Could be mistaken, but I recall hearing that 1020 kHz is the last 'true' clear channel per the FCC and KDKA is the only QRO signal authorized after dark in that allocation..........
Unfortunately, the days of truly clear channels
are long gone, even 1020 (KDKA's frequency).
I just spent a few minutes looking up stations that are now licensed to operate at night on 1020 kHz and this is what I found:
KOAN 1020 kHz Night DA EAGLE RIVER, AK 10.0 kW
KTNQ 1020 kHz Night DA LOS ANGELES, CA 50.0 kW
WURN 1020 kHz Night DA KENDALL, FL 0.98 kW
KJJK 1020 kHz Night DA FERGUS FALLS, MN 0.37 kW
KMMQ 1020 kHz Night DA PLATTSMOUTH, NE 1.4 kW
KCKN 1020 kHz Night DA ROSWELL, NM 50.0 kW
KOKP 1020 kHz Night DA PERRY, OK 0.25 kW
KWIQ 1020 kHz Night DA MOSES LAKE NORTH, WA 0.44 kW
The DA means Directional Antenna and their authorized powers are listed at the end of each line. There are also stations in Canada and Mexico authorized to operate at night on 1020, which are not listed.
I can still remember when there were about two dozen truly clear channels at night (early 60's). Living on the West Coast at the time, it was easy for me to hear these stations from Chicago, Boston, New York, Pittsburgh, Cincinnati, etc., almost every night in California. By the 1970's most, if not all of these "clear channel"
stations were forced to share their frequencies with others at night...