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Hello and welcome to the board,
First, there are some decisions to be made. AM or FM?
The range from a legal AM part 15 tx. with an effective antenna exceeds that of a legal FM tx. (tx. is an abbreviaton for transmitter). The catch here is to set up and tune an effective antenna for AM. This is difficult and probably not worth the effort if this is a temporary installation. A “legal” FM antenna is a short whip or wire with no ground required. On FM, the FCC regulates the field strength and not the antenna, where on AM the antenna system is limited to 3 meters. With FM, the “legal” antenna needs to be determined by the manufacturer to provide the proper field strength. The FM-25 for example uses a 19″ whip which is supplied with the kit.
Think about the venue. Do patrons need to listen as they park or do you intend them to listen as they move about? If just for parking, the short range of a legal FM tx. will work considering that automobile radios have good sensitivity. If they move around, especially using portable radios, the range will be about 200 to 400 feet from the transmitter.
If you use FM, locate the transmitter as close to the expected audience as possible (in other words, in the center).
As a start, check out the SSTRAN website for AM and the Ramsey Electronics site for the FM-25. These companies both offer kits. If you are not comfortable with kit building, there are a few others that sell FCC certified transmitters but the cost is higher. One company that comes to mind is Rangemaster and they have a website.
Hope this helps.
As you narrow your decisions such as AM or FM then it becomes easier. Many on this board, I am sure, will be willing to advise.