[onetwentythree]

The antenna is steered to point towards the DSN antenna on the earth. A larger moving mass would make it harder to maintain telescope pointing while the antenna is moving.

In reality, the antenna pointing is 'paused' during each science observation, unless pointing is needed due to the length of the observation.

[nextaccountic]

Oh, the antenna doesn't need to just point in the general direction of Earth, it needs to point to somewhere in the surface. That makes sense, having a narrower beam would save power and achieve higher bitrates

Does this mean it has only a 12-hour window to transmit? Or there's multiple antennas on Earth?

[onetwentythree]

It uses NASA’s Deep Space Network. There are three stations around the globe (California, Australia, and Spain), spaced so that there is near continuous coverage for deep space missions. JWST points it’s antenna at the station that is currently in view.

However, the ground stations are shared between many missions, so they are not available for JWST all the time. Expectation is that JWST gets 8-12 hours/day of DSN time.