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Yuri Blanarovich wrote:
Howdy antenna aficionados! Wanna FIGHT? Here is another misconception propagated by "guru" W8JI, like "there is no electrostatic shielding, the grounded piece of tubing (shield) IS the antenna." Let the games begin! (I will hold my horses for a while :-) As posted on reflector: . . . I don't find anything incorrect with Tom's response. What did you find in it that wasn't accurate? Roy Lewallen, W7EL |
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GREAT! I even got a spin-off thread! just the kind thing to keep a cold,
wet, windy day interesting! "Tom Ring" wrote in message .. . Yuri Blanarovich wrote: Howdy antenna aficionados! Wanna FIGHT? Yuri More mixer and more ice would serve you well. tom K0TAR |
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Tom, this is well written and devoid of any antagonism towards anyone.
If anybody wants to dispute any point then all relavent data is in place in your posting and thus forces all who disagree to stay on subject without the need for extraneous data when debating their differences. There will ofcourse, be some that will be more interested in a fight or profanity in the absence of comunicable knoweledge , but you are well positioned to just stand by your posting without retaliating in kind. Well done Art |
FIGHT? Here is another W8JI myth bone!
Tom, this is well written and devoid of any antagonism towards anyone.
If anybody wants to dispute any point then all relavent data is in place in your posting and thus forces all who disagree to stay on subject without the need for extraneous data when debating their differences. There will ofcourse, be some that will be more interested in a fight or profanity in the absence of comunicable knoweledge , but you are well positioned to just stand by your posting without retaliating in kind. Well done Art |
FIGHT? Here is another W8JI myth bone!
"art" wrote in message
oups.com... Tom, this is well written and devoid of any antagonism towards anyone. If anybody wants to dispute any point then all relavent data is in place in your posting and thus forces all who disagree to stay on subject without the need for extraneous data when debating their differences. There will ofcourse, be some that will be more interested in a fight or profanity in the absence of comunicable knoweledge , but you are well positioned to just stand by your posting without retaliating in kind. Well done Art Here is the exchange on the subject from TopBand reflector: K3BU (...) and W8JI responses: Tom is confusing Faraday shield with Electrostatic shield and whole reasoning that the grounded shield of small loop antenna is THE antenna is all wrong. Wire loops inside the electrostatic shield are perfectly OK to receive the RF and ARE the antenna. It's a very well known property that nothing passes through the walls of a shield more than several skin depths thick. This is because skin effect keeps the current in the outside layers and the core of the shield wall is dead. This is the very thing that allows our coaxial lines to behave like three conductors, a center conductor, a inner wall, and an outer conductor. The physical behavior of a shield does not change with application. Electrostatic shield in small loop antennas reduces the interference, electrical noise locally generated (prevalent electrical fields). Not so Yuri. First an electrostatic field by definition is a non-changing field. Static is stationary or unchanging, and things that aren't changing can't make RF noise. (Here he is confused about electrostatic shield, "electrostatic field" and electrical field and just like with loading coils case, confusing the issue with behavior of ALL shields, Faraday, Electrostatic, coax, etc. applied to a wrong case. - Yuri) The field from an accidental transmitter (noise source) is just like the field from any intentional signal source like a transmitter. There is absolutely nothing that says the field has a high field impedance (electric field dominant). Even if it was a high impedance at the source, just 1/10th wave or so from the source the field would change to a low impedance. We can't filter noise by virtue of field impedance or a shield. Even if we could, the noise source in the nearfield would randomly field dominant depending on distance and source charateristics. The only thing the shield can do at radio frequencies is change the system balance. 73 Tom |
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Yuri,
It seems to me that when "W8JI" is associated with something, you assume immediately that it is wrong. If you were to read Ronold W. P. King's explanation about small loop antennas in "Transmission Lines, Antennas and Waveguides", would you be any more apt to believe it? How about Glenn S. Smith's discussion of them in Johnson and Jasik's "Antenna Engineering Handbook" (second edition)? Each of those begins with a reasonably detailed description of an "unshielded" loop and moves on to a "shielded" loop. In addition, can you expain to us how the current on the wires on the inside of the shield is NOT balanced by an equal current in the opposite direction on the inside surface of the shield? Please tell us in detail just what currents are where on the shielded loop. If you are going to try to tell us that some explanation is in error, please provide us with enough detail that we can make up our own minds. So far, all I've seen here is some vague reference to confusion about shields. The descriptions in each of the two references I gave above are far more detailed than what you have posted here, either of your own or of W8JI's, and I find them both enlightening--they are slightly different from each other--but both detailed enough that you can make up your own mind about what's really going on, and not have to read ranting generalities or statements with nothing to back them up. Cheers, Tom |
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Nice try Yuri, but you can see no one is buying into your theories.
A shield works the way a shield works. The time-varying fields, once the frequency is high enough so the shield is several skin depths thick, really isolates everything from passing through the shield. The primary coupling is via the voltage across the gap and the current flowing around that edge. The shield is the actual antenna. Not the conductor inside the shield. Noise is NOT any particular field impedance. There is nothing that says noise is electric field dominant at the radiator, and if it was just a few feet away (about 1/10th wave) it would change anyway. If you disagree with how a shield works or if you think electrical noise is a high field impedance or electric field dominant, then you should say why or how that is true. 73 Tom |
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"K7ITM" wrote in message oups.com... Yuri, It seems to me that when "W8JI" is associated with something, you assume immediately that it is wrong. That's what might seem to you, but I point out gross misinformation, when I come across it. I express my opinion based on what I know or believe. I could be wrong and I gladly get educated. Mostly, if I see, measure or touch something, I believe it to be right. Mumbo-jumbo "scientwific explanation", taking off on tangent to justify the fallacy don't cut it with me. If you were to read Ronold W. P. King's explanation about small loop antennas in "Transmission Lines, Antennas and Waveguides", would you be any more apt to believe it? How about Glenn S. Smith's discussion of them in Johnson and Jasik's "Antenna Engineering Handbook" (second edition)? Each of those begins with a reasonably detailed description of an "unshielded" loop and moves on to a "shielded" loop. I don't have the King's book, in Jasik's the treatment of small loops and shielded loops is dealing with some "medieval" designs. The closest to my version is his Fig. 5.23a showing balanced shielded loop. But then the 5-23bdoesn't make much sense to me, having small loop on the front of reflector, when the small loop has the minimum of radiation along the axis through the loop, and he places the reflector in the minimum - null direction? The way they show the loops, half of loop solid wire, half coax line, creates confusion what is antenna, what is shield, or perhaps combines them. I have not used those designs. In addition, can you expain to us how the current on the wires on the inside of the shield is NOT balanced by an equal current in the opposite direction on the inside surface of the shield? Please tell us in detail just what currents are where on the shielded loop. If you are going to try to tell us that some explanation is in error, please provide us with enough detail that we can make up our own minds. So far, all I've seen here is some vague reference to confusion about shields. The descriptions in each of the two references I gave above are far more detailed than what you have posted here, either of your own or of W8JI's, and I find them both enlightening--they are slightly different from each other--but both detailed enough that you can make up your own mind about what's really going on, and not have to read ranting generalities or statements with nothing to back them up. Cheers, Tom I will not get tangled into currents, phasors, but describe my design of small shielded loop antenna that I used on 160m and this should perhaps shed some light on the controversy. I used 1/2" copper water tubing (non ferrous material passing the magnetic field) for circular loop about 4 foot diameter. At the top the loop had gap, at the bottom it was mounted in small metallic box. Loop, box and mast were all DC connected and grounded. Mast was about 5 ft high, with Ham-m rotor at the base to rotate the contraption. This formed Electrostatic shield for the antenna. From the connection box I threaded three turns of electrical house wire #12 and across the ends connected mica trimmer capacitor C1 (abt 1200 pF?) to resonate the three wire loop antenna at 1.830 kHz). Not connected to anything else, nor ground or loop. Then I threaded one turn of the same #12 wire as a coupling turn. One end was connected to the coax braid, the other end through another mica trimmer capacitor C2 to the center conductor of the coax. Floating, not grounded or connected to other loop or tubing. I tuned the C1 to resonate the three turns at the desired frequency and C2 to provide 50 ohm match to coax. Circuit wise this mirrors the LC parallel tuned circuit with link coupling and provide better signal than other published designs. I tried version of this without copper tubing shield and with. I had local AC power line noise (within fractions of wavelength) and shielded loop attenuated the local noise. The way I see this works, the three turns were the antenna, it was tunable across the band. The "link" coupling allowed to keep the symmetry of antenna and provided some isolation for common mode currents between the antenna and coupling (well known in LC tuned circuit with link coupling.). The copper tubing was ELECTROSTATIC SHIELD which let's the EM waves pass through. If the copper tubing IS the antenna, then how does it work? Short, grounded in the center bent dipole? Then the radiation pattern should have maximum perpendicular to the plane of the loop/dipole. But the antenna has NULLS in that direction, corresponding to the properties of the 3 plus 1 wire loops. You scientwists can play games with theories how it should behave, but the reality again shows how it behaves. Anyone can build the antenna as I described and VERIFY it. Wire loops without electrostatic shield tubing still work the same way as with the shield. So which IS antenna? Another description of the subject antenna is at http://www.tpub.com/content/antennaa...-352-14_31.htm 73 Yuri Blanarovich, K3BU, VE3BMV |
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I haven't gone through this in detail yet, but one misconception is glaring:
Yuri Blanarovich wrote: . . . I used 1/2" copper water tubing (non ferrous material passing the magnetic field) for circular loop about 4 foot diameter. . . If you believe that, it's no surprise that you're having difficulty understanding how a shielded loop works. It's not hard to demonstrate that the (time-varying) magnetic field doesn't penetrate a non-ferrous shield, if you believe (correctly) that a time-varying magnetic field will produce a current on a nearby conductor. Simply put an oscillator or signal source into a copper box -- you can solder one op out of PC board material. Run some wires all around the inside which carry the oscillator signal, putting them as close to the shield wall as you like. Put a battery inside the box to power the oscillator and seal the box up. Then sniff around the outside of the box with any kind of magnetic field detector you can devise. If you have a little potted oscillator of some kind, you should be able to do this in a couple of hours at most. Or, just connect your rig to a good dummy load with some double shielded coax and sniff around the outside of the copper coax shield. If you put the detector just outside the shield, the current on the inside of the shield will be much closer to the detector than the current on the center conductor. So if the shield is transparent to a magnetic field, your detector should go wild. (Make sure the rig is very well shielded, though, so no common mode currents make their way from the rig to the outside of the shield.) Alternatively, if you'll spend some time with a good electromagnetics text learning about eddy currents and the like, you'll understand why you'd be wasting your time with those experiments. Once you're convinced that the shield blocks the magnetic as well as electric field, you'll have to revise your theory on how a shielded loop works. And you'll find that Tom's explanation is correct. Roy Lewallen, W7EL |
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Yuri Blanarovich wrote:
"K7ITM" wrote: It seems to me that when "W8JI" is associated with something, you assume immediately that it is wrong. That's what might seem to you, but I point out gross misinformation, when I come across it. What gets missed quite often in these discussions is that everyone agrees on 99 44/100 percent of the technical issues and we tend not to discuss those issues. We only discuss the 56/100 percent of the issues upon which we disagree. It is akin to the arguments between Einstein and Bohr. I suspect that no two people here on r.r.a.a are in 100% agreement on everything. -- 73, Cecil http://www.qsl.net/w5dxp |
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Cecil Moore wrote: Yuri Blanarovich wrote: "K7ITM" wrote: It seems to me that when "W8JI" is associated with something, you assume immediately that it is wrong. That's what might seem to you, but I point out gross misinformation, when I come across it. What gets missed quite often in these discussions is that everyone agrees on 99 44/100 percent of the technical issues and we tend not to discuss those issues. We only discuss the 56/100 percent of the issues upon which we disagree. Make no mistake about it, I disagree with everything Yuri has posted about the physics behind a "shielded loop". I certainly don't want to be considered to be 99% in agreement with anyone who thinks a time-varying magnetic field can pass though a highly conductive copper wall, or any wall that is several skin depths thick, just as I don't want to be 99% in agreement with anyone who thinks a loading coil "replaces" or has the phase shift or "current drop" of missing electrical degrees. The basic physical properties have to be understood before I'd be largely in agreement. If basic building blocks are wrong, our idea of how the worlds works must also be very distorted. 73 Tom |
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wrote:
Cecil Moore wrote: What gets missed quite often in these discussions is that everyone agrees on 99 44/100 percent of the technical issues and we tend not to discuss those issues. We only discuss the 56/100 percent of the issues upon which we disagree. Make no mistake about it, I disagree with everything Yuri has posted about the physics behind a "shielded loop". That's part of the 56/100 percent of the issues upon which you disagree. ... I don't want to be 99% in agreement with anyone who thinks a loading coil "replaces" or has the phase shift or "current drop" of missing electrical degrees. Just proves that no individual is right 100% of the time. 75m bugcatcher coils obey the laws of physics and thus suffer a delay in the real world. Hint: Contrary to the lumped circuit model, *everything* has a delay in the real world. The only place coils don't have a delay is in your mind where miracles and magic are possible. The loading coil causes a phase shift in accordance with the laws of physics. The stinger causes a phase shift in accordance with the laws of physics. The impedance discontinuity between the coil and stinger causes a phase shift in accordance with the laws of physics. There are NO missing degrees. When you comprehend how an electrical 1/4WL stub can be 19 degrees of 450 ohm line plus 18 degrees of 50 ohm line and be physically 0.1 WL long, then you will have comprehended reality. -- 73, Cecil http://www.qsl.net/w5dxp |
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Roy wrote, "It's not hard to demonstrate that the (time-varying)
magnetic field doesn't penetrate a non-ferrous shield, if you believe (correctly) that a time-varying magnetic field will produce a current on a nearby conductor." Yes, it's all easy to demonstrate. It's used in practice all the time: the shielding in a transmitter, the aluminum shield cans around IF and RF coils, the copper strap around a power transformer (used specifically to lower the external magnetic field around the transformer, so it won't couple into low-level audio circuits or affect colors on a color CRT). And indeed it all agrees with theory. For this one, you need little more than Faraday's Law of Magnetic Induction. It's fine with me if there are people who don't want to be bothered with theory, but if they profess that something works by means different from the theory that I understand and which agrees with the observations I make, they shouldn't expect me to believe them without putting some very serious effort into explaining why the accepted theory is wrong. I believe Yuri when he tells me his antenna works. But I'm not buying into his explanation of HOW it works. Cheers, Tom |
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"K7ITM" wrote in message
ups.com... Roy wrote, "It's not hard to demonstrate that the (time-varying) magnetic field doesn't penetrate a non-ferrous shield, if you believe (correctly) that a time-varying magnetic field will produce a current on a nearby conductor." Yes, it's all easy to demonstrate. It's used in practice all the time: the shielding in a transmitter, the aluminum shield cans around IF and RF coils, the copper strap around a power transformer (used specifically to lower the external magnetic field around the transformer, so it won't couple into low-level audio circuits or affect colors on a color CRT). And indeed it all agrees with theory. For this one, you need little more than Faraday's Law of Magnetic Induction. It's fine with me if there are people who don't want to be bothered with theory, but if they profess that something works by means different from the theory that I understand and which agrees with the observations I make, they shouldn't expect me to believe them without putting some very serious effort into explaining why the accepted theory is wrong. I believe Yuri when he tells me his antenna works. But I'm not buying into his explanation of HOW it works. Cheers, Tom I am not selling explanations how it works. I understand your and Roy's points. I am not claiming to try to formulate the infinitesimal theory of wasaaaap and I didn't try that with loading coils. Ensuing discussions helped me to better understand the mechanaism of how things work, the theory and how can I better apply them. I thank you for that. What I have problem with someone claiming shield is not a shield (Why do they bother calling it shield or shielded loop?), when I saw the shielding properties of it in the vicinity of the local interfering signals. It performs as a shield to the antenna that is wound inside. Tom categorically denies SHIELD, it IS the ANTENNA he claims. (Like there is no current drop along the loading coil! - The gospel from the all-knowing guru.) What I have problem with someone claiming the small loop antenna (three plus one turn) is not the antenna, but when I remove the shield, the "not antenna" is still THE ANTENNA. I am not arguing the mechanics or theory behind how the shield works, it may be transparency to magnetic field, it may be the voltage generated in the gap, bla, bla... Based on my experience with the said antenna, I concluded that wire loops are THE antenna, shield works as an electrostatic shield. I know that if I stick oscillator inside of 10' of 1/2" tubing, I will get hardly or no signal out. I know if I bend that tubing into a circle with gap and stick wire loop antenna inside, I can get signals out of that "shielded" antenna and can attenuate close by interfering signals. Shielding doesn't MAKE my antenna work (it works without shield too), shield enhances its rejection/shielding properties in near fields. I know there are small loops and there are small shielded loops and they work and I have proved it. Just don't tell me it is called shield because it is antenna, or that antenna inside the shield doesn't work, or shield doesn't shield from electrostatic fields, or that my antenna I described doesn't work as I described. Tom may pontificate his ideas to his worshippers, but I don't swallow that. I point out my, and who else cares, disagreement, especially when I see his "ideas" migrating into ham literature. Go ahead with your but, but, butts..... 73 Yuri Blanarovich, K3BU, VE3BMV |
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Cecil,
Now we have special case of biiiig coils being antenna here. Let the games begin! Yuri "Cecil Moore" wrote to W8JI: When you comprehend how an electrical 1/4WL stub can be 19 degrees of 450 ohm line plus 18 degrees of 50 ohm line and be physically 0.1 WL long, then you will have comprehended reality. -- 73, Cecil http://www.qsl.net/w5dxp |
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"Roy Lewallen" wrote in message ... I haven't gone through this in detail yet, but one misconception is glaring: Yuri Blanarovich wrote: . . . I used 1/2" copper water tubing (non ferrous material passing the magnetic field) for circular loop about 4 foot diameter. . . If you believe that, it's no surprise that you're having difficulty understanding how a shielded loop works. It's not hard to demonstrate that the (time-varying) magnetic field doesn't penetrate a non-ferrous shield, if you believe (correctly) that a time-varying magnetic field will produce a current on a nearby conductor. Simply put an oscillator or signal source into a copper box -- you can solder one op out of PC board material. Run some wires all around the inside which carry the oscillator signal, putting them as close to the shield wall as you like. Put a battery inside the box to power the oscillator and seal the box up. Then sniff around the outside of the box with any kind of magnetic field detector you can devise. If you have a little potted oscillator of some kind, you should be able to do this in a couple of hours at most. That is called Faraday shield and does not function as Electrostatic shield. Or, just connect your rig to a good dummy load with some double shielded coax and sniff around the outside of the copper coax shield. If you put the detector just outside the shield, the current on the inside of the shield will be much closer to the detector than the current on the center conductor. So if the shield is transparent to a magnetic field, your detector should go wild. (Make sure the rig is very well shielded, though, so no common mode currents make their way from the rig to the outside of the shield.) Alternatively, if you'll spend some time with a good electromagnetics text learning about eddy currents and the like, you'll understand why you'd be wasting your time with those experiments. I learned about shieldings, Faradyas, I use them, in equipment design, in RF and harmonics suppression, I built shielded room for university. But I also know the difference between the Farady shield and Electrostatic shield and seen them work. Maybe lumping all shields is as no good as lumping all coils ain't no good? Once you're convinced that the shield blocks the magnetic as well as electric field, you'll have to revise your theory on how a shielded loop works. And you'll find that Tom's explanation is correct. Roy Lewallen, W7EL Roy, I have magnetothermia machine which is about 200 W push-pull power generator at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30 inch in circumference. Loop wire, antenna (center conductor of coax) is fed from the plates of two tubes, shield is open at the far end and grounded at the exit from the enclosure. I get those 200 W heating my body tissue with magnetic field. Maybe it has something to do with shielding being a fraction of a wavelength distance from the radiator and the properties of the magnetic and electric components in the antenna reactive near field region? I know that this loop radiates along its circumference, not just from the gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk? You seem to associate and stick to wrongos and I am sorry you find their explanations correct, for the reality proves them wrong. 73 Yuri Blanarovich, K3BU |
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Yuri,
Just once, and I'm done with this. Someone somewhere along the line mistakenly called it a shield. They didn't understand how it works and what's important. Get over it. Look at just the "shield" with no wires inside. Isn't that exactly a single turn loop antenna? Isn't the feedpoint the gap in the loop? If you put a wire (or several wires) through the inside of that tube you used to call the shield, they just pick up the signal from the feedpoint. Consider a single wire through the tube. There is a voltage across the gap, the feedpoint of the loop. Since there is essentially no voltage drop along the wire in the center, across the distance of the gap in the tube, then the voltage across the gap must appear as transmission line voltage across the coaxial feedlines which are made up of the wire and the inner surface of the tube. If you've arranged things symmetrically, then the total gap voltage will divide equally between the two. Then it's just standard coaxial lines from there to where you connect your receiver, or where you put a tuned tank. Or if you have multiple wires through the tube, the net transmission line current divides among them. And you can resonate them with a capacitor, but that doesn't make them have antenna currents on them. If you have another way to analyze it accurately, fine. I don't care. My way works for me, and it does not disagree with the _performance_ I've seen you post about. It does disagree with the _theory_ you've suggested. As for WHY adding the "shield" helps get rid of local e-field noise (from sources less than a few wavelengths away, which at VLF might be kilometers), and why the nulls are more perfect, it's because symmetry is CRITICAL for that performance, and adding the outside tube allows you to make a more perfectly symmetrical loop than you can practically accomplish with just wires and all the tuning stuff you hang off it. If you are VERY careful to keep things symmetrical, you can also do it without the tube. But it takes amazingly little imbalance to screw things up. Dat's it in a nutshell. Cheers, Tom |
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Yuri Blanarovich wrote:
. . . Tom may pontificate his ideas to his worshippers, but I don't swallow that. I point out my, and who else cares, disagreement, especially when I see his "ideas" migrating into ham literature. . . . But Tom's explanation is correct. It's consistent with theory; alternate explanations aren't. If you're really interested in learning how a "shielded" loop works and won't accept Tom's explanation because it came from Tom, you can find a similar explanation in a number of reputable texts. I'll gladly provide references, if you ask before I leave for Dayton. Once you gain an understanding of some basic electromagnetic principles, the correctness of the explanation will be obvious. Oh, and don't worry about Tom's ideas migrating into the literature. They were already in the literature well before any of us were born. Roy Lewallen, W7EL |
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Yuri Blanarovich wrote:
. . . I learned about shieldings, Faradyas, I use them, in equipment design, in RF and harmonics suppression, I built shielded room for university. But I also know the difference between the Farady shield and Electrostatic shield and seen them work. Maybe lumping all shields is as no good as lumping all coils ain't no good? Sorry, you're not making much sense to me. You said that a non-ferrous shield is transparent to a (time-varying) magnetic field. The experiments I proposed illustrate that this is false. This has nothing to do with what name you attach to a shield. Roy, I have magnetothermia machine which is about 200 W push-pull power generator at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30 inch in circumference. Loop wire, antenna (center conductor of coax) is fed from the plates of two tubes, shield is open at the far end and grounded at the exit from the enclosure. I get those 200 W heating my body tissue with magnetic field. Hm. How do you know it's from just the magnetic field? This is really interesting. Just a couple of postings ago, you said that a non-ferrous shield is transparent to a magnetic field. Now you say that a magnetic field is heating your body. Do you have some embedded steel shrapnel or something making your body ferrous, or do you just eat lots of nails and scrap metal? Maybe it has something to do with shielding being a fraction of a wavelength distance from the radiator and the properties of the magnetic and electric components in the antenna reactive near field region? What has? The heating? That's due to the lossiness of bodily fluids in the presence of either time-varying magnetic or electric fields. I know that this loop radiates along its circumference, not just from the gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk? If you'll read what Tom has posted, or a description in any good text, you'll find that the whole circumference of a "shielded" loop radiates. The field comes from current on the outside of the "shield", not from some field penetrating the shield. That's my theory. It's the same as Tom's, and that of every respected author I've read. You seem to associate and stick to wrongos and I am sorry you find their explanations correct, for the reality proves them wrong. Reality proves Newton wrong -- any fool can see that moving objects come to rest on their own. There's no conflict between theory and reality -- just between theory and people's interpretations of what they're seeing. I'll stick with the theory that's been known and confirmed for over a century. People with alternate theories, like yours, will have to provide some extraordinary proof to sway my thinking. It seems you're more interested in proving Tom to be wrong about something -- anything! -- than taking the effort to really understand what's actually happening. So nothing else I can post will help you. I hope the lurkers have gotten something from this, though. Roy Lewallen, W7EL |
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I suspect
that no two people here on r.r.a.a are in 100% agreement on everything. Surely you jest... :) Fight! Fight! Fight! Kinda reminds me of Beevis and Butthead after eating too much chocolate.. K7ITM pretty much boiled it down to the raw minerals by noting that the usual "shielded loops" only advantage is the oft improved balance. I've already been through all this mess testing them here... And I've proven to myself that an open wire loop can be just as good as a "shielded loop" just as long as balance is taken care of. It's the balance that matters. If the two types are equally balanced, and the same size, they will act the same. The rest is just fodder for bored old farts on a newsgroup. Of course, many won't agree with me, and this would include Yuri, since he believes a shielded loop is quieter than an open loop. But I don't care. It's a free country. Or I think it is... Sometimes I wonder these days with all these goofballs we have in DC running the show. MK |
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Cecil Moore wrote:
I suspect that no two people here on r.r.a.a are in 100% agreement on everything. I disagree! heh ;^) - 73 de Mike KB3EIA - |
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Roy Lewallen wrote:
If you'll read what Tom has posted, or a description in any good text, you'll find that the whole circumference of a "shielded" loop radiates. The field comes from current on the outside of the "shield", not from some field penetrating the shield. That's my theory. It's the same as Tom's, and that of every respected author I've read. Game, Set, and Match, Roy. The explanation and the everyday application of the concept of non-ferrous shielding are both simple and elegant. Seems like the thread stopper to me! I suspect it will continue anyhow.... 8^) - 73 de Mike KB3EIA - |
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Mike Coslo wrote:
The explanation and the everyday application of the concept of non-ferrous shielding are both simple and elegant. I'm fairly ignorant when it comes to shielding. Do the magnetic fields from a magnet penetrate copper? Do the magnetic fields from 60 Hz devices penetrate the shield on coax? -- 73, Cecil http://www.qsl.net/w5dxp |
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wrote in message oups.com... And I've proven to myself that an open wire loop can be just as good as a "shielded loop" just as long as balance is taken care of. It's the balance that matters. If the two types are equally balanced, and the same size, they will act the same. 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. The rest is just fodder for bored old farts on a newsgroup. Of course, many won't agree with me, and this would include Yuri, since he believes a shielded loop is quieter than an open loop. But I don't care. 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. I emphasize, that electrostatic shield on the loop antenna is effective on close proximity radiation, within some fractions of a wavelength from the source of interference/signal. It does not (significantly) affect band noise or distant noise/signals. Anyone who can build shielded loop and test it within local arcing source or test transmitter, can see the attenuation of the said noise. So shield works as a electrostatic shield, if you guys like it or not, or refuse to admit. It is not that I believe in that, I have experienced it, seen it, measured it and it works, it is there and anoyne can verify that, contrary to "theories" of those who "figured" it can't be. Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields shunting the field to ground. Capacitor's one "plate" is the interference/signal source (antenna) - other "plate" is the el. static loop shield, grounded, shunting electrical fields to ground and preventing from entering the antenna. (Something like that). Sooo, antenna works without shield (not just my assertion), but when you insert it in the shield then shield becomes W8JI antenna. 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? It's a free country. Or I think it is... Sometimes I wonder these days with all these goofballs we have in DC running the show. MK We were better off with Clintonistas having orgies in WH while Bin Ladin turbanites were running around, blowing up Americans and using our flight schools, our planes to demonstrate their "religion of peace" in NYC WTC inferno? Let's stick to some reality in antennas. Yuri, K3BU |
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On Wed, 17 May 2006 11:30:55 -0400, "Yuri Blanarovich"
wrote: Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields There's a very simple test of this "shield." It relates to experience and doesn't need for you to go to the library. 1.) Tack a wire across the gap. Q. Do you still have signal? A. No!? None???? Extra Credit Question: Did the wire make the "shield" better, or worse? 73's Richard Clark, KB7QHC p.s. from your experience, the answer to the initial question above may vary. If in fact it does, it may bring new material for discussion. |
FIGHT? Here is another W8JI myth bone!
"Mike Coslo" wrote in message ... Roy Lewallen wrote: If you'll read what Tom has posted, or a description in any good text, you'll find that the whole circumference of a "shielded" loop radiates. The field comes from current on the outside of the "shield", not from some field penetrating the shield. That's my theory. It's the same as Tom's, and that of every respected author I've read. Game, Set, and Match, Roy. The explanation and the everyday application of the concept of non-ferrous shielding are both simple and elegant. Seems like the thread stopper to me! I suspect it will continue anyhow.... 8^) - 73 de Mike KB3EIA - Before you pronounce your verdict, why don't youze guyze build the shielded loop antenna as I described and test it. Try version without shield, see what IS antenna, and try the same antenna with shielded loop. Then run electric drill or another source of arcing or interference in the vicinity and see if there is shielding effect or not. Then pronounce your verdict and pontificate on how electrostatic shields suppose to work. Otherwise you look silly like W8JI cult worshippers. 73 Yuri, K3BU |
FIGHT? Here is another W8JI myth bone!
"Richard Clark" wrote in message ... On Wed, 17 May 2006 11:30:55 -0400, "Yuri Blanarovich" wrote: Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields There's a very simple test of this "shield." It relates to experience and doesn't need for you to go to the library. 1.) Tack a wire across the gap. Q. Do you still have signal? A. No!? None???? Agree! That makes it Faraday shield, which stops any signal from entering inside of the tubing. I never asserted that Faraday shield or closed metallic enclosure passes any signals or fields. We are talking about electrostatic shield, which if removed, antenna works without change, you put it back, it still works the same way plus it rejects in its reactive near field region electrical field interference. If it was to be antenna, then when removed, the rest should stop working as an antenna, or what is the theory? Extra Credit Question: Did the wire make the "shield" better, or worse? It turned it to Farady shield and prevented signals from exciting the antenna inside. Extra Credit Question for professor: Q1: If electrostatic shield is added to small loop antenna and it attenuates the interference or signals from its vicinity, does it perform the function of a shield or antenna? Q2: Can the piece of tubing that is grounded by its outside surface, acts as a capacitor's plate and provide the path to ground for electric field in vicinity? 73's Richard Clark, KB7QHC p.s. from your experience, the answer to the initial question above may vary. If in fact it does, it may bring new material for discussion. I just wish that points of discrepancy were addressed, rather than parties taking off on tangents fitting their convinctions and trying to weasel out of the wrong statements. 73 Yuri, K3BU |
FIGHT? Here is another W8JI myth bone!
On Wed, 17 May 2006 12:17:07 -0400, "Yuri Blanarovich"
wrote: Extra Credit Question: Did the wire make the "shield" better, or worse? It turned it to Farady shield and prevented signals from exciting the antenna inside. 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? 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? 73's Richard Clark, KB7QHC |
FIGHT? Here is another W8JI myth bone!
Yuri Blanarovich wrote: That makes it Faraday shield, which stops any signal from entering inside of the tubing. I never asserted that Faraday shield or closed metallic enclosure passes any signals or fields. We are talking about electrostatic shield, which if removed, antenna works without change, you put it back, it still works the same way plus it rejects in its reactive near field region electrical field interference. If it was to be antenna, then when removed, the rest should stop working as an antenna, or what is the theory? Yuri, A shield is a shield. People made some very good posts explaining how the "shield" works, and there was nothing wrong with my original explaination. You stated the shield is an "electrostatic" shield and I pointed out a static field does NOT cause noise. Static is by definition stationary or non-varying. The only reason the shield affects the noise, as I and others have pointed out, is the shield changes the balance of the system. The shield IS the actual portion of the antenna that receives the signal, whether that signal is noise or an intentional desired signal. The entire shield can be dispensed with without any change in the system so long as the system remains in balance, and that is quite possible to do. As a mater of fact if a non-symmetrical "shield" is added over a balanced system it will decrease balance and make the system more susceptable to noise because the feedline will become part of the actual antenna. You might look for a copy of "Fundamentals of Electricity and Magnetism" (McGraw-Hill). This entire book deals with basic field behavior and entire chapters explain in detail what everyone is saying. As Roy pointed out, I didn't make this stuff up. It has been in print since the 1800's and the electric field effects first experimented with around 600 BC (although it was the 1600's before serious experiments were done). There's nothing impossible about what you may have observed but the reasoning you gave and statements about my explaination being in error are wrong. There is absolutely nothing that causes noise to electric field dominant and the shield absolutely does not "filter" the time-varying electric field from the time-varying magnetic field. The shield IS the actual antenna and the stuff inside it, once inside it, is excited only by the gap. Nothing at frequencies of interest passes through the shield walls.This is a very well-known behavior and why so many immediately disagreed with your description. 73 Tom |
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Yuri, K3BU wrote:
"Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields." Terman on page 1049 of his 1955 edition writes: "Such a shield ensures that all parts of the loop will always have the same capacitance to ground irrespective of the loop orientation in relation to neighboring objects." Yuri is consistent with Terman, and that is liable to be better than a bible because it is provable and demands no faith. If wrong, it will be rewritten with corrections. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Yuri Blanarovich wrote:
"Mike Coslo" wrote in message ... Roy Lewallen wrote: If you'll read what Tom has posted, or a description in any good text, you'll find that the whole circumference of a "shielded" loop radiates. The field comes from current on the outside of the "shield", not from some field penetrating the shield. That's my theory. It's the same as Tom's, and that of every respected author I've read. Game, Set, and Match, Roy. The explanation and the everyday application of the concept of non-ferrous shielding are both simple and elegant. Seems like the thread stopper to me! I suspect it will continue anyhow.... 8^) - 73 de Mike KB3EIA - Before you pronounce your verdict, why don't youze guyze build the shielded loop antenna as I described and test it. Try version without shield, see what IS antenna, and try the same antenna with shielded loop. Then run electric drill or another source of arcing or interference in the vicinity and see if there is shielding effect or not. Then pronounce your verdict and pontificate on how electrostatic shields suppose to work. Otherwise you look silly like W8JI cult worshippers. Yuri, you is way too intense! I don't pontificate, and my silliness is genetic, not involved in any worship of W8JI. I very much expect that any effects that you see may be due to another cause than what you attribute it to. I don't know if your antenna is not completely shielded along it's entire circumference or not. I wonder if you could put your antenna inside a Faraday cage and see different results. Perhaps even try the unshielded antenna in the Faraday cage. Unshielded antenna in cage should equal shielded loop in open. If it doesn't, I'd look for a problem in the experiment first, not a problem in the theory. - 73 de Mike KB3EIA - |
FIGHT? Here is another W8JI myth bone!
Yuri Blanarovich wrote:
"Richard Clark" wrote in message ... On Wed, 17 May 2006 11:30:55 -0400, "Yuri Blanarovich" wrote: Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields There's a very simple test of this "shield." It relates to experience and doesn't need for you to go to the library. 1.) Tack a wire across the gap. Q. Do you still have signal? A. No!? None???? Agree! That makes it Faraday shield, which stops any signal from entering inside of the tubing. Makes it a short! I never asserted that Faraday shield or closed metallic enclosure passes any signals or fields. Aren't both conditions shields?. One just has a short. We are talking about electrostatic shield, which if removed, antenna works without change, you put it back, it still works the same way plus it rejects in its reactive near field region electrical field interference. If it was to be antenna, then when removed, the rest should stop working as an antenna, or what is the theory? Extra Credit Question: Did the wire make the "shield" better, or worse? It turned it to Farady shield and prevented signals from exciting the antenna inside. Extra Credit Question for professor: Q1: If electrostatic shield is added to small loop antenna and it attenuates the interference or signals from its vicinity, does it perform the function of a shield or antenna? Q2: Can the piece of tubing that is grounded by its outside surface, acts as a capacitor's plate and provide the path to ground for electric field in vicinity? 73's Richard Clark, KB7QHC p.s. from your experience, the answer to the initial question above may vary. If in fact it does, it may bring new material for discussion. I just wish that points of discrepancy were addressed, rather than parties taking off on tangents fitting their convinctions and trying to weasel out of the wrong statements. Give it a few more posts, Yuri, and it will turn into standing waves in coils again!! ;^) - 73 de Mike KB3EIA - |
FIGHT? Here is another W8JI myth bone!
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FIGHT? Here is another W8JI myth bone!
Tom, W8JI wrote:
"There is absolutely nothing that causes (sic) noise to electric field dominant and the shield absolutely does not "filter" the time-varying electric field from the time varying magnetic field." A "Faraday shield" is designed to allow magnetic field coupling while disallowing electric coupling. See page 38 of Terman`s 1955 edition: "It is possible to shield slectrostatic flux without simultaneously affecting the magnetic field by surrounding the free 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." I`ve previously described the Faraday picket fences or Faraday screens used in the medium wave broadcast stations where I worked that were used to avoid capacitive coupling to the antennas while permitting magnetic coupling. Capacitive coupling would favor harmonics of the operating frequency. These are undesirable. The Faraday screen effectively rejects the capacitive coupling. It shorts the lightning strikes to ground too. In a Faraday screen one end of pickets or wires is grounded. Their other ends are open-circuited. So, circulating current can`t flow through the wires. Thus, no counter-field can be generated to oppose magnetic coupling but capacitive flux lines land on the wires and are shorted to ground. It all works very well. Look at Terman`s shielded loop on page 1048 of his 1955 editiomn. There`s a gap in the shield opposite the feedpoint. The gap prevents current circulation in the loop shield thereby making it permeable to magnetic coupling while shorting the electric field to ground. Therefore, this loop cover is a Faraday screen. Why should we care if noise comes from near or far? The near field has 3 components. See "TV and Other Receiving Antennas" by Arnold B. Bailey. The first near field component is produced by the electric vector and decays by the cube of the distance. The second is the induction field and decays as the square of the distance. The third is the radiation field electric vector which becomes the volts per meter at a great distance. This decays inversely with distance and its power decays as the square of the distance. 6 dB every time the distance doubles. Point is we don`t have to get very far from a noise source to make a big improvement in noise received, especially if we avoid electric field coupling which decays especially fast in the near field. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
Richard Harrison wrote: Yuri is consistent with Terman, and that is liable to be better than a bible because it is provable and demands no faith. If wrong, it will be rewritten with corrections. If you read what has been said here very carefully you will find Yuri claims the shield "blocks electric fields" or stops "electrostatic fields". This is the effect of a Faraday cage or shield. I did not claim that effect. Terman certainly did not. It is a folklore or Ham-myth that only appears in amateur circles. What others (including Terman exactly as you quoted) are trying to tell Yuri is the shield ONLY affects balance. The shield IS the actual antenna element that does the radiating. That is written in a half dozen engineering good engineering references. That is how ANY shield behaves. Read he http://en.wikipedia.org/wiki/Faraday_cage or he http://members.aol.com/omlcgm/deteck...gy/faraday.htm or any of dozens of other places. Unless an off-the-wall hobbyist publication or rouge opinion, you will see everyone agrees. What Terman said is absolutely correct. The "shield" (when properly constructed) balances the capacitance of the antenna to earth. It does not "stop" the electric field. It certainly does not filter a time-varying electric field because doing so would by definition of Maxwell's equations (which everyone who isn't a CFA or EH antenna quack agrees are true) also stop the time varying magnetic field. As everyone (including Terman) has tried to explain, the shield only affects balance. The shield HAS to be the actual antenna element because by definition of ALL the peer-reviewed textbooks published to date as well as any description of coaxial cables the inner shield wall is isolated from the outside by the skin depth of that wall. This is so very simple to prove, it only takes a moment. It doesn't even take exotic test gear. These experiments were done in the 18th century with very crude instruments. You can take a solid copper sheet for example and place a small loop antenna near that "wall". If you probe current on the wall near the loop on the loop side, you will find a current maximum right under that loop. VERY easy to see. 100% repeatable. Now if you move the probe to the other side of the wall you will find current MINIMUM at the sheet center and increasing towards two of the edges. This is a TOTALLY open wall with no seal, it isn't even a box. Shields a few skin depths thick are a virtually perfect barrier to both magnetic and electric fields. This is true for densely woven coaxial cable shield or even thin aluminum shields, metallic sheets, or any good conductive wall. Saying Terman supports anything to the contrary only proves someone is misquoting or misunderstanding plain English, since Terman is a very clear writer. There is no way Terman failed basic physics and his peer reviewed textbooks are wrong. Yuri may need to read some basic textbooks, I'd be happy to copy the applicable pages if there isn't a good library nearby. It is essential to get the basics down solid. 73 Tom |
FIGHT? Here is another W8JI myth bone!
"Richard Harrison" wrote in message ... Yuri, K3BU wrote: "Electrostatic shields work on principle of capacitance plate, being grounded and side exposed to electrical/electrostatic fields." Terman on page 1049 of his 1955 edition writes: "Such a shield ensures that all parts of the loop will always have the same capacitance to ground irrespective of the loop orientation in relation to neighboring objects." Yuri is consistent with Terman, and that is liable to be better than a bible because it is provable and demands no faith. If wrong, it will be rewritten with corrections. Best regards, Richard Harrison, KB5WZI Richard, thanks for the reference and support, coming from one who had his hands "dirtied" with the antennas. Terman also says just a sentence befo "Errors from unbalance can be minimized by using circuit arrangements that are symmetrical with respect to ground, such as shown in Fig. 26-27b. It is also helpful to enclose the loop in an electrostatic shield, such as metal housing broken by an insulated bushing, as show schematically in Fig. 26-27c." and then sentence quoted above. Clearly, the shield is functioning as an electrostatic shield, providing symmetry and is not acting as "W8JI Antenna". Loops are the antenna, shield is the SHIELD, contrary to W8JI proselytizing. Small loops are the antennas, with or without the shield. Electrostatic shield is a shield, provides symmetry for the antenna and helps to reject, shunt the interference from the sources in the proximity of the antenna by its virtue of the capacitance to the ground. Terman didn't say: "yo stupid, you don need no stinkin' loops, jus' use the shield as antenna" :-))) 73 Yuri Blanarovich, K3BU |
FIGHT? Here is another W8JI myth bone!
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." That is a description of the shield on Terman`s direction finding loop. The loop has a gap in the shield opposite its feedpoint. The gap prevents current from circulating around the loop shield and thus prevents creation of an opposing magnetic field by the shield to the incident field acting on the loop. The grounded shield nevertheless terminates electric flux shorting it to ground. The loop shield is thus a true Faraday screen, not a Faraday car body or screened room. Best regards, Richard Harrison, KB5WZI |
FIGHT? Here is another W8JI myth bone!
"Mike Coslo" wrote in message ... Yuri Blanarovich wrote: "Mike Coslo" wrote in message ... Roy Lewallen wrote: If you'll read what Tom has posted, or a description in any good text, you'll find that the whole circumference of a "shielded" loop radiates. The field comes from current on the outside of the "shield", not from some field penetrating the shield. That's my theory. It's the same as Tom's, and that of every respected author I've read. Game, Set, and Match, Roy. The explanation and the everyday application of the concept of non-ferrous shielding are both simple and elegant. Seems like the thread stopper to me! I suspect it will continue anyhow.... 8^) - 73 de Mike KB3EIA - Before you pronounce your verdict, why don't youze guyze build the shielded loop antenna as I described and test it. Try version without shield, see what IS antenna, and try the same antenna with shielded loop. Then run electric drill or another source of arcing or interference in the vicinity and see if there is shielding effect or not. Then pronounce your verdict and pontificate on how electrostatic shields suppose to work. Otherwise you look silly like W8JI cult worshippers. Yuri, you is way too intense! I don't pontificate, and my silliness is genetic, not involved in any worship of W8JI. Sorry! I didn't mean you specifically, jus' generally those who worship W8JI gospels. I very much expect that any effects that you see may be due to another cause than what you attribute it to. I don't know if your antenna is not completely shielded along it's entire circumference or not. I wonder if you could put your antenna inside a Faraday cage and see different results. Perhaps even try the unshielded antenna in the Faraday cage. Of course it will not work, Faraday cage - shield, shields all RF. Unshielded antenna in cage should equal shielded loop in open. If it doesn't, I'd look for a problem in the experiment first, not a problem in the theory. It is electrostatic shield, not "shielded, closed" loop shield. Antenna will still work the same inside the cage, just will not receive any signals if they are not passed through the cage. I am not overthrowing legitimate theories, I am describing what I observed and objecting to call the shield an antenna, when it isn't!!! - 73 de Mike KB3EIA - I have no problem with theories, I have problem with silly claims that shield is an antenna. I described my experiments, explained behavior and performance of the shielded loop in the near field interfering signals/noise. Build it, if you have problem with local noise, you would see the benefit of the electrostatic shield on the suppression of it and on symmetry and deep nulls on other signals. Shield is a shield and not antenna. Rest of mumbo-jumbo is twist away from the subject and attempt to legitimize wrongoooo! 73 Yuri Blanarovich, K3BU |
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