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It seems to me that you’re taking an awful chance if you put your livelihood into multiple transmitters in a single site install, and then get it shot down by the FCC.
Here’s what I’d do.
First, I wouldn’t quit my day job just yet. You should try things out first, particularly if you’ve never done Part 15 broadcasting before, to make sure that you’re going to get what you think you’re going to get out of it.
Why not set up 2 transmitters, not on the same site, perhaps 1/2 mile away from each other? That should keep you away from any grey areas in the FCC rules with multiple transmitters. You’ll have to decide whether to have them on different frequencies (in which case you don’t have to worry about synching them) or on the same frequency (then you’ll have to synch them – I’d recommend some sort of wireless link that doesn’t randomly introduce delays) and then experiment. You’ll be able to see the various ranges you can achieve to car radios and home radios. You’ll be able to see if you can synch them if you go that way and what the issues are (i.e., do you have to resynch everything if the power goes down). Or if you go with different frequencies, you’ll be able to see if that is a deal killer in your specific application.
I’d also really recommend looking at setting up an Internet stream, which could be used to listen in to your station on the fringe areas. Again, you can experiment – it isn’t all that hard to do, and can increase your listenership substantially (which increases ad sales).
You’ll make a (relatively) minimal investment overall and get much more ‘hard’ information than just theorizing here. If it doesn’t work, then you won’t have lost everything (you can always sell the equipment and get back some of your money).
Then, if you decide to go for it, you can use the results to determine exactly how many transmitters you require, and move forward.
One thing that I’ve learned about Part 15 broadcasting over the years – you actually have to do it. You can’t change the laws of physics but we’re dealing with such minimal power levels, and restrictive rules, that what might generally work in RF may not work here (factors that don’t do much when you’re dealing with full size antenna systems and thousands of watts can have major effects in Part 15).