Microstrip Directional Coupler

Model

A directional coupler is a passive device which couples part of the transmission power by a known amount out through another port. When the power coupled out to coupled and transmitted ports is half the input power (i.e. 3 dB below the input power level), such a coupler is referred to as a 3 dB coupler. The first figure shows the geometry of a microstrip 3 dB directional coupler. The thickness of the substrate is 0.794 mm and the relative dielectric constant is 2.2. The structure has two parallel main transmission lines and two coupling embranchments with lengths of one-forth wavelength. Due to their structural simplicity and easy fabrication, the directional couplers are widely used in many microwave circuits. Edge sources are used to feed the microstrip line at all ports.

Simulation

The directional coupler is a four-port structure. A template broadband simulation is prepared by using four edge sources and four ports. Then, the S-parameter Simulation Setup tool is applied to build four simulations to extract the four-port S-parameters of the coupler. The maximum grid step is 1 mm and the minimum grid step is 0.05 mm in the frequency region of interest. The grid is refined on the edges of the coupler. The UPML absorbing boundary conditions are used to truncate the domain.

Results

The four-port S-parameters of the coupler is obtained by using the S-parameter Evaluation tool. The bottom figure shows simulated frequency response of the 3dB directional coupler. At around f = 6.5 GHz, S11 amd S21 have the lowest values which imply that the structure is matched and port 2 is isolated. At around the same frequency, the magnitudes of both S31 and S41 become 3 dB as expected.

Ref. Wu Ren, et. al., The Analysis of 3dB Microstrip Directional Coupler in Time-varying Media by FDTD Method, Proc. 2nd International Conference on Microwave and Millimeter Wave Technology, 2000.