Spin wrote:
So in receiving the AM BCB frequencies in the united states, a vertical
antenna is 'generally' preferable over a horizontal antenna? The above is
based on having a receiver without any internal antenna.

Not only the USA but world wide -- the laws of physics are the same
everywhere. During the day, propagation at BCB frequencies is by surface
wave. Only vertically polarized waves propagate this way, so all AM
broadcast stations use vertical antennas to produce vertically polarized
waves. These are best received with a vertically polarized antenna.

At night, AM broadcast signals can be propagated relatively long
distances by being reflected or refracted by the ionosphere. When that
happens, the polarization is rotated in a seemingly random fashion, so
horizontally polarized receiving antennas aren't at such a disadvantage.

Roy Lewallen, W7EL