Abstract: The present invention relates to directional couplers for microwave signals, and in particular to a combline directional coupler (301) for use at millimetre wavelengths. The combline microwave coupler (301) comprises a first transmission line (202), and a second transmission line (203), each transmission line including along its length a series of stubs (206, 207), the stubs of the first and second transmission lines being oriented in a combline pattern with each other such that in use microwave energy passing along one transmission line is coupled to the other. The coupler also includes four ports one at each end of the transmission lines for coupling microwave signals into or from said transmission lines, including at the first end of the combline pattern a first port to the first transmission line, and a second port to the second transmission line, and including at the second end of the combline pattern a third port to the first transmission line and a fourth port to the second transmission line.