NEC-LIST:Current-sensing loops on AM broadcast towers

Background/assumptions:

In the multi-tower array antenna of a typical MF AM broadcast
station, one sees what appear to be current (or circumferential
B-field) sensing loops on the towers at a certain height. I assume
that signals are carried from these loops by transmission lines to a
convenient common location where they are phase- and
amplitude-compared to determine the relative magnitudes and phases of
the currents in the towers, to achieve the desired directional
pattern of radiation from the array; and that couplers at the bases
of the towers are adjusted toward this end.

As, e.g., a NEC simulation will show, the complex amplitude of the
current varies with height differently in each tower (except in
trivial symmetrical cases) -- which is why it may be insufficient or
at least less straightforward to measure the currents just at the
bases of the towers.

Questions:

1. What rule(s) govern(s) the heights of these loops?

2. In the FCC licensing data published on the web, do the
tower-excitation amplitude and phase data refer to the positions of
these loops?

-Chuck

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