These directional couplers calculations are based on textbook equations and conditions where all four ports are assumed to be terminated in impedances that are exactly matched to the characteristic impedance of the coupler, and that perfectly linear operation occurs. The "Directivity" term accounts for non-ideal isolation between the coupler input and the isolated port. The "Mainline Loss" term exists to account for resistive losses in the coupler (a specification often provided by the manufacturer). Here are definitions of the terms used.
Input (Port 1) | Main system signal input |
Transmitted (Port 2) | Main system signal output |
Coupled (Port 3) | Power sampled from Port 1 (forward, or incident) |
Isolated (Port 4) | Power sampled from Port 2 (reverse, or reflected) |
Coupling Factor | Amount of power sampled relative to the input |
Mainline Loss | Resistive losses not due to coupling |
Directivity | Difference between Port 3 and Port 4 |
Isolation | Difference between Port 1 and Port 4 |
Coupling Losses Only | Theoretical reduction in output power relative to the input due to power lost to the coupled and isolated ports. |
