At the time, Kevin, 1580 and 1570 were co-owned. I worked for them for a few years around 1970, DJ-News-MD. Adams-Getchall were the owners.
I don't know if they were originally co-owned back when the 'second' station signed on, though -- or which one came on first. I'd be just guessing that 'WPAC' 1580, the big 10,000 watt omni, was the first to be established. They had two sticks, by which to pull the station's signal in to protect CBJ in Quebec during critical hours. 'WHRF' 1570 Riverhead broadcast from an old clam shack or potato storage shed* next to the Flanders Drive-In theatre. They also had two towers, and was a strict directional 1000 watt daytimer pulled in away from Montreal's big CKLM on 1570 and aimed somewhat southeast. The WHRF signal fell off rapidly as you drove due west out or Riverhead proper.
But 'WPAC' 1580 had a terrible signal. Their standalone building and towers were inland, in Medford, in pine barrens. The only real overlap of their signals was the south shore stretch of the William Floyd Pkway, where a decent analog car radio tuned to 1575 kHz would get an echo effect during when both stations carried their American Entertainment newscasts.
When I worked for Adams-Getchall, their two FMs were also co-owned, of course. They were 106.1 in Patchogue and 103.9 in Riverhead.
Contemporary with them was the Island Broadcasting System's short-spaced omnis1370 WALK in Patchogue and WRIV 1390 in Riverhead! The I.B.S only had the one FM, though -- WALK-FM on 97.5.
Both radio companies treated their FM's like dummy loads for some 6-7 years after the FM began to emerge. Both WPAC-FM and WALK-FM used to sign off around midnight -- and both in mono. Plus, 'WHRF'-FM reportedly was cranking out 160 watts ERP instead of the 3000 or so it was entitled to use.
But you are correct ..... 21 miles between 1580's and 1570's transmitter sites ..... terrible ground conditions ...... yet, no real overlap.
* WHRF's bunker once was the birthplace of a litter of kittens underneath the transmitter rack. One morning on the air, the DJ was flipping through the copybook doing a live read, and audibly had the snot scared out of him, on AM and FM, when a furry paw behind the book reached over and slapped the pages. I won't reveal his name; let's just say the guy who shreiked 'Eaaggggh!!!!!' was the station PD, Don Cannon (later to star in Philly radio for decades).
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