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The term Faraday screen, sield, or cage, has been applied to perforated
continuous sheets of metal in this thread. That`s generally wrong. A Faraday screen, shield, or cage is a network of parallel wires or strips connected together at one end but disconnected from each other at their opposite ends. I`ve worked at several broadcast stations which used Faraday screens at every tower to magnetically couple the tower to the feedline while eliminating all capacitive coupling. The purpose is to disadvantage harmonic coupling to the tower in which capacitance favors due to lowered capacitive reactance at the harmonic frequencies. At the stations, two coupled coils are used. They are close together and share the same axis. Between the two coil forms is erected a heavy plate sliced with parallel cuts. These start at one edge of the plate but end before reaching quite to the other edge. The purpose is to eliminate capacitive coupling between the coils but to allow tight magnetic coupling between the coils. In the broadcast station they also have another salutary effect. The tower`s lightning strikes nearly all are terminated on the Faraday screen and kept out of the radio equipment. A Faraday screen, shield, or cage is a network of parallel wires or strips connected on one end but disconnected from each other at their opposite ends. It`s similar to a conductive comb. The connected ends of the wires are usually grounded. The open-circuit wires prohibit circulating current from wire to wire. Fields of the induced current would cancel the field of the inducing current thus canceling inductive coupling. Due to the gaps, the screen is transparent to the magnetic field but the wires capture the electrostatic lines of force and eliminate capacitive coupling through the screen. A screen properly grounded at both ends of the wires sliminates magnstic and electrostatic coupling. It is a shield but not a Faraday shield. A continuous conducting shield is not a Faraday shield, even if perforated with small holes. A lot of screened rooms have been constructed from copper window screen. It decouples the contentents of the room from the whole world when done correctly. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
A Faraday screen, shield, or cage is a network of parallel wires or strips connected together at one end but disconnected from each other at their opposite ends. I`ve worked at several broadcast stations which used Faraday screens The special comb-like structure that Richard describes, which is deliberately constructed to block electric fields but transmit magnetic fields, is normally called a Faraday "screen" - but not a cage. The term Faraday "cage" is reserved for a complete conducting enclosure that blocks both electric and magnetic fields from entering the interior. The rest of the discussion is about how well an incomplete or penetrated enclosure might work as a Faraday cage. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Ian White, G3SEK wrote:
"---but not a cage." A cage according to my American dictionary is: "A boxlike receptacle or enclosure for confining birds or other animals, made with openwork of wires, bars, etc." Ian sent me to my dictionary of electronics which reads: "Faraday cage-See Faraday Shield" Usage varies from place to place. I don`t know if I`m vindicated or stand corrected. Best regards, Richard Harrison, KB5WZI |
In message , Richard
Harrison writes Ian White, G3SEK wrote: "---but not a cage." A cage according to my American dictionary is: "A boxlike receptacle or enclosure for confining birds or other animals, made with openwork of wires, bars, etc." Ian sent me to my dictionary of electronics which reads: "Faraday cage-See Faraday Shield" Usage varies from place to place. I don`t know if I`m vindicated or stand corrected. Best regards, Richard Harrison, KB5WZI Was it not an Ice Bucket which Faraday used to demonstrate the fact that the electrostatic charges repelled each other as far as possible, and therefore stayed on the outside of the bucket? The inside was electrically dead. Ian. -- |
On Tue, 28 Dec 2004 21:46:06 +0000, Ian Jackson
wrote: Was it not an Ice Bucket which Faraday used to demonstrate the fact that the electrostatic charges repelled each other as far as possible, and therefore stayed on the outside of the bucket? The inside was electrically dead. Hi Ian, Maybe it was part of an office party. Anyway, Gauss demonstrated that electric charge FIELD LINES prefer as much separation as possible (which conforms to your charges being repelled). With a curvature, the field line normal to the surface will either cause line crowding or line spreading depending upon the geometry. With a positive curvature (the outside of a conducting shell) the lines spread; with a negative curvature (the inside of a conducting shell) the lines converge. Given that the bucket is conductive inside and out, he demonstrated that line proximity within the bucket drove the charges outside. This is not quite an issue of charges being repelled as far as possible, or they would be uniformly distributed inside and out. By the same logic (and experience), charge will accumulate on the surface at the smallest radius - hence the points on lightning rods. By extension, this is also the source of capacitor failure at either the edges (smallest radius of a plate) or in surface burrs. HCJB, in Quito, suffered from corona discharge and converted to loops (misnomer, actually box), they still suffered when the corners (smallest radius) supported the same discharge (being corner fed). They shifted to a center feed point and put the hi voltage nodes at the middle of a wire span. 73's Richard Clark, KB7QHC |
In message , Richard Clark
writes By the same logic (and experience), charge will accumulate on the surface at the smallest radius - hence the points on lightning rods. By extension, this is also the source of capacitor failure at either the edges (smallest radius of a plate) or in surface burrs. Again scraping the very bottom of the memory banks, I seem to recall that when lightning rods were first used (in the late 1700s), the British used sharp points. The French, in the spirit of one-upmanship, decided that theirs should have brass balls. DOH!!! Ian. -- |
"Ian Jackson" wrote , Richard Clark writes By the same logic (and experience), charge will accumulate on the surface at the smallest radius - hence the points on lightning rods. By extension, this is also the source of capacitor failure at either the edges (smallest radius of a plate) or in surface burrs. Again scraping the very bottom of the memory banks, I seem to recall that when lightning rods were first used (in the late 1700s), the British used sharp points. The French, in the spirit of one-upmanship, decided that theirs should have brass balls. DOH!!! Ian. -- Very interesting! However the American Benjamin Franklin's pointed lightning rods (it was not a British design) was never scientifically challenged until a couple of years ago. Scientists have now shown that blunt-tipped air terminals are attached by lightning with significantly higher frequency than sharp tipped rods are. Pretty amazing that it took over 230 years to "discover" this! So scrap the concept that a sharp edge attracts charges, at least it does not attract lighting, the ultimate charge. http://www.usatoday.com/weather/reso...-rod-tests.htm http://www.esdjournal.com/articles/f...n/franklin.htm http://www.mikeholt.com/news/archive...tningblunt.htm etc, etc Jack Painter Virginia Beach VA |
On Tue, 28 Dec 2004 18:56:13 -0500, "Jack Painter"
wrote: "Ian Jackson" wrote , Richard Clark writes By the same logic (and experience), charge will accumulate on the surface at the smallest radius - hence the points on lightning rods. By extension, this is also the source of capacitor failure at either the edges (smallest radius of a plate) or in surface burrs. Again scraping the very bottom of the memory banks, I seem to recall that when lightning rods were first used (in the late 1700s), the British used sharp points. The French, in the spirit of one-upmanship, decided that theirs should have brass balls. DOH!!! Ian. -- Very interesting! However the American Benjamin Franklin's pointed lightning rods (it was not a British design) was never scientifically challenged until a couple of years ago. Scientists have now shown that blunt-tipped air terminals are attached by lightning with significantly higher frequency than sharp tipped rods are. Pretty amazing that it took over 230 years to "discover" this! So scrap the concept that a sharp edge attracts charges, at least it does not attract lighting, the ultimate charge. http://www.usatoday.com/weather/reso...-rod-tests.htm http://www.esdjournal.com/articles/f...n/franklin.htm http://www.mikeholt.com/news/archive...tningblunt.htm etc, etc Jack Painter Virginia Beach VA Jack, All three references are of the same article. Note the rebuttals at the end of one of them. I would also find it hard to believe that ANY rods on a 12000 foot mountain were not hit in 7 years! That study would suggest that pointed rods were excellent lightning repellers and would protect things from being struck. Exactly what Franklin first thought. If not excellent repellers then it would be highly suspect of the placement of the pointed rods on the mountain. 73 Gary K4FMX |
Gary, K4FMX wrote:
"I would find it hard to believe that ANY rods on a 12,000 foot mountain were not hit in seven years." I saw a PBS program tonight on people scaling the highest peak in Antarctica. It may never have been struck by lightning in modern times. I spent two six-month hitches for my company on Tierra del Fuego. Not quite Antarctica, but still so cold that lightnning is unknown on the island. Best regards, Richard Harrison, KB5WZI |
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