Dave Pitzer wrote:
"WABC (New York City) comes in poorly at night even though its xmtter is
only 80 miles away."

80 miles is within groundwave range of WABC. 770 KHz should reach that
far in the daylight hours as well as at night. At night, the ionosphere
reconfigures allowing a reflected wave from the station to reach your
receiver. While the groundwave is subject to ittle if any variation, the
skywave is subject to many variations. Thats just the beginning of the
problem. All U.S. stations share their assignments with other stations
operating on the same frequencies. If the desired station is not
interfered between its own groundwave and skywave (you are out of
groundwave range) it is still likely to be interfered by other stations
sharing its frequency or even interfered by adjacent channel stations.
When these undesired stations fade in, they can suppress reception of
the desired station by taking over the receiver`s automatic gain control
circuit.

To accomodate multiple assignments on a single carrier frequency, FCC
often requires new assignees to include a null toward one or more
aditional existing stations, especially at night, in the new station`s
radiation pattern. If the reduction in potential interference is deemed
insufficient, the new station may be required to reduce power as well as
switch directional patterns during nighttime hours. The new station may
be the oldest station on the frequency if it has decided to relocate or
make some other modification to its license terms.

So, WABC may have directional coverage of its area which it switches at
nightfall and daybreak. I have an FCC book showing all the assignments
but it is very out of date.

Best regards, Richard Harrison, KB5WZI