Does anyone know where I can obtain a plot of the antenna pattern for
a major (50,000 watt) commercial broadcast station?

www.radiolocator.com (where you'll find it, and any other AM station in
the US and
Canada)
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The contours shown on Radiolocator.com are based BOTH on the radiation
pattern of the transmit antenna system, and ground conductivity along
various radials for 360° around the transmit site.

If conductivity was a constant throughout the groundwave coverage area of
the station (almost never happens), then from the "patterns" on
Radiolocator - to the extent they are accurate - you could calculate the
actual shape of the transmit antenna radiation pattern, remembering that
field strength changes as the square root of changes in radiated power.

RF

Visit http://rfry.org for FM broadcast RF system papers.