May 25th 06, 05:01 PM
posted to rec.radio.amateur.antenna
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FIGHT? Here is another W8JI myth bone!
On Thu, 25 May 2006 08:40:26 -0700, Roy Lewallen
wrote:
Richard Clark wrote:
On 25 May 2006 03:38:14 -0700, wrote:
There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.
Will you give me an example where the electric field is zero and all
coupling is via magnetic flux?
Tom,
As this has already been discussed not but two postings ago, the
posting your responded to, why are you asking for that content again?
I was going to ask the same question but Tom beat me to it. And I must
have missed the example, too. Would you be so kind as to repost it?
Roy Lewallen, W7EL
On Wed, 24 May 2006 12:11:52 -0700, Roy Lewallen
wrote:
Richard Clark wrote:
. . .
Richard's applications and illustrations do not push this boundary. In
fact, Ramo et. al distinctly offer the case of "electrostatic
shielding" and clearly support the separation of magnetic and electric
flux (fields). . .
Can you direct me to where in the text they do so? All I've found is a
short section (5.28) on "Electrostatic Shielding" where they explain
that introducing a grounded conductor near two others will reduce the
capacitive coupling between them. Obviously this will alter the local
E/H ratio, but in no way does it allow an E or H field to exist
independently, even locally, let alone at any distance.
Hi Roy,
Article 5.12 "Circuit Concepts at High Frequencies or Large
Dimensions"
Figure 5.28(a) shows a complete shielding. Of course this is entirely
electric, and arguably magnetic. However, magnetic flux can penetrate
thin shields, electric flux cannot.
This is part and parcel to the world of isolated and shielded
circuits. The electrostatic shields are as effective as they are
complete in their coverage. Their contribution is measured in mutual
capacitance between the two points being isolated. With a drain wire
to ground, and a low enough Z in that wire, then that mutual
capacitance tends towards zero (however, near zero is a matter of
degree as I've offered in past discussion).
Figure 5.28(a) shielding is quite common in medical circuit design,
and mutual capacitance does equal zero; and yet signals and power pass
in and out through magnetic coupling. Isolated relays are a very
compelling example of magnetic transparency in the face of total
electric shielding.
Magnetic shielding operates through reflection or dissipation
(absorption loss due to eddy currents). This loss is a function of
permeability µ. Unfortunately, permeability declines with increasing
frequency, and with declining field strength. Basically, all metals
exhibit the same characteristic µ above VLF; hence any appeal to
"magnetic materials" used to build antennas is specious.
This is not to say the magnetic shield is ineffective, merely derated
seriously from what might be gleaned through poor inference by reading
µ values from tables.
However, it is quite obvious that transformer inter stage shielding
and the faraday shield found in AM transmitters is not seeking to
optimize this attenuation, far from it. Thus the degree in isolation
is found in the ratio of the mutual capacitance between the two points
before and after shielding; and the attenuation in magnetic flux
induction introduced between the two circuits after shielding.
Returning to Ramo, et. al, the introduction of a partial shield.
Figure 5.28(c) is effective insofar as its ability to reduce mutual
capacitance.
73's
Richard Clark, KB7QHC
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