Patent ID: 8803633

Claim:
A directional coupler comprising: a main line including a signal input port and a signal output port; and a sub line that includes a coupling port and an isolation port and that is coupled to the main line through electric field coupling and magnetic field coupling, wherein a series capacitor is connected to only one of the signal output port and the coupling port, wherein the signal output port includes a first parasitic inductance and the coupling port includes a second parasitic inductance, and a capacitance value Cx of the series capacitor is determined as follows: for a capacitance value C 1 that would cause resonance with the first parasitic inductance at a desired frequency and for a capacitance value C 2 that would cause resonance with the second parasitic inductance at the desired frequency, the capacitance value Cx of the series capacitor is set smaller than or equal to the capacitance value C 1 or smaller than or equal to the capacitance value C 2 .