The reason of your question is probably one of the four parameters of the coupler:
- Directivity or Isolation
- VSWR
- Coupling coefficient
- Insertion loss
Directivity is a measure of how well a coupler can isolate two signals. Directivity, therefore, sets the limits on how accurately a coupler can perform a specific measurement. Ideally, one would like to measure the magnitude of the reflected signal alone.
However, because of directivity, the reflected signal is combined with a small, undesired portion of the incident signal. It is important to emphasize that the magnitude of this reverse-coupled signal depends strictly on the directivity of the coupler.
regards