I assume the complete name of the product used in your applications is Micropilot M FMR250 - non-contact radar product from E+H. As far as I am aware, E+H provides a purge port on their free space radars to address exactly this particular issue - dust build-up on the horn launch surface and they advertise this feature in their literature. Do you use the air purge on this product?
If you used that already and still have problems, then the alternative non-contact level measurement technologies to use are laser or ultrasonic:
1. Laser will have no problem with dust build-up, because it is non-intrusive to the process mounting and the lens are protected by dust tubes. The only limiting factor for this technology is the dust density and time dust present. If your process is as such that dust is present only during fill cycle and then settles (this operation type is typical for storage type silos), then the laser is a good solution for this application.
2. Applying ultrasonic requires careful consideration of all application and mounting specifics (reposing angles, position of feedstream, mounting nozzle location etc.). Ultrasonic usually handles OK with a dusty environment as the transducer sound emitting surface vibrates while the sound wave is launched and tends to clean itself from dust particles. Nevertheless, to apply ultrasonic in dry bulk applications require a lot of expertise and field experience.
I would suggest the following:
1. Check first the purge option for FMR250
2. If you have been already through step 1, then consider alternative technology trial (laser or ultrasonic)