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

Richard Harrison wrote:
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.

Me neither! The main lesson is that we have to be careful to define what
we mean, because there's a strong risk that other people might
understand something different.

Faraday cages are used at CERN and other large particle accelerators, to
keep the sensitive particle detectors isolated from the pulsed megawatts
of RF energy that are kicking the particles around the ring. CERN is an
international facility, so each country has its own experiments using
separate Faraday cages.

Several years ago, I needed to call a friend who was working at CERN.
Someone picked up the phone, and a voice said "British Cage".

"Well," I thought, "that certainly puts us in our place..."


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek