If you are researching FDR soil moisture sensor technology, there are three distinct variants that are almost never properly compared, despite producing wildly different real world results.
Budget basic capacitive FDR sensors are the most widely available low cost option, extremely popular for hobbyist and DIY projects. However they have one major well documented limitation: very high salinity sensitivity, which renders readings unreliable in fertilized agricultural soil.
Resonant capacitive FDR, best known from the TEROS 12, uses a fixed 70MHz design. This is the current industry standard for professional field monitoring, offering exceptional long term stability, minimal interference and very low drift.
Finally lab grade swept frequency FDR is the only technology that remains accurate in extremely high salinity soils, and is primarily used for bound water and soil physics research.