"Richard Harrison" wrote
Nearly every medium wave broadcast station uses a
Faraday screen at every tower between the primary and
secondary of an air-core coupling transformer... Because,
without the screen, capacitive coupling to the tower would
favor harmonics of the broadcast frequency over its
fundamental frequency and make compliance with
FCC rules difficult.
____________

Must politely disagree with that last part -- and probably the first part as
well, at least for modern ACU (Antenna Coupling Unit) designs used in MW
broadcasting.

As a condition of the FCC or other qualification needed legally to offer the
tx for sale, MW broadcast transmitters must meet their harmonic suppression
specs by themselves. They are not permitted to use ACU Faraday screens or
other external means in doing so.

A MW ACU is optimised for maximum power transfer between the transmission
line and the tower for the carrier and its sidebands, however it almost
never uses coupled coils to do that. It uses T, L or Pi networks.

But with or without a Faraday screen, an ACU can couple no more harmonic
energy to the radiator than exists, with respect to the carrier, at the
input of the ACU -- which already meets harmonic suppression specs.

RF

Richard Fry wrote:
"As a condition of the FCC or other qualification needed to legally
offer the tx for sale, MW broadcast transmitters must meet their
harmonic suppression specs by themselves."

It`s true. They are type accepted and don`t produce excess harmonics at
their outputs. The coupling system does not suppress the fundamental but
may further suppress the harmonics.

I`m sorry to misspeak. Never the less, a capacitor`s impedance is
inversely proportional to frequency. Its elimination as a coupling to
the antenna eliminates a preference for higher frequencies in the
coupling system. A pi or T network with shunt capacitance and series
inductance favors the fundamental frequency over its harmonics. These
aren`t required to meet specs but they further reduce harmonic radiation
from the radio station.

The Faraday screen is common in radio stations. It was put there not to
affect the antenna match but only to eliminate capacitive coupling to
the antenna. It also serves as a path to earth for many antenna
lightning strikes as evidenced by numerous pits and metal hrom them
splashed about the enclosure.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
The Faraday screen is common in radio stations. It was put there not to
affect the antenna match but only to eliminate capacitive coupling to
the antenna. It also serves as a path to earth for many antenna
lightning strikes as evidenced by numerous pits and metal hrom them
splashed about the enclosure.
______________

This must be a very dated observation. In my experience, and as confirmed
to me this morning by colleagues and consultants who design such networks,
MW broadcast Antenna Coupling Units commonly do not contain Faraday screens.

Lightning protection for most MW broadcast antenna systems is provided by
some combination of a small inductance in series with the tower feed (copper
tubing with a helical loop or two in it to connect the ACU output to the
tower feedpoint), an arc gap across the tower base to ground adjusted to
flash over at the lowest practical peak voltage, and a static drain choke
(path with high Z for RF, but low DC resistance to ground).

RF

Richard Hry wrote:
"---MW broadcast Antenna Coupling Units commonly do not contain Faraday
screens."

My observation is from a previous century. Faraday screens were used in
stations in which I worked.

My response was to the question: Can the E and H fields be separated?
Yes they can, and the Faraday screen is a way to eliminate coupling the
E field while coupling the H field. On the decoupled side of the shield,
induced magnetic lines immediately produce electric potential
differences and E fields. Little changes and nothing has been lost
except for capacitive coupling between circuits on opposite sides of the
Faraday screen. Breaks in the screen prevent circulating current which
would generate opposition to magnetic coupling. This is demonstrated as
effective since ancient times in electricity. Electric lines find a
ground return in the Faraday screen and go no further.

A shield without breaks allows current circulation which generates an
opposing force (Lenz`s law) and nullifies the induction. The continuous
conducting screen also provides a grounded termination for the electric
lines and blocks their passage too.

Capacitive coupling through a hole in a continuous shield can allow the
E field to be coupled while eliminating magnetic coupling.

This does not say there is any merit to the E/H Antenna, about wehich I
am ignorant.

Best regards, Richard Harrison, KB5WZI