Art, KB9MZ wrote:
"Do any commercials use a rotating coil to vary the coupling?"

Lots of ingenuity and variety have been used, and I`ve not seen it all.
But, I haven`t seen a swinging link out at the tower where the Faraday
shield is used in medium wave broadcast stations.

MW BC stations don`t ordinarily change frequencies or antennas except
for some day/night changes done by remote switching. For initial power
division among several towers, there are power and phasing networks back
at the station house. There is a "dog house" at each tower to house
matching and coupling networks. Coupling includes the Faraday shield
between primary and secondary coils.

The shield hides one coil from the other for electrostatic lines of
force. The shield is often a grounded metal plate. It is ineffective in
blocking magnetic coupling because it is only grounded on one end, and
has parallel slots perpendicular to the grounded end. These slots
prevent circulating current in the plate. Circulating current in the
plate would produce a counter-EMF which would neutralize the magnetic
field of the primary coil and its coupled energy in the secondary. The
slots make the shield plate pemeable to the magnetic field, but not
permeable to the electrostatic field.

Sometimes, individuual wires are used as a Faraday shield in place of a
slotted plate. The wires may work better at reducing circulating
current, but the slotted plate obviously works well enough for many
transmitting stations.

Best regards, Richard Harrison, KB5WZI