Analysis And Design Of Stripline Directional Coupler

Directional couplers are widely used as an integral part of many microwave networks and systems, such as amplitude controller, balanced amplifier, power splitter and combiner. They have matched characteristics at all four ports, making them ideal for insertion in a circuit or subsystem.Firstly, the odd-mode and even-mode network analysis of symmetrical coupled transmission line is described. Scatter matrix of backward-wave directional coupler composed of symmetrical coupled transmission lines is then determined. Important characteristics of directional coupler such as phase shift, port isolation and propagation delay are also analyzed.Secondly, the synthesis procedures of single-section and three-section symmetrical directional coupler are introduced. The computation method of physical dimensions for directional coupler with broadside-coupled offset stripline configuration is also given.Thirdly, we focus on the design and implementation of a 3-dB broadband stripline directional coupler. To achieve strong coupling of 3dB, two symmetrical couplers (a single-seciton coupler and a three-section coupler) with weaker coupling are cascaded in tandem. Ideal transmission line circuit element values and physical dimensions of these two couplers are computed respectively. A model of the tandem coupler is set up using an electromagnetic simulation tool—HFSS to correct the design parameters. A 3-dB broadband directional coupler is constructed according to the parameters. The measuring results of the coupler show that it matches our design requirements well. This directional coupler is characterized by its high power capacity (>800W), small size, good amplitude and phase balance over a broad band frequency range (100~500MHz).Finally, modifications are made in order to obtain better amplitude balance and to make the coupler operate at lower frequencies (30~150MHz).