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)

For continuous level measurement in silos with powders, there are two choices for level measurement:
1. Contact level measurement method - this method is very accurate and will avoid errors which may occur during silo fill or empty cycle. A typical device used for such measurement is GWR (Guided Wave Radar). Level measurement accuracy is +/-5 mm (+/-0.2 in.). This method is cost effective.
Limitations: Dielectric constant of the measured material - if very low (<2), this may present a problem for the GWR device. Another limitation is the silo height - if silo height exceeds 15 m/50 ft, usually the advice is to check how well re-enforced is the silo roof - the pull forces on the GWR cable during material discharge can be very high and this may have an impact on the silo roof integrity.
Plum Bob - this product is a mechanical on-demand measurement device with good accuracy. Not advisable to be used during silo fill cycle.
Here we will not discuss another common method for measurement - Load Cells - as this technology comes with a higher price tag, it also measures weight instead of level and to be effective must be installed before the silo is erected.
2. Non-contact level measurement method - the advantage of this method is the easy installation and almost no need for preventive maintenance. One disadvantage is that with powders, especially during silo fill cycle, the generated dust may introduce errors and impact the measurement reliability. Many end-users, however, will look first to this method to find their level measurement solution as non-contact avoids lot of post-installation issues.
Products used: Laser, Ultrasonic, Open Air Radar. Note: it is not advisable to apply Ultrasonic products in pressurized silos (pressure exceeding 1.5 bar/22 psi)
To determine which technology/product is the most applicable, the application specifics must be reviewed.
Conclusion: If accuracy and measurement reliability are the two most important criteria, then GWR should be applied if limitations (see above) are not met.

Laser technology has two major differences when compared to the present ultrasonic and open air radar product markets:
1. The laser can measure any solids surface at any angle.
2. The laser measures with a very narrow beam with virtually no divergence.