Yes, the transmission method can have an advantage over the reflection method in detecting small variations in the dielectric permittivity of the sample under test. This is because the transmission method provides access to both S11 and S21 parameters, while the reflection method only provides access to S11.

In the reflection method, the S11 parameter is related to the reflection coefficient of the sample, which depends on the dielectric permittivity and thickness of the sample. The sensitivity of the reflection method to small variations in the dielectric permittivity depends on the magnitude of the reflection coefficient, which can be low for thin or low-permittivity samples. This can make it challenging to detect small changes in the dielectric properties of the sample using the reflection method.

In contrast, the transmission method measures both the incident and transmitted waves, allowing for the direct determination of the complex transmission coefficient S21. This provides additional information about the attenuation and phase shift of the wave through the sample, which can be used to calculate the complex permittivity of the sample. The sensitivity of the transmission method to small variations in the dielectric permittivity depends on the magnitude of the transmission coefficient, which can be higher than the reflection coefficient for certain samples and frequencies. This can make it easier to detect small changes in the dielectric properties of the sample using the transmission method.

However, it is important to note that the choice of measurement method depends on the specific application and the characteristics of the sample being measured. The transmission method can be more sensitive to variations in the dielectric permittivity, but it may also require more complex experimental setups and calibration procedures. The reflection method, on the other hand, is simpler and more straightforward, and may be sufficient for certain applications.