Miniaturised rat-race coupler with second and third harmonic suppression using synthesised transmission lines

A miniaturised rat-race coupler capable of suppressing the second and third harmonics is proposed. The coupler is designed with slow-wave synthesised transmission lines using two π-networks in cascade. Each π-network is formed by a parallel LC tank and two shunt capacitors, and is associated with a transmission zero for harmonic attenuation. The two transmission zeros embedded in the synthesised line can be controlled independently. The proposed rat-race coupler, centred at 2.4 GHz with an operational bandwidth of 18.8%, and at least 21-dB suppression at the second and third harmonics, features a compact footprint, which is only 12.9% the size of its conventional counterpart.