Some more:
The directional antenna lets the TCAS computer transmit
interrogations and receive replies on one of four antenna beams.
The TCAS processor electronically points the antenna beam in one of
four different directions to find the bearing of intruder airplanes.
The antenna does not have to move. The TCAS adjusts the drive level
and phase of each of the four antenna elements in the directional
antenna to point the beam.
TCAS transmits 1030 MHz messages from the top and bottom TCAS
antennas to interrogate ATC transponders in other airplanes. The
TCAS receives 1090 MHz messages from ATC transponders through the
top and bottom directional antenna.
Later in the troubleshooting, it states those resitance values mentioned in my first reply, fo the four segments of the antenna:
C4 (RED) 9.7K TO 10.3K OHMS
C3 (BLUE) 6.46K TO 7.14KOHMS
C2 (BLACK) 3.7K TO 4.1KOHMS
C1 (YELLOW) 1.7K TO 1.89K