Pin diode-based variable directional coupler

Analysis and design of a novel variable directional coupler based on a parallel line directional coupler using the even-odd mode technique are proposed. In this coupler, the variation of coupling is achieved using a PIN diode, connected to the nominally coupled port of a parallel line directional coupler, while the output is derived from the normally isolated port. To verify the theory, a parallel microstrip line coupler is fabricated on a substrate with ε _r = 3.66, h = 762 μm, and tanδ = 0.006 and measured. The measured results show a variation in the transmission coefficient of nearly 20 dB, while at the same time, the change in the direct arm insertion loss is lower than 0.2 dB.