Design of an Ultra Compact 180-Degree Ring Coupler With Harmonic Suppression Using Composite Lines for Modern RF Systems

This paper presents lumped reactive capacitors and inductors to create a compact filtering hybrid ring coupler (HRC) with a broad suppression band. The traditional 180-degree ring coupler has six long $\lambda $ /4 branches. In the proposed coupler, six compact proposed branches are applied instead of these six long $\lambda $ /4 branches, resulting in size reduction and harmonic suppression. The designed HRC works at 800 MHz and provide 220 MHz operating bandwidth from 690 MHz from 910MHz, which is equal to 27.5% FBW. The designed HRC has acceptable performance in operating frequency and higher frequencies. The designed 180-degree ring hybrid coupler (HRC) achieves a broad stop-band ranging from 1.7 GHz to 5 GHz, effectively rejecting $2^{\mathrm {nd}}$ to $6^{\mathrm {th}}$ harmonic. The proposed HRC is designed, simulated and fabricated, which the measured results validate the simulations. In the pass band, the proposed hybrid coupler exhibits less than 0.2 dB insertion loss, over 26 dB return loss, and 49 dB isolation measured at 800 MHz. Moreover, the conventional 800 MHz HRC occupied $0.5~\lambda \times 0.25~\lambda $ (140.7 mm $\times 70.8$ mm), while size of the proposed HRC is 10.7 mm $\times 6.7$ mm ( $0.038~\lambda \times 0.024~\lambda $ ), which provides 99% size reduction.