As a matter of fact AM BC stations, despite their large ground
systems, abandoned the 5/8th wave many years ago. They
found in the real world use of 5/8th waves instead of extending
coverage they reduced coverage.
____________

The h-plane inverse field of a 5/8-wave AM BC vertical working against 120
buried radials each at least 1/4-wave long is calculated in theory and
measured in practice as having the greatest possible field per unit of
radiated power of any non-sectionalized radiator height, no matter what the
earth conductivity at the antenna site.

But the 5/8 wave BC vertical does have a discrete, high angle sidelobe that,
at night, can interfere with its own groundwave over an annular zone
starting a few hundred miles from the antenna. Very distant coverage is
provided by low-angle skywave (less than about 30 degrees), and is not
affected because the groundwave is gone at those distance ranges.

But this is the reason that 24-hr, 50 kW AM BC stations use a radiator
typically around 195 degrees. Its h-plane inverse field (the groundwave) is
not quite as great as from a 5/8-wave, but it doesn't develop that
high-angle lobe. It is popularly called an "antifade" radiator.

RF (WJR staff engineer, 1960s)