The "stripline" in all avionics equipment is an assembly that operates in the microwave frequency range (usually.3 GHZ to 30 GHZ) in the RF assembly of the unit. It contains all stages that operate in that range, such as Local Oscillators, RF amplifiers, heterodyne mixers, transmission lines (called waveguides).
Avionics components such as DMEs, transponders, radio altimeters, and weather radars commonly include stripline assemblies.