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"Cecil Moore" wrote in message et... 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 Cecil, Here's a link to an interesting post on TowerTalk by Jim K9YC discussing the subject of shielding effectiveness of coax at very low frequencies: http://lists.contesting.com/_towerta.../msg00663.html 73, Mike, W4EF.............................................. ....... |
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Were you interested in a field coaxial to the axis of the line, or one
that is transverse? Perhaps I could get my assistant, Beaker, to run some tests for us. He sometimes get a little, ah, involved with his experiments, though, so it may take some time. Regards, Bunsen Cecil Moore wrote: 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 |
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On Tue, 23 May 2006 15:31:13 -0700, Roy Lewallen
wrote: 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? Hi All, Really, this contretemps seems to be over a matter of scale and application. Ramo, Whinnery, and Van Duzer make clear distinctions between mutual couplings and radiative couplings. Most of the discussion in this and related threads appear to discard these distinctions. Richard's application of screened air linked couplers and using the illustration of power transformers is found in "Fields and Waves..." by these authors: "Where there is a component of the electric field in phase with the current, the integral of the electric field cannot be considered either as a pure "capacitive" or "inductive" voltage drop since there will be real energy transfer (radiation) from these terms." Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). And so as to anticipate the conundrum of the "static" in electrostatic, the authors show no issue. However, they do provide a rational warning: "It often happens that electrodes, although grounded for direct current, may be effectively insulated or floating at radio frequencies because of impedance in the grounding leads. In such cases the new electrodes do not accomplish their shielding purposes but may in fact increase capacitive coupling." Insofar as Yuri's complaint, it is an ego trip that wholly ignores the scales of wavelength, the application of materials, the nature of balance, and the misapplication of mutual coupling to explain far field effects. In short, he has been bitten by the "lumped vs. distributed" distinction once again. The only saving grace of his argument may be found in that there are two forms of the "shielded dipole" where one supports Tom's claim, and the other support's Yuri's. Unfortunately, as correct as Richard's examples are, they too are misapplied to the "shielded dipole." The "shielded dipole" may be small in relation to wavelength, but its response mechanism is NOT found by using mutual coupling math, but rather through radiation math. 73's Richard Clark, KB7QHC |
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Richard Clark wrote:
. . . Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . Can you direct me to where in the text they do so? All I've found is a short section (5.28) on "Electrostatic Shielding" where they explain that introducing a grounded conductor near two others will reduce the capacitive coupling between them. Obviously this will alter the local E/H ratio, but in no way does it allow an E or H field to exist independently, even locally, let alone at any distance. Roy Lewallen, W7EL |
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On Wed, 24 May 2006 12:11:52 -0700, Roy Lewallen
wrote: Richard Clark wrote: . . . Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . Can you direct me to where in the text they do so? All I've found is a short section (5.28) on "Electrostatic Shielding" where they explain that introducing a grounded conductor near two others will reduce the capacitive coupling between them. Obviously this will alter the local E/H ratio, but in no way does it allow an E or H field to exist independently, even locally, let alone at any distance. Hi Roy, Article 5.12 "Circuit Concepts at High Frequencies or Large Dimensions" Figure 5.28(a) shows a complete shielding. Of course this is entirely electric, and arguably magnetic. However, magnetic flux can penetrate thin shields, electric flux cannot. This is part and parcel to the world of isolated and shielded circuits. The electrostatic shields are as effective as they are complete in their coverage. Their contribution is measured in mutual capacitance between the two points being isolated. With a drain wire to ground, and a low enough Z in that wire, then that mutual capacitance tends towards zero (however, near zero is a matter of degree as I've offered in past discussion). Figure 5.28(a) shielding is quite common in medical circuit design, and mutual capacitance does equal zero; and yet signals and power pass in and out through magnetic coupling. Isolated relays are a very compelling example of magnetic transparency in the face of total electric shielding. Magnetic shielding operates through reflection or dissipation (absorption loss due to eddy currents). This loss is a function of permeability µ. Unfortunately, permeability declines with increasing frequency, and with declining field strength. Basically, all metals exhibit the same characteristic µ above VLF; hence any appeal to "magnetic materials" used to build antennas is specious. This is not to say the magnetic shield is ineffective, merely derated seriously from what might be gleaned through poor inference by reading µ values from tables. However, it is quite obvious that transformer inter stage shielding and the faraday shield found in AM transmitters is not seeking to optimize this attenuation, far from it. Thus the degree in isolation is found in the ratio of the mutual capacitance between the two points before and after shielding; and the attenuation in magnetic flux induction introduced between the two circuits after shielding. Returning to Ramo, et. al, the introduction of a partial shield. Figure 5.28(c) is effective insofar as its ability to reduce mutual capacitance. 73's Richard Clark, KB7QHC |
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Richard Clark wrote: Figure 5.28(a) shows a complete shielding. Of course this is entirely electric, and arguably magnetic. However, magnetic flux can penetrate thin shields, electric flux cannot. Only when the shield is thin compared to the skin depth. When the shield is thick relative to skin depth nothing gets through. This is very easy to prove. I have been making measurements of a ~ .032 inch thick copper wall and with 0dB reference on a small resonant pick up loop my analyzer is in the noise (-90dB) on the side directly opposite that loop. The same is true for a direct soldered connection to the wall on each opposite side. One inch to the side on the same side levels are -10dB. That would be a two inch long path shorted by the copper the entire way. Go to the direct opposite side through only .032 thick copper and levels are not even detectable. This is part and parcel to the world of isolated and shielded circuits. The electrostatic shields are as effective as they are complete in their coverage. Their contribution is measured in mutual capacitance between the two points being isolated. I don't have that reference and so cannot see that shield, but the only thing the shield can do is reduce field impedance by changing the ratio of electric to magnetic fields. In order to take either one to zero the other must also be at zero. I think the confusion comes from misapplying a grid forming a shunt capacitance to reduce direct capacitance between two objects (forming a "T" divider) to the shielded loop antenna or shielded link. Consider a loop of any size, even a link in a tank coil. That conductor has a field impedance and radiation characteristics largely set by the diameter of the coil. Once we put a wall around that conductor more than a few skin depths thick NOTHING goes through that wall. The "shield" actually becomes the coupling coil, the link inside simply develops a voltage across the shield to drive that external coil. Both electric and magnetic fields are present on the outer wall of the shield, and while they may be modified by shield balance they really are not much different than we had with just the inner conductor alone. We really just change the balance. With the grid, we have substantial air gap segmenting the "wall". Naturally the coupling mechanism is different than we have with a solid wall. We, in effect, have dozens of long gaps. Each conductor in that grid is indeed excited by the magnetic and electric fields, and each conductor has a potential difference across area and a current flowing. Part of the field, both electric and magnetic, leaks through. Part is reradiated by the currents and voltages in the grid. I think at some point of time MRT or Dave Saloff patented a right angle grid of two layers with opposing ends in each adjacent conductor in each layer grounded that I designed. The idea was to more evenly distribute the fields and prevent "hot spots". This was for a medical application. Rest assured this applicator produced both time-varying electric and magnetic fields, but the balance was so much improved tuning was more stable. The improved balance and evenly distributed field meant the feedline did not radiate any significant amount when brought near the patient, unlike a regular multiple turn loop. I still have some prototype applicators here, as well as the field measurements required by the FDA. The applicator actually had to match with lowest return loss into the buttocks of an average size 30 year old female. 73 Tom |
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FIGHT? Here is another W8JI myth bone!
There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common induction that occurs on the very short scales I've offered. Will you give me an example where the electric field is zero and all coupling is via magnetic flux? 73 Tom |
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FIGHT? Here is another W8JI myth bone!
Richard Clark wrote:
On 25 May 2006 03:38:14 -0700, wrote: There are too many contra-examples too sustain your point. What you are talking about is radiation, this does not account for common induction that occurs on the very short scales I've offered. Will you give me an example where the electric field is zero and all coupling is via magnetic flux? Tom, As this has already been discussed not but two postings ago, the posting your responded to, why are you asking for that content again? I was going to ask the same question but Tom beat me to it. And I must have missed the example, too. Would you be so kind as to repost it? Roy Lewallen, W7EL |
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On Thu, 25 May 2006 08:40:26 -0700, Roy Lewallen
wrote: Richard Clark wrote: On 25 May 2006 03:38:14 -0700, wrote: There are too many contra-examples too sustain your point. What you are talking about is radiation, this does not account for common induction that occurs on the very short scales I've offered. Will you give me an example where the electric field is zero and all coupling is via magnetic flux? Tom, As this has already been discussed not but two postings ago, the posting your responded to, why are you asking for that content again? I was going to ask the same question but Tom beat me to it. And I must have missed the example, too. Would you be so kind as to repost it? Roy Lewallen, W7EL On Wed, 24 May 2006 12:11:52 -0700, Roy Lewallen wrote: Richard Clark wrote: . . . Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . Can you direct me to where in the text they do so? All I've found is a short section (5.28) on "Electrostatic Shielding" where they explain that introducing a grounded conductor near two others will reduce the capacitive coupling between them. Obviously this will alter the local E/H ratio, but in no way does it allow an E or H field to exist independently, even locally, let alone at any distance. Hi Roy, Article 5.12 "Circuit Concepts at High Frequencies or Large Dimensions" Figure 5.28(a) shows a complete shielding. Of course this is entirely electric, and arguably magnetic. However, magnetic flux can penetrate thin shields, electric flux cannot. This is part and parcel to the world of isolated and shielded circuits. The electrostatic shields are as effective as they are complete in their coverage. Their contribution is measured in mutual capacitance between the two points being isolated. With a drain wire to ground, and a low enough Z in that wire, then that mutual capacitance tends towards zero (however, near zero is a matter of degree as I've offered in past discussion). Figure 5.28(a) shielding is quite common in medical circuit design, and mutual capacitance does equal zero; and yet signals and power pass in and out through magnetic coupling. Isolated relays are a very compelling example of magnetic transparency in the face of total electric shielding. Magnetic shielding operates through reflection or dissipation (absorption loss due to eddy currents). This loss is a function of permeability µ. Unfortunately, permeability declines with increasing frequency, and with declining field strength. Basically, all metals exhibit the same characteristic µ above VLF; hence any appeal to "magnetic materials" used to build antennas is specious. This is not to say the magnetic shield is ineffective, merely derated seriously from what might be gleaned through poor inference by reading µ values from tables. However, it is quite obvious that transformer inter stage shielding and the faraday shield found in AM transmitters is not seeking to optimize this attenuation, far from it. Thus the degree in isolation is found in the ratio of the mutual capacitance between the two points before and after shielding; and the attenuation in magnetic flux induction introduced between the two circuits after shielding. Returning to Ramo, et. al, the introduction of a partial shield. Figure 5.28(c) is effective insofar as its ability to reduce mutual capacitance. 73's Richard Clark, KB7QHC |
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"Roy Lewallen" wrote:
I was going to ask the same question but Tom beat me to it. And I must have missed the example, too. Would you be so kind as to repost it? Not sure of the context but ideally at a voltage node in an unterminated transmission line, the E-field is very close to zero while almost all of the EM energy exists in the H-field. According to "Optics", by Hecht, the same thing can happen in free space with light waves. -- 73, Cecil, W5DXP |
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Sorry, this does not contain an example of a (time-varying) electric or
magnetic field in the absence of the other. Such a condition is, in fact, impossible. Richard Clark wrote: . . . Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . Can you direct me to where in the text they do so? All I've found is a short section (5.28) on "Electrostatic Shielding" where they explain that introducing a grounded conductor near two others will reduce the capacitive coupling between them. Obviously this will alter the local E/H ratio, but in no way does it allow an E or H field to exist independently, even locally, let alone at any distance. Hi Roy, Article 5.12 "Circuit Concepts at High Frequencies or Large Dimensions" Figure 5.28(a) shows a complete shielding. Of course this is entirely electric, and arguably magnetic. However, magnetic flux can penetrate thin shields, electric flux cannot. We've been talking about *time-varying* fields, and must have forgotten to explicitly state that qualification. The figure in question deals with static fields. Time-varying electric flux can indeed penetrate thin shields of finite conductivity, although the E/H ratio within the shield is very small. If a shield could block time-varying electric fields, the time-varying magnetic field which remained would create an electric field. A time-varying magnetic fields creates a time-varying electric field and vice-versa; this is dictated by Maxwell's equations. The answer to question 4.06d in Ramo, et al, "Can a time-varying field of any form exist in space without a corresponding electric field? Can a time-varying electric field exist without the corresponding magnetic field?" is no. A gapless shield made of a perfect conductor of any thickness will completely block both electric and magnetic fields. This is part and parcel to the world of isolated and shielded circuits. The electrostatic shields are as effective as they are complete in their coverage. Their contribution is measured in mutual capacitance between the two points being isolated. With a drain wire to ground, and a low enough Z in that wire, then that mutual capacitance tends towards zero (however, near zero is a matter of degree as I've offered in past discussion). Figure 5.28(a) shielding is quite common in medical circuit design, and mutual capacitance does equal zero; and yet signals and power pass in and out through magnetic coupling. Isolated relays are a very compelling example of magnetic transparency in the face of total electric shielding. The mutual capacitance at DC equals zero. Time-varying electric fields penetrate the shield if it's thin in terms of skin depth. Magnetic shielding operates through reflection or dissipation (absorption loss due to eddy currents). This loss is a function of permeability µ. Unfortunately, permeability declines with increasing frequency, and with declining field strength. Basically, all metals exhibit the same characteristic µ above VLF; hence any appeal to "magnetic materials" used to build antennas is specious. This is not true. Metals do indeed exhibit varying permeabilities at RF and above. This can be illustrated by a number of means, a common one being the efficacy of a powdered iron core. Electric field shielding also operates through reflection and dissipation. Permeability affects both, because of its effect on material wave impedance and skin depth. This is not to say the magnetic shield is ineffective, merely derated seriously from what might be gleaned through poor inference by reading µ values from tables. Permeability does indeed change with frequency for a variety of reasons. Consequently, some intelligence (and often measurement or guesswork) has to be used to determine what it will be at the frequency in question. However, it is quite obvious that transformer inter stage shielding and the faraday shield found in AM transmitters is not seeking to optimize this attenuation, far from it. Thus the degree in isolation is found in the ratio of the mutual capacitance between the two points before and after shielding; and the attenuation in magnetic flux induction introduced between the two circuits after shielding. Returning to Ramo, et. al, the introduction of a partial shield. Figure 5.28(c) is effective insofar as its ability to reduce mutual capacitance. Indeed it is. This is not, however, an example of a (time-varying) magnetic or electric field existing in isolation. I readily agree that a static electric or magnetic field can exist in isolation from the other, as I'm sure all other participants to this discussion do. But not time-varying ones. You can greatly change the E/H ratio, but you can't make it zero or infinite. And whatever you do will have only a local effect -- the ratio will rapidly approach the intrinsic Z of the medium as you move away from the anomaly which modified the ratio. Rapidly, that is, in terms of wavelength -- it can be quite a physical distance at very low frequencies. Roy Lewallen, W7EL |
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On Thu, 25 May 2006 21:38:45 -0700, Roy Lewallen
wrote: Time-varying electric flux can indeed penetrate thin shields of finite conductivity, although the E/H ratio within the shield is very small. A gapless shield made of a perfect conductor of any thickness will completely block both electric and magnetic fields. Hi Roy, Given the vast gulf that separates these two observations above, and the oblique reply in general that does not flow from your previous question that I responded to.... It seems you are answering a topic I have not entered into, or restating what I've already offered. 73's Richard Clark, KB7QHC |
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Richard Clark wrote:
On Thu, 25 May 2006 21:38:45 -0700, Roy Lewallen wrote: Time-varying electric flux can indeed penetrate thin shields of finite conductivity, although the E/H ratio within the shield is very small. A gapless shield made of a perfect conductor of any thickness will completely block both electric and magnetic fields. Hi Roy, Given the vast gulf that separates these two observations above, and the oblique reply in general that does not flow from your previous question that I responded to.... It seems you are answering a topic I have not entered into, or restating what I've already offered. Sorry, once again I miss your point. I maintain that time-varying electric and magnetic fields cannot exist independently, while you claim that they can. Tom and I asked for an example of a case where they do, and your response did not contain such an example. Roy Lewallen, W7EL |
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On Fri, 26 May 2006 11:10:28 -0700, Roy Lewallen
wrote: while you claim Hi Roy, The courteous thing would be to quote me directly rather than paraphrase me obliquely. Respond to the posting you find objectionable. 73's Richard Clark, KB7QHC |
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On Fri, 26 May 2006 16:40:46 -0700, Roy Lewallen
wrote: Hi Roy, But here's what you've said, and with which I disagree What appears to be the only content you disagree with: There are too many contra-examples too sustain your point. What you are talking about is radiation, this does not account for common induction that occurs on the very short scales I've offered. And: Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . We never actually get to what it is that is disagreeable do we? This is merely the window dressing for backing into an oblique translation: Am I mistaken, then? Who can tell but you? It is, after all, your statement that you disagree. We can only guess. Were you agreeing all along that a time-varying electric or magnetic field can't exist independently and therefore there can't be completely inductive (H field) or capacitive (E field) coupling? A 30 word speech dressed as a question is not clear writting. :-) Agreeing all along? No, I am never in the habit of agreeing all along. A time-varying electric or magnetic field can't exist independently? Fields in free space are intimately joined and inseparable. There can't be completely inductive (H field) or capacitive (E field) coupling? If I am not mistaken, this is the same question again. Do you in fact see any difference between the two that merits the boolean AND? Should I anticipate other philosophical questions such as Are you agreeing all along about conductivity and Ohm's law? Let me shock you and say NO so as to not deflate others' anticipation. I bet they will know how to pin me down. ;-) 73's Richard Clark, KB7QHC |
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Wow, you got me there. I'm so used to communicating with engineers that
I was actually expecting a direct and coherent response. Silly me. There was one clear and unambiguous statement in your response, though: Fields in free space are intimately joined and inseparable. So we don't disagree after all. I see now that in your previous postings "contra-examples" really means "supporting examples", and "Ramo et. al . .. . clearly support the separation of magnetic and electric flux (fields)" really means they reject it. You can really do amazing things with the English language. I'm in awe. Roy Lewallen, W7EL Richard Clark wrote: On Fri, 26 May 2006 16:40:46 -0700, Roy Lewallen wrote: Hi Roy, But here's what you've said, and with which I disagree What appears to be the only content you disagree with: There are too many contra-examples too sustain your point. What you are talking about is radiation, this does not account for common induction that occurs on the very short scales I've offered. And: Richard's applications and illustrations do not push this boundary. In fact, Ramo et. al distinctly offer the case of "electrostatic shielding" and clearly support the separation of magnetic and electric flux (fields). . . We never actually get to what it is that is disagreeable do we? This is merely the window dressing for backing into an oblique translation: Am I mistaken, then? Who can tell but you? It is, after all, your statement that you disagree. We can only guess. Were you agreeing all along that a time-varying electric or magnetic field can't exist independently and therefore there can't be completely inductive (H field) or capacitive (E field) coupling? A 30 word speech dressed as a question is not clear writting. :-) Agreeing all along? No, I am never in the habit of agreeing all along. A time-varying electric or magnetic field can't exist independently? Fields in free space are intimately joined and inseparable. There can't be completely inductive (H field) or capacitive (E field) coupling? If I am not mistaken, this is the same question again. Do you in fact see any difference between the two that merits the boolean AND? Should I anticipate other philosophical questions such as Are you agreeing all along about conductivity and Ohm's law? Let me shock you and say NO so as to not deflate others' anticipation. I bet they will know how to pin me down. ;-) 73's Richard Clark, KB7QHC |
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On Fri, 26 May 2006 21:18:13 -0700, Roy Lewallen
wrote: There was one clear and unambiguous statement in your response, though: Fields in free space are intimately joined and inseparable. Hi Roy, This statement inspired you to manufacture the following as being my meaning? So we don't disagree after all. I see now that in your previous postings "contra-examples" really means "supporting examples", and "Ramo et. al . . . clearly support the separation of magnetic and electric flux (fields)" really means they reject it. 73's Richard Clark, KB7QHC |
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I added some stuff on my webpage that relates to this thread.
It might help explain how a shield works, or at least give people ideas on how to make their own measurements. http://www.w8ji.com/skindepth.htm It's VERY clear nothing goes directly through a wall. 73 Tom |
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On 27 May 2006 19:35:12 -0700, wrote:
I added some stuff on my webpage that relates to this thread. It might help explain how a shield works, or at least give people ideas on how to make their own measurements. http://www.w8ji.com/skindepth.htm It's VERY clear nothing goes directly through a wall. Hi Tom, The stuff you've got there is nice, but the accompanying discussion has very little context if you haven't been part of the threads here. In fact, and only from my recollection, your descriptions were better described in this group. At the page however, I really don't see what you are trying to do in the "How does a shield work." The "Faraday Screens" was interesting. It would be more interesting with the data you imply, but that is hardly necessary as there is so much literature in how it works. 73's Richard Clark, KB7QHC |
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