It was built by Major Edwin Amstrong, the inventor of the band, way back in 1937, just north of NYC. And it's quite an impressive sight even all these years later.

"The Armstrong or Alpine Tower is the site of the world’s first FM broadcast station, W2XMN. The call sign is still etched in cement above the front door of the Major’s original station building on the site today."

It also served as a back-up for a plethora of New York City TV stations, which were knocked off the air when the Twin Towers went down in the Sept. 11th attacks. 

You can see it (and read more about it) here.