A 15W transmitter into a bad antenna is worse than a 1W transmitter into a good antenna. For 100 km range, you need:
A 100 km range for an FM transmitter requires professional-grade equipment rather than simple DIY circuits, as achieving this distance typically necessitates of power and a high-altitude antenna tower. Most hobbyist "long-range" FM transmitter circuits only reach 1 to 5 km . Long-Range FM Transmitter Diagrams