Design of Ultrawideband Directional Coupler Using Coaxial Balun and Resistive Network

In this letter, a novel type of directional coupler based on a coaxial balun and a resistive network is presented. The inner conductor of the coaxial line is directly connected to the input and output ports, while energy is coupled to the coupled port through a power-division network employing three resistors. Circuit analysis is performed using voltage and current relationships, from which the coupling coefficient and insertion loss (IL) are derived. By appropriate selection of the resistance values, constant coupling level, high directivity, and excellent port matching are achieved simultaneously. For proof of concept, a design example is proposed, simulated, and fabricated. Good agreement between the simulated and measured results demonstrates that the proposed design achieves an ultrawideband operating bandwidth (100 kHz–9 GHz), a constant coupling coefficient ($16.75~\pm ~0.75$dB), a high directivity (>24 dB), as well as excellent impedance matching at all ports (>20 dB), which well validates the design methodology.