Radioastronomy Antennas
the term "Astronomy" call for passive sky observation. Some "active" scientific project are called "radar-astronomy". The transmission of high power signal for SETI or communication with human interplanetary missions is called "deep space communication".
In Astronomy , also in radioastronomy, it's important both sensitivity and angle of view.
To increase sensitivity, if the radiosource emission is a black body (in most cases are black body) increase collecting Area, decrease Noise Temperature, increase Bandwidth and increase observing time.
So the first answer to your question is: the antennas are largest as possible. 10 m - to 100 m antennas are typically paraboliod with F/D=0.3 - 0.4.
But for the beam of view, to be scientifically competitive, no one antenna is pratical !
Astronomers combine (in phase) the signals coming from far antennas located around the world, to get a "virtual" antenna that have a extremally thin lobe of interference (i.e. 0.001 arcsec).
The tendency for the new one single single antenna are growing into the following main directions:
-Larger, fully steareable paraboloid
-Multibeam.
Spacecraft Radiotelescope.
The multibeam is close displacement of an array of illuminators on the focal plane (focal zone) of the dish. In this manner the observer may look simultaneously to many points on the sky.
With the spacecraft antennas, the observer may look at frequency masked by the atmosphere.
But remember: every radiotelescope is unique, there isn't a "typical" mode to build antennas.
The 4K cooled Maser ( 40+ y.o. tecnhology) is unsurpassed in low noise, but is narrowband.
The tecnology used to receive signals is criogenically cooled GaAs or InP
HEMT LNA. Observers may get Maser like performances with BW large up to 1/2 octave and more.