Aniruddhan S

The concept of electrical balance is used to achieve wideband isolation in an asymmetric 2.4 GHz directional coupler. An intuitive approach using impedance transformation property of transmission lines is used to develop the theory for designing arbitrary-coupling-ratio directional couplers. A coupler based on this theory is designed and fabricated on a two-layered RO4003C PCB. The coupler achieves a return loss of better than 10 dB over a 1.7-2.7 GHz range. The transmission coefficients of the strongly and weakly coupled ports are around-3.5 dB and-5 dB respectively, at 2.4 GHz. The isolation is better than 25 dB over a 1.4-3 GHz range.

The theory presented by Kumar et al. is based on the assumption that edge-coupled transmission lines (TLs) have infinite even-mode characteristic impedance and is clearly mentioned. Simulation is done to show that better than 10 dB matching can be achieved with a practical TL without using additional TL sections as long as impedance transformed by the even-mode of TL is much lower than the port impedance. The addition of extra quarter-wave TL sections can further improve the matching.

A directional coupler using edge coupled lines is designed and fabricated on a two-layer PCB with isolation being achieved through electrical balance. Measured results show 4dB coupling and 34dB isolation at a frequency of 2.5GHz with 16% matching bandwidth (-10dB). Isolation varies between 31dB to 36dB in the 2.3GHz to 3GHz frequency range. © 2014 IEEE.