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When using multiple transmitters for talk radio it's actually better to have them relatively off-frequency by about 10 Hz or so. Doing so avoids any audio phasing between the transmitters.
I'm the voice of authority here, I was Chief Engineer of WLLH Lowell, MA and WLLF Lawrence, MA in the later 1980's. WLLH/WLLF were about 25 miles apart, both on 1400kHz with the same programming. They operated with special HP 105AR precision crystal oscillators which were set about 10Hz apart. I "tuned" the oscillators by getting halfway between Lowell and Lawrence and having my assistant adjust the one in Lowell for the slight beat. By doing this, there was no location in the "mush" zone between the transmitters where one got phased audio and the 10Hz beat was almost unnoticeable on talk material.
It does help to have professional broadcast level audio processing that keeps the audio level pretty dense at all times, that hides the beat very effectively. I personally prefer Orban 9100/9200's for AM processing. The old Innovonics MAP processors are pretty good for the money and a Texar or Gentner AM Prism isn't half bad either.
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