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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 |
FIGHT? Here is another W8JI myth bone!
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 |
FIGHT? Here is another W8JI myth bone!
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 |
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