A horn antenna suitable for use with a level measurement device and having a composite emitter structure. The emitter structure or assembly comprises an emitter and a plug. The emitter provides the process interface and is formed from a material having properties which include microwave transparency, chemical resistance and/or mechanical strength. The plug is isolated or partitioned from the process interface. The plug is formed from a material different from the emitter and exhibits the properties of microwave transparency and/or mechanical strength. According to another aspect, the level measurement device includes a coupling mechanism which allows the removal of the horn antenna independently of the emitter.