Design and Simulation of Ultra-Wideband 3 dB Hybrid Tandem Coupler

In this paper, the design and simulation of 140-240MHz, 3dB hybrid coupler using multi-element coupled lines and its applicability in development of mock-up fast RF matching network is discussed. In mock-up RF matching network, 3dB hybrid coupler is used to divide the RF power coming from the RF source and to couple reflected power to isolated port for the protection of source. A single quarter wavelength coupled element provides narrow frequency band, thus for the broadband applications, multi-element coupled lines are used where several number of quarter wavelength element are cascaded in a specific manner. In multi-element coupled lines, coupling of middle element is found to be much higher as compared to side elements and provide small gap between the coupled lines. The increase in number of elements provide very narrow coupling gap and at a certain limit its fabrication becomes very difficult. To achieve the wider coupling gap in middle element, two broad band 8.34dB multi-elements sections are connected in tandem to have overall 3dB coupling. Initially a narrow band single element 3dB hybrid coupler is designed, simulated and presented in detail. It provides very narrow frequency band which is not sufficient for the application of mock-up RF system and therefore, ultra-wide band hybrid coupler is designed. Here, result of both the devices is presented in comparative manner.