I'm just guessing, Schmave, that WRVA and its concentric two-tower pattern is not so much there to protect anyone during the day as much as it is to serve that Richmond-Norfolk corridor.
Back in 'the day', I'd been with the Folks vacationing in Norfolk. I gotta cudgel my memory some. But in 1962 and 1963 -- on a GE clock radio that sat atop the kitchen fridge in 'Happy Days' -- I'd get a weak WGAI from NC, a weak WMBG from Williamsburg, loud locals WTAR 790 and Soul WRAP 850, local country WCMS 1050, a pretty solid WRVA, and so on.
An old National Radio Club AM log book lists 1140 as a 'U.S. and Mexican Clear' frequency.
But protecting Mexico in the DAY?
Even today, that NW-SE stretch between Richmond and Norfolk includes the bulk of people along that artery. The 'sides' of the pattern are more sparse than what is directly along what is now I-64. And a two-tower directional signal isn't as fierce as the 3- or 4-tower system. Perhaps someone here knows if WRVA originally had to pull it in, mildly, at night, and their engineers decided that, population-wise, they'd do very well to use the same coverage during the day.
Happy New Year, Schmave and the other swell folks on this forum!