Antenna, Transponder, Stub
This unit is a bit on the expensive side.  I recommend the RAMI-AV-22 instead.

Frequency: 1030-1090 MHz (Transponder Band)
Connector: BNC, female
Rated for: 300 Knots TAS
Transponder antenna with top loaded stub monopole.
Antenna radiator is mechanically captivated and is machined from solid brass for impact resistance.
Contact points are made from beryllium copper.
Metallic parts are plated with bright nickel for corrosion protection.
Mounts through a single 0.600" diameter mounting hole. 

Dimensions (See drawing at right)

Specifications

Lead time: 1 week