4.  ATC Antenna___________
 A.  Two ATC antennas are installed in the airplane. The antennas are on the top and the bottom of the airplane. Two rf switching relays connect both antennas to the active transponder. The top antenna is located on the centerline of the fuselage at station 430.25. The bottom antenna is located on the centerline of the fuselage at station 355. The antennas are omnidirectional L-blade type antennas. Each antenna can transmit and receive signals for the active transponder. The active transponder automatically uses the antenna with the greatest signal strength.
 5.  Operation_________
 A.  Mode-A and Mode-C
 (1)  There is no power switch on the ATC control panel. Power is applied to the system when the ATC-1 and ATC-2 circuit breakers on the P6-1 and P18-1 panels (fig. 2) are closed. On systems with two antennas, power is aplied to both ANTENNA TRANSFER RELAYs when the ATC ANTENNA SW circuit breaker on the P6-1 panel (fig. 2) is closed.
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A 737-300/400/500MAINTENANCE MANUAL

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737-300/400/500MAINTENANCE MANUAL
 (2)  
The desired ATC system (No. 1 or No. 2) is set with the transponder select switch. The 1 position causes the control panel to send an 'ON' discrete to transponder No. 1 if the AIR/GND RELAY is in AIR mode. The 2 position causes the control panel to send an 'ON' discrete to transponder No. 2 if the AIR/GND RELAY is in AIR mode. The STBY position causes the control panel to send a STANDBY discrete to both transponders.

 (3)  
The desired 4-digit airplane identity code is set with the code switches. Each of the switches controls a digit in the code display window.

 (4)  
The desired altitude reporting condition is set with the altitude reporting switch. The '1' position causes the altitude reporting function to come on and sets the altitude data source to ADC 1. The '2' position causes the altitude reporting function to come on and sets the altitude data source to ADC 2.

 (5)  
If the altitude reporting function is on, then the ATC system will reply to mode-A and mode-C ground station interrogations. If the altitude reporting function is off, then system will reply to mode-A interrogations. Mode-S replies are not affected by the condition of the mode switch.

 (6)  
The identity code and mode configuration information is sent to the transponders through ARINC busses and discretes. The 4-digit code identifys the airplane to the ground station for mode-A and mode-C interrogations.

 (7)  
When the airplane leaves the ground, the AIR/GND relay is set to AIR. This sends a discrete signal to the control panel. The control panel then sends an 'ON' discrete to the selected transponder. At this point, the system will respond to ground station interrogations.

 B. Mode-S Operation
 (1)  Mode-S functions operate on an airplane if the mode-S programming plug is configured with a unique mode-S address. If all the programming pins are grounded (they have a bit value of one), then there is no unique address and the mode-S functions do not operate. The mode-S code is hard wired on mode-S equiped airplanes. The identity code set on the ATC control panel has no effect on the mode-S address code of the airplane. Mode-S functions in the airplane are automatic and are not configured by the pilot.
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737-300/400/500MAINTENANCE MANUAL
 C. Self-Test Operation
 (1)  Both ATC transponders have Built-In Test Equipment (BITE). A BITE test can be started if the transponder to be tested has power. The BITE test is started when the TEST switch on the transponder face is pushed. The BITE test does the following:
　
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