"Dave Pitzer" wrote
... Is there any place I can find polar graphs of commercial
broadcast station's antenna patterns?

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Dave,

Here http://www.radio-locator.com/ is a link to a website with calculated
coverage areas/contours for US AM broadcast stations. The contours are
based on their licensed radiation patterns (directional or not), AND ground
conductivities for the geographic regions concerned.

The polar radiation patterns of these stations most probably don't look much
like these plots, because of the heavy influence that ground conductivity
has on received field strength along the various azimuth bearings.

Even the real coverage contours of AM broadcast stations using omni antennas
usually are anything but omni, due to the effects of varying ground
conductivities around their various azimuth sectors and ranges.

Have fun.

RF