Antenna, Transponder, Blade Style with Remote Connector
This unit is a bit on the expensive side.  I recommend the RAMI-AV-74 instead.

Frequency: 950 to 1220 MHz (DME Band) and 1030-1090 MHz (Transponder Band)
Connector: BNC (female) on a 6 inch coaxial extension cable
Rated for 400 Knots TAS at 25,000 ft.
Broadband and rugged antenna designed for DME or transponder use.
Antenna assembly encased in a glass reinforced polyester molded shell.
This model offers standard two stud mounting, where the studs areextended to 1 inch in length. 

Dimensions (See drawing at right)

Specifications

Lead time: 1 week