Once I moved to Ohio from Saint Pete, I did my best to DX 620 [WSUN] at night along with other Tampa Bay stations. I thought Ohio radio sucked but that was just my unfamiliarity with it. Sometimes, very faintly, I could hear it as it phased in and out with WTMJ. Then WRJZ in Knoxville, TN bumped up to 5000 watts nights from 500 [if I remember correctly] and even with the nightime null to the northeast it knocked any chance of me hearing 620 from Saint Pete. outta the sky. All I could pick up anymore was WTMJ and WRJZ faintly. I remember going by those two towers straddling W. Gandy Blvd so many times as a kid.If we look at the first, 620 in Tampa/St Petersburg, we see that a directional system allowed a very good local service in Tampa without harming the coverage of WTMJ in Milwaukee. Directional systems allowed many very viable stations to share channels. Today's issue is that urban sprawl has outgrown those 1930's and 1940's setups.
Actually, many if not most original daytimers were set up to add a signal to a larger market where all the fulltime options were gone. Examples would be KOWH in Omaha (the original Top 40 station), 740 in Avalon / Los Angeles, CA, 1190 in New York City.