
Roy Lewallen, W7EL wrote: "Am I mistaken, but is this not a clear statement that a copper or aluminum shield will block magnetic flux along with explanation of why it happens?" Yes. And now the rest of the story which I`ve already posted several times. At the bottom of page 38 in Terman`s 1955 edition; "It is possible to shield slectrostatic flux WITHOUT simultaneously affecting the magnetic field by surrounding the space to be shielded with a conducting cage that is made in such a way as to provide NO low-resistance path for the flow of eddy currents, while at the same time offering a metallic terminal upon which electrostatic flux can terminate." I invited readers to look at page 43 in the same book whe "A grid of wires such as shown in the accompanying figure will provide electrostatic shielding WITHOUT magnetic shielding---." I also said that similar grids (metal picket fences) were used in AM broadcast stations I`d worked in to eliminate capacitive coupling to the antennas which would otherwise favor harmonics of the broadcast frequency. W8JI is yet desiring to drive a stake in the heart of the "myth" that E&H fields are separable if even for an instant. He is dead wrong. Certainly the E or H field can disappear for part of a cycle. This happens routinely in a transmission line or in free space, as the energy is swapped back and forth between the two components. Tom has never said this is not so. And for whole cycles you can locally change the ratio of E/H, but you cannot separate them. Maxwell's equations show this. And if you do change the ratio of E/H, the normal free space ratio is restored within a small distance. The magnetic field alone does quite well in transferring all the wave`s energy through a special transformer which completely bars the electric field. Of course, current in the transformer`s secondary produces a voltage and the E-field is immediately restored. Yes. And does this restored E field not occupy the space between the winding and the wire grid? If so, how can you tell that the grid has "stopped" the E field if it exists on both sides of the grid? This is not witchcraft. In free-space the electric field and the magnetic field are repeatedly exchanging all the energy back and forth. It keeps the wave going. With this I totally agree. That explanation is related to why you can't simply remove one component or the other. At a short or open on a transmission line energy is not lost. It is merely transferred for an instant into the surviving field. Similarly, all the energy can be transferred through the electric field with zero magnetic coupling. Imagine two separately shielded coils. Now, use a coupling capacitor to transfer the energy from one coil to the other. Voila! E-field transfer with zero magnetic coupling. It`s no myth. It`s a fact. It's a myth that there's no magnetic field in the space between a capacitor's plates. Roy Lewallen, W7EL |
FIGHT? Here is another W8JI myth bone!
Roy, W7EL wrote:
"It`s a myth that there`s no magnetic field in the space between a capacitor`s plates." Maxwell`s great speculation was that "displacement current", as between a capacitor`s plates, produced magnetic flux as does conduction current. His speculation is now proved. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote:
Cecil, W5DXP wrote: "Does a 60 Hz magnetic field travel virtually unimpeded through a coaxial shield?" Surely many readers have toiled with shielded audio cables similar to coax and they know the answer is yes, if the shield is broken (this often happens at the ends of the cable). Best regards, Richard Harrison, KB5WZI The skin depth of copper at 60 hertz is supposed to be 8.53mm. That's too thick to make a practical shield. No wonder they're having problems. 73, Tom Donaly, KA6RUH |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote:
Roy, W7EL wrote: "It`s a myth that there`s no magnetic field in the space between a capacitor`s plates." Maxwell`s great speculation was that "displacement current", as between a capacitor`s plates, produced magnetic flux as does conduction current. His speculation is now proved. Yes. So how does a capacitor between two inductors constitute "E-field transfer with zero magnetic coupling" as you stated? Roy Lewallen, W7EL |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote:
Roy Lewallen, W7EL wrote: "Am I mistaken, but is this not a clear statement that a copper or aluminum shield will block magnetic flux along with explanation of why it happens?" Yes. And now the rest of the story which I`ve already posted several times. At the bottom of page 38 in Terman`s 1955 edition; "It is possible to shield slectrostatic flux WITHOUT simultaneously affecting the magnetic field by surrounding the space to be shielded with a conducting cage that is made in such a way as to provide NO low-resistance path for the flow of eddy currents, while at the same time offering a metallic terminal upon which electrostatic flux can terminate." I invited readers to look at page 43 in the same book whe "A grid of wires such as shown in the accompanying figure will provide electrostatic shielding WITHOUT magnetic shielding---." I also said that similar grids (metal picket fences) were used in AM broadcast stations I`d worked in to eliminate capacitive coupling to the antennas which would otherwise favor harmonics of the broadcast frequency. W8JI is yet desiring to drive a stake in the heart of the "myth" that E&H fields are separable if even for an instant. He is dead wrong. The magnetic field alone does quite well in transferring all the wave`s energy through a special transformer which completely bars the electric field. Of course, current in the transformer`s secondary produces a voltage and the E-field is immediately restored. This is not witchcraft. In free-space the electric field and the magnetic field are repeatedly exchanging all the energy back and forth. It keeps the wave going. At a short or open on a transmission line energy is not lost. It is merely transferred for an instant into the surviving field. Similarly, all the energy can be transferred through the electric field with zero magnetic coupling. Imagine two separately shielded coils. Now, use a coupling capacitor to transfer the energy from one coil to the other. Voila! E-field transfer with zero magnetic coupling. It`s no myth. It`s a fact. Best regards, Richard Harrison, KB5WZI When did you perform this experiment, Richard? (Perfectly shielding two coils and then coupling them with a capacitor.) And how did you manage to shield them if, as you seem to think, the magnetic fields are capable of penetrating the shield? 73, Tom Donaly, KA6RUH |
FIGHT? Here is another W8JI myth bone!
Roy Lewallen wrote:
It's a myth that there's no magnetic field in the space between a capacitor's plates. What quantum particles support that magnetic field? -- 73, Cecil http://www.qsl.net/w5dxp |
FIGHT? Here is another W8JI myth bone!
Tom Donaly wrote:
The skin depth of copper at 60 hertz is supposed to be 8.53mm. That's too thick to make a practical shield. No wonder they're having problems. So 60 Hz magnetic fields penetrate shielded coax? -- 73, Cecil http://www.qsl.net/w5dxp |
FIGHT? Here is another W8JI myth bone!
"Cecil Moore" wrote in message om... Tom Donaly wrote: The skin depth of copper at 60 hertz is supposed to be 8.53mm. That's too thick to make a practical shield. No wonder they're having problems. So 60 Hz magnetic fields penetrate shielded coax? -- 73, Cecil http://www.qsl.net/w5dxp Cecil, if ever I had the feeling that I was about to answer a loaded question, this is it, but here goes anyway - "Yes, I believe a 60 Hz magnetic field impinging on a piece of shielded coax would penetrate the shield of that coax significantly if the shield were made of a non-ferrous conductor." 73, Mike W4EF............................................. |
FIGHT? Here is another W8JI myth bone!
Michael Tope wrote:
"Cecil Moore" wrote: So 60 Hz magnetic fields penetrate shielded coax? Cecil, if ever I had the feeling that I was about to answer a loaded question, this is it, but here goes anyway - "Yes, I believe a 60 Hz magnetic field impinging on a piece of shielded coax would penetrate the shield of that coax significantly if the shield were made of a non-ferrous conductor." It's not a loaded question. I just always assumed that coax would shield the system from 60 Hz noise and I guess I was wrong. -- 73, Cecil http://www.qsl.net/w5dxp |
FIGHT? Here is another W8JI myth bone!
"Richard Harrison" wrote Similarly, all the energy can be transferred through the electric field with zero magnetic coupling. Imagine two separately shielded coils. Now, use a coupling capacitor to transfer the energy from one coil to the other. Voila! E-field transfer with zero magnetic coupling. It`s no myth. It`s a fact. Best regards, Richard Harrison, KB5WZI I wonder how the gurus 'splain the different behavior of vertical vs. horizontally polarized antennas, where E field determines the way signals reflect and form the pattern. If E and H fields are so "inseparable" there should be no difference, right? They are ignoring behavior of E and H fields within the near field of antennas and workings of the shield. Another example of shield's performance was when I had small shielded loop next to the Beverage. The combination gave better S/N performance and better signal levels than each of them alone. Again, shield performing shielding function in the vicinity of near field of both antennas. Loops were tunable and performing as an antenna, shield was shielding from the near by interference and providing symmetry. FACTS - verifiable not subject to wild speculations about current crawling around the edge to inside of tubing, or antenna inside of electrostatic shield quitting to work as antenna because of "W8JI shield is antenna teachings". -- Yuri Blanarovich, K3BU, VE3BMV |
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