What is the shape and location of a transponder antenna?

The transponder antenna typically features a short blade or a wire with a ball end, and is commonly mounted on the bottom of the fuselage. This design and placement are optimal for receiving and transmitting signals efficiently without interference from the airflow over the aircraft.

The antenna's position on the underside of the fuselage helps ensure that it can effectively communicate with ground stations as well as other aircraft, as signals can be transmitted downward into the airspace below. The compact design of this type of antenna also minimizes drag and impact on the aircraft's aerodynamic performance.

Other configurations like a cylindrical shape on top or a flat rectangular shape in the tail do not accurately represent standard transponder antenna designs, as they would not provide the necessary performance or aerodynamics suited for effective transponder operation. The disk shape on the wing is more indicative of different types of antennas, such as for satellite communications, rather than the specific functions served by a transponder antenna.