Design of ultra-wideband 3dB tandem hybrid coupler using multi-element coupled lines

Abstract

Dissertation presents the design and simulation of 50W, 140-240MHz, 3dB hybrid coupler using multi-element coupled lines and its applicability in development of mockup 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 it also provides essential protection to the RF generator by coupling of reflected power to isolated port. A single quarter wavelength coupled element provides narrow frequency band and is used in development of narrow band 3dB hybrid coupler. For the broadband applications, multi-element coupled lines are used where several number of quarter wavelength element are cascaded in a specific manner. Coupling of each of the cascaded element is calculated using theory of equal ripple polynomial i.e. based on curve fitting method. This theory is also explained in detail in thesis and values of coupling for 3-elements coupled lines are derived. 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. Therefore, development of broadband 3dB coupler using single section of multi-element coupled lines could not be possible due to fabrication constraints. 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, ultrawide band hybrid coupler is designed. Here, result of both the devices is presented in comparative manner.