The Robert C. Byrd Green Bank Telescpoe is located in the national radio quiet zone in West Virginia. It is the largest steerable instrument in the world, with a dish 300 feet in diameter and height 485 feet above the ground. I've used this telescope to observe 21cm radiation from neutral hydrogen in nearby galaxies hosting an active galactic nucleus.
The 305m (1000ft) Arecibo Telecope was once the world's largest single-aperture telescope. It has since been overtaken by the Five Hundred Meter Aperture Spherical Telescope (FAST) in China. Arecibo is of the premier instruments for extragalactic radio astronomy. I've also used this telescope for 21cm observations with the Arecibo Legacy Fast ALFA (ALFALFA) survey, a huge collaboration of astronomers measuring distances, masses, and velocities of thousands of galaxies in the local universe.