>From: "Tom Rauch" <w8ji@contesting.com>
>...It is a very well known physical property of a "shield" more
>than several skin depths thick that essentially nothing goes
>through that shield. It's a Faraday cage, and when the
>time-varying electric field goes to zero so does the
>magnetic field.
Tom:
One question that some readers of this forum might ask: If that is the
case, how does an inductively coupled network, where two resonant inductors
are separated by a Faraday shield, transfer energy?
Recall the old B&W HDVL series plug-in coils with swinging-link coupling.
Originally, link coupling was accomplished with a small pick-up link
(usually no more than 4 turns) mounted in close proximity to the resonant rf
tank coil in the amplifier. The mutual coupling was varied by adjusting the
physical distance between the two coils. When harmonic TVI became a
problem, the link was often shielded from the main tank coil with a Farady
cage. This usually consisted of a metal tube that completely surrounded the
turns of the link coil. To prevent the Faraday shield from acting as a
shorted turn, a small gap was cut in the shield, in a manner identical to
the gap used with the shielded loop antenna. The purpose of the Faraday
shield was to prevent capacity coupling between the two coils from
transferring harmonic energy from the transmitter's tank coil to the antenna
feedline. But if "essentially nothing goes through that shield," how is the
fundamental-frequency energy from the tank coil transferred to the pickup
link?
My explanation is that in the case of the link-coupled tank circuit, we are
talking about what is essentially a transformer, where the primary and
secondary coils are magnetically coupled, and the shield prevents capacity
coupling. But with the shielded loop antenna we are considering pickup of
signals from an electromagnetic wave that is propagating through space, not
merely a rapidly varying electrostatic field or magnetic field. The
shielded loop does not resolve the radio wave into distinct electrical and
magnetic components; instead, the varying electrical and magnetic fields are
inherent by-products of the electromagnetic wave as it propagates.
Therefore, if the Faraday cage blocks the electrostatic field of a radio
wave, the accompanying magnetic component is also blocked.
My take is that this also explains why the alleged operating principle of
the so-called "E-H" and "cross-field" antennas is bogus.
However, to me this explanation is less clear in the case of the common-mode
vs differential currents on a piece of coax. Since the common-mode current
on the outside of the coax shield of a transmitting antenna is there due to
excitation of the metal conductor by a generator of a high-frequency
alternating current, and not the propagation of a radio wave through space,
why would the rf on the outside of the coax shield behave more like a radio
wave, than like a varying magnetic (or electrostatic) field. I'd be
interested in your comments.
73,
Don k4kyv
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