Don't know if this is any help - from the 737CL AMM:
3 . Directional Antenna (Fig. 2)
A. The directional antenna is an electronically steerable phased array that
has four top loaded monopole elements. The directional antenna is
connected to the TCAS computer with four coaxial cables that have TNC
plugs on the antenna end.
B. The TCAS uses two directional antennas. One is installed on top and the
other is installed on the bottom of the airplane. These antennas
transmit to and receive signals from intruder airplanes.
C. Each of the four elements of the directional antenna has a resistor that
goes from the antenna element to ground. Each of the four resistors has
a different value. The TCAS Processor regularly does a continuity test
on the antenna ports and will see the correct resistance value if the
port is not shorted and not open.