Why are radio telescopes usually shaped like large dishes?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

Radio telescopes are shaped like large dishes because they utilize a parabolic antenna to collect and focus incoming cosmic radio waves onto a small receiver, known as a feed.1

This design, often called a radio reflector, functions similarly to a satellite dish. Because cosmic radio sources are extremely weak, the large surface area of the dish is essential to gather enough signal for detection. The parabolic shape ensures that incoming radiation is concentrated at a specific focal point, where it can be captured and transferred to a sensitive radio receiver for analysis.1