The beam steering circuits send the signals to the four antenna elements. During transmit, the beam steering circuit controls the phase relationships of the four RF outputs. The whisper/ shout attenuator makes the output power smaller or larger based on the control signals from the transmitter.
During receive, the phase of the received signals is different in each element. These beam steering circuit sends the signals to the receiver.
The TCAS computer receives a discrete input from the landing gear lever switch. When the landing gear lever is down, the TCAS computer uses all four antenna elements equally for the bottom antenna. Thus, the bottom antenna operates as an omnidirectional antenna.