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However, if you leave IBOC out for a moment, with the newer PLL synthesized transmitters are 3 adjacent channels needed to provide a clear signal?
When you throw IBOC in the answer is of course no, but that’s a different topic.
If you could cut it down to a single adjacent channel, then there would be room for more LPFM licensed stations that could serve the public. But money talks, and LPFM walks. Even if it was 2 adjacent channels there would be room for many 10 watt (effective) channels for community radio to use. From the location where our school is, we could hit most of what we want with 2 watts (FM) or under on a simple low gain omni antenna.
The problem with research like this is that you have to get qualified people with qualified equipment. Those people and that equipment cost money. I can tell you that even if I fired up on any of the channels in my market, the other broadcasters would crush the signal from my Ramsey FM25A, I would estimate about 10 feet at most under those conditions.