Lunar Module Rendezvous Radar Antenna
The Lunar Module's Rendezvous Radar Antenna was mounted on the top of the LM's ascent stage, directly above the hatch. The antenna is visible in my pictures of LM-2, LM-9, and LTA-8. I also have additional pictures of LM Rendezvous Radar Antennas at the Air Zoo and at Stafford Air & Space.
The rendezvous radar was used after the astronauts had successfully landed on the moon, performed their exploration, and wished to return to the Command Module (although it was also used during the descent to the lunar surface to supply tracking data for any required abort maneuver).
The rendezvous radar, operated in conjunction with a radar transponder in the CSM, was used acquire and track the CSM before and during rendezvous and docking. The radar provided the guidance system and in-cabin displays with line-of-sight range, rate, and angle data with respect to the CSM. The CSM's transponder received the LM's radar signal, modified it, and then transmitted it back. By providing the rendezvous radar with a discrete radar signal to acquire and track, the transponder made the CSM appear as the only object in the rendezvous radar's field of view [text adapted from Apollo Operations Handbook: Lunar Module LM-6 and Subsequent: Volume 1: Subsystems Data (LMA790-3-LM 6)].