Ken Norris, you have again caught me up in reality, vs. my colorful 3D imagination.
TRUE (like you say) as one moved from 1680 to 1670 the radio would not adjust. So, I was only thinking of stationary listeners;
TRUE (like you say) for listeners in the weak in-between spot there would only be two weak, noisy choices. Would anyone tolerate that?
BUT in my most recent post I made the point that the exact same audio would NOT necessarily be necessary... that sure is redundant the way I said that... because the listener would only hear one of the audios or the other. The two audios could be out of step without penalty.
Reality and truth have things in common.
What I mean is, you are more right than I am, but I am not completely wrong.
With respect and love to the Members for having such wonderful tolerance for those of us who think before speaking with limited success.....
I should stop while I'm ahead, but I'm not ahead yet.
IRCC, one reason the FCC doesn't license AM stations for adjacent frequencies in the same coverage area is because normal bleed tolerances would cause interference issues. So, you would definitely not want to broadcast out of sync program material on adjacent frequencies. Close proximity to the second frequency while listening to the first will be horrible.
I recently built an antenna system for operating at 1630khz at my studio, which is approximately .5 mi. from the TX on my boat at 1650khz. It doesn't work to my satisfaction. Even though they are 2 channels apart (1640khz is in between), the 1630khz signal, which is weaker due to poorer grounding, allows my car receiver to pick up some signal from the 1650khz station. Since the audio is out of sync between the two, the interference is unacceptable.
I still have more work to do, but it looks like I'll have to find another frequency, or move the stations much further apart, which isn't an option right now. I wish the TH could transmit on 1710kHz. There is nothing heard on that frequency anywhere on the island, day or night.
BTW, 1710khz operation under Part 15 has a hard field strength limit spec, but if you calculate it using the formula for any other freq in the band, it ends up the same.
I said "the 1630khz signal, which is weaker due to poorer grounding, allows my car receiver to pick up some signal from the 1650khz station."
That's incorrect. What I meant to say was this: The 1650khz signal gets interference from the 1630khz signal when I'm about a block away from the 1630khz station.
I don't know how that would relate to varying power and proximity distances when talking about high power stations.
Another slightly different point rises out of this discussion which I don't think has been exactly talked about.
Results may vary based on different radio receivers. I have a Sangean ATS 505 which is so precise on AM that it pickups no bleedover from adjacent channels. I am transmitting at 1680, the radio is 5-feet from the antenna, but I can listen to 1670 or 1690 with no problem, and they're both two states away.
However, in the bathroom another radio picks up any frequency that's near the selected frequency, because it has AFC (automatic frequency control), so adjacent channels become a problem.
Still, in my earlier portrayal, I was assuming the two transmitters on adjacent frequencies would be located on different properties in different homes, some distance apart. We should therefore emphasize that being too close to each other is what makes adjacencies inadvisable.