Coupling Coefficient

The ratio in dB of the incident power fed into the main port to the coupled port power when all ports are terminated by reflection less terminations.

Coupling Tolerance

The allowable unit-to-unit variation in nominal coupling.

Coupling Flatness

The maximum peak-to-peak variation in coupling coefficient that may be expected over a specified frequency range.

Coupling Loss

The reduction in power available to the main-line output due solely to power transferred to the coupled line. It is equal to the theoretical minimum main-line insertion loss.

Main-Line Loss

The change in load power, due to the insertion of the coupler in a transmission system, with reflectionless terminations connected to the ports of the coupler. The main-line loss includes the effect of power transferred to the coupled line.


The difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction. In the case of a bi-directional coupler, an alternative definition is the difference in dB of the power output of the two coupled ports, when power is transmitted in a constant direction on the main-line. Reflectionless terminations are assumed to be connected to all ports.

Average Power

The CW average power handling capability with one-way transmission through the main line of the coupler under matched load conditions.


The standing-wave ratio at any port of a coupler is specified for the case of reflectionless terminations at all other ports. As with any device, VSWR is a measure of the quality of match relative to a given characteristic impedance.


The range of frequencies over which performance falls within specific limits.

Bi-Directional Coupler

Another name for a 4-port coupler; that is, a single coupler having no internal termination. It is intended to allow forward and reflected signals to be sampled simultaneously.

Dual Directional Coupler

The combination of two 3-port couplers having their main lines cascaded, and their internally terminated ports facing each other at the interface between the couplers. This provides bi-directional coupler action, but with independent use of the coupled ports: A mismatched load applied to either of them will not affect the other.

Browse all RF couplers