Sorry, this does not contain an example of a (time-varying) electric or
magnetic field in the absence of the other. Such a condition is, in
fact, impossible.

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.

We've been talking about *time-varying* fields, and must have forgotten
to explicitly state that qualification. The figure in question deals
with static fields. Time-varying electric flux can indeed penetrate thin
shields of finite conductivity, although the E/H ratio within the shield
is very small. If a shield could block time-varying electric fields, the
time-varying magnetic field which remained would create an electric
field. A time-varying magnetic fields creates a time-varying electric
field and vice-versa; this is dictated by Maxwell's equations. The
answer to question 4.06d in Ramo, et al, "Can a time-varying field of
any form exist in space without a corresponding electric field? Can a
time-varying electric field exist without the corresponding magnetic
field?" is no.

A gapless shield made of a perfect conductor of any thickness will
completely block both electric and magnetic fields.


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.

The mutual capacitance at DC equals zero. Time-varying electric fields
penetrate the shield if it's thin in terms of skin depth.

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 true. Metals do indeed exhibit varying permeabilities at RF
and above. This can be illustrated by a number of means, a common one
being the efficacy of a powdered iron core.

Electric field shielding also operates through reflection and
dissipation. Permeability affects both, because of its effect on
material wave impedance and skin depth.

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.

Permeability does indeed change with frequency for a variety of reasons.
Consequently, some intelligence (and often measurement or guesswork) has
to be used to determine what it will be at the frequency in question.

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.

Indeed it is. This is not, however, an example of a (time-varying)
magnetic or electric field existing in isolation.

I readily agree that a static electric or magnetic field can exist in
isolation from the other, as I'm sure all other participants to this
discussion do. But not time-varying ones. You can greatly change the E/H
ratio, but you can't make it zero or infinite. And whatever you do will
have only a local effect -- the ratio will rapidly approach the
intrinsic Z of the medium as you move away from the anomaly which
modified the ratio. Rapidly, that is, in terms of wavelength -- it can
be quite a physical distance at very low frequencies.

Roy Lewallen, W7EL