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#51
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Richard
I think you are confusing low frequency aplications with high frequency aplications. A flat sheet allows formation of ground loops that then form their own emissions. This is not desirable in high frequency aplications and thus a common short circuit to ground for discharge is required. Best regards Art ichard Clark wrote in message . .. On 19 Dec 2003 19:51:26 -0800, (Art Unwin KB9MZ) wrote: Can't see how a Faraday shield can be used if they are link coupled Hi Art, Nearly every power transformer on this planet uses a faraday shield between link coupled circuits. Those that don't (and they got to be dirt cheap from a garage shop operation) suffer from it too (as does the user). This is an old, old topic that Richard Harrison, KB5WZI, has described, explained, discussed to considerable bandwidth that should serve as a basis for your research in Google. 73's Richard Clark, KB7QHC |
#52
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#53
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#54
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Richard you are a smart guy with respect to radio matters so you must
be misinterpreting things that are being said on this thread That commercial radio stations use tines is evidence to me that they serve a purpose i.e. electrostatic shielding, not everything, just electrostatic stuff ( ignoring harmonics e.t.c. ) As for low frequency use, that would include audio, not just 50/60 which you refer to 'power . Now you say look at Terman and I have, not only the section on transformers but on coupling circuitry...way back to the days of spark gaps and ticklers, each of which are treated differently by not only Terman but also by others. You will also note that audio cables have sheet like covering just like books do with TOTAL shielding. Pretty much all books on fields and waves have a section of total shielding of nearby circuits with special reference to TOTAL enclosure and the effects on circuits or inductances that are so enclosed. With all that being said and getting back to the initial area of discussion where Richard alluded to the picket fence. Why do you think that the broadcast industry hung on to the ungainingly picket fence after all these years and why would it not descriminate with regard to other influences ? Best regards Art Richard Clark wrote in message . .. On 22 Dec 2003 21:06:44 -0800, (Art Unwin KB9MZ) wrote: Richard I think you are confusing low frequency aplications with high frequency aplications. A flat sheet allows formation of ground loops that then form their own emissions. This is not desirable in high frequency aplications and thus a common short circuit to ground for discharge is required. Best regards Art Hi Art, I am confusing nothing. A current cannot be induced without a path, if you do not provide any more than one terminus, there is no path EXCEPT TO GROUND and it is a common shared by both halves of the magnetic linked circuits (read your Terman). This means the circuits on both sides of the shield are electrically decoupled from each other and are driving ground directly - capacitively (the electric field). The sole purpose of the Power Transformer's Faraday Shield is EXACTLY for snubbing VLF/MF/HF/VHF/UHF coupling (there's no purpose to killing 50/60 Hz fields with a Faraday Shield, that just doesn't make sense in the first place). The guff you are repeating is the ill-understood concepts of UHF/SHF circulating currents that could only be developed if the wavelength scale can support it. Are you talking about 900 MHz applications with 3 inch Faraday screens? I think not. There's no mystery about this, dimension and frequency drive all such issues. 73's Richard Clark, KB7QHC |
#56
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Art, KB9MZ wrote:
"That commercial stations use tines is evidence to me that they serve a purpose, i.e. electrostatic shielding, not everything, just electrostatic stuff." That is exactly correct. The magnetically coupled coils have a rather broadband response, but the coupling coefficient is less than 1. If you took the advice of others in this thread and checked Cebik`s pages, he advises that incomplete coupling (k1) does not insure inefficiency. High efficiency is available with loose coupling though throughput may be constricted. Reduced magnetic coupling may make additional coupling from capacitance bypassing the transformer throughput more significant. Impedance of the gratitous capacitance, bypassing the coupling transformer, at the 2nd harmonic frequency is only 1/2 the impedance of that at the fundamental frequency. Likewise, at the 3rd harmonic frequency, impedance of the capacitive reactance is only 1/3 that at the fundamental frequency. The stray coupling capacitance amounts to a high-frequency boost circuit which is thwarted by the Faraday screen. Faraday screening to eliminate priority harmonic coupling to the antenna is an important advantage in harmonic suppression. I think the Faraday screen is an expected feature of coupling to the antenna tower in medium wave broadcasting. My copy of Terman isn`t at hand now, but I recall that he treats skin effect, iron-core transformer Faraday screens, and air-core transformer Faraday screens. Richard Clark is right. The implementations are slightly different depending if it`s an iron-core low frequency transformer or an air-core high-frequency transformer. The aim is the same, eliminating capacitive coupling between the transformer coils. At short wave frequencies there are other practical ways to rid antennas of harmonic content. I recall King, Mimno, and Wing describing tuned transmission line traps to expel harmonics. Kraus says antenna useful bandwidth is generally a matter of both pattern and impedance. In a thin dipole, Kraus says the pattern usually changes slowly with frequency, so likely it`s impedance variation which limits the useful bandwidth. In a fat antenna, or in a conical, horn, lens, rhombic, and some others, the impedance is so well behaved that the pattern variation may limit the useful bandwidth. Kraus` 1950 2nd edition of "Antennas" includes several transmission line tricks for matching short wave antennas over a wider bandwidth. See item 14-24 (Matching Arrangements) on page 434. Best regards, Richard Harrison, KB5WZI |
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