Directional Coupler Miniaturization Research

Directional couplers, as one of the most used passive components, are widely usedin the field of microwave and millimeter wave circuits, such as balanced mixers, phaseshifters, power amplifiers, etc. With the rapid development of wireless communicationsystems and the increasing demand for the portable equipments, people propose ahigher requirement for the size of a coupler. Especially when coupler is used in lowoperation frequency applications, traditional couplers are always unacceptable large forany practical application. So it is essential to carry an in-depth study on miniaturizationof couplers.This paper starts from basic theory. At first, it outlines analysis and miniaturizationmethod. Then two kinds of design theory and methods are introduced to designminiaturization3dB-directional coupler. The first one is traditional miniaturizationmethod that utilizing lumped-elements or open-stubs to design compact coupler. Theanalysis theory and software simulation results are given for this method.A new method using fractal coupled lines to design compact coupler is proposed inthis paper. By introducing second order Gosper island and first order Hilbert curves, theproposed couplers are miniaturized. The compact coupler featuring first order Hilbert ismeasured by a vector network analyzer and the measured results accord with simulationresults well. Return loss is better than19dB. Isolation is better than20dB, through portand coupling port amplitude unbalance is-3.4±0.5dB, phase unbalance is90°±2°.Through the above work, the methods of using lumped-elements, open-stubs andfractal coupled lines to miniaturize couplers are proved. Among the methods mentionedabove, couplers featuring fractal coupled lines are able to make further miniaturizationof the couplers designed by traditional methods and can be put into practicalapplications.