Recently i am working with non contact radar level transmitter. My process fluid is of high dielectric constant, slurry, will be agitated, there will be vapor. Now i have two offer from two different vendor, one with low operating frequency (<10GHz), and other is of high operating frequency (>75 GHz). Now i am getting confused as there is huge difference in operating frequency. Please clarify me regarding this issue in which case i should use high and low frequency radar transmitter.
Your application has lots of problems. It may be that the way to decide is to ask each vendor for a demo installation and see which unit works best. You are trying to measure a non existing level-- what the level in the tank would be if it wasn't being agitated. Nothing will do a really wonderful job at doing this measurement. You have vapor, you have slurry, which is agitated, and throws material onto the devices in the tank, and that would include the measuring instrument.
If I were you, I'd do a test installation.
There is a huge difference between the frequency bands. Basically, the difference is the beam angle, beam spread, and the size of the antenna. When you have a radar with low-frequency the beam angle is around 20 degrees, radars working with 26GHz can have a beam angle of 8 to 22 degrees, radar with high-frequency such 75 or 80GHz, they're gonna have small beam angles and antenna.
I wrote an article with more details, you can read here > https://visaya.solutions/article/radar-level-transmitters-frequency-bands/
Furthermore, I have many product reviews comparing radars you can find them here > https://visaya.solutions/category/devices/product-review/
The frequency itself is less important that the service conditions you've described.
You'll be experiencing all sorts of issues unrelated to radar non-contact measurements, but then all measurements will have problems.
But some vendors are saying for turbulence or vapor and condensation application low and mid frequency instrument works much better. As the signal with high frequency and low wavelength scattered with the wave of the surface and returning signal become weaker.
On the other hand high frequency transmitter has shorter antenna size, small beam angle, and shorter beam width. This feature permits some installation facilitates also.
so what frequency instrument should i choose in case of this application? should i trust the vendors as they are using both technologies over the years and years? or i should go in details?
"As the signal with the high frequency and low wavelength scattered with the wave of the surface and returning signal become weaker." This approach is a sales pitch from a particular American company because they don't have high-frequency radars.
Their comment makes sense, however, there are a lot of application with VEGA, Endress or Siemens High-frequency radars.
You can test the radar before you buy one.