Single-layer balanced branch-line coupler

Balanced branch-line couplers become more and more popular in modern communication systems due to their advantages of noise immunity, low electromagnetic interference and convenient connection with other balanced circuits or antennas. The conventional branch-line couplers are usually single-ended designs, which are based on four quarter-wavelength microstrip lines. Four additional baluns should be added at the four ports for the application of balanced systems, which will lead to large size. Therefore, it is very important to propose one single device achieve both differential-mode power dividing and common-mode suppression. The reported balanced branch-line couplers are all two-layer designs. In this paper, a single-layer balanced branch-line coupler is proposed based on four quarter-wavelength and four half-wavelength transmission lines. Furthermore, in order to reduce the size of the circuit, a compact single layer balanced branch-line coupler is proposed by T-junction structures.