Like for liquids there are also advantages and disadvantages for solids depending on the respective frequency.
In general there is not just “one right frequency” for all applications.
The advantages of the 80GHz radar in solids are: a longer measuring range and a smaller beam angle at same antenna size.
This allows easier installation and commissioning. The smaller beam angle allows also to measure closer to the bottom of the silo.
The antenna design of the radar can be more compact as well.
The wave length depends directly on the frequency. The higher the frequency the shorter the wave length. This has direct impact on the reflection of the respective media. With a higher frequency the damping in media is stronger.
The shorter wave length of the 80 GHz brings also disadvantages. For example in strong dust. Here a 26 GHz parabolic antenna performs better.
Conclusion: if you have a narrow and tall silo the 80GHz radar has clear advantages and cost benefits. For example no big expensive parabolic antenna is required.
If space is not a problem and a combination of low DC and strong dust occurs, a 250mm parabolic antenna with a 26GHz radar is the better choice.