The Propagation Characteristics Of Air-filled Ridged Substrate Integrated Waveguide And The Design Of Directional Coupler

Substrate Integrated Waveguide(SIW)has attracted much attention since it first introduced in 2001.As it greatly improves the performance of planar transmission lines in terms of Q factor and loss while retains the advantages of low-cost and easy integration.However,one of the well-documented SIW limitations is that dielectric loss of SIW contributes the largest component of transmission loss at high frequencies.With the aim of reducing the dielectric loss of normal SIW,several special SIW with dielectric removed have been reported over the past years,including the air-filled substrate integrated waveguide(AFSIW)and the empty substrate integrated waveguide(ESIW).In fact,the ESIW devices are not compact as the corresponding SIW devices due to the absence of a dielectric.On the other hand,the normally usable bandwidth of the AFSIW and ESIW is about one octave,which has potential to be increased for some wideband applications.Ridge waveguide is an effective approach to decrease cut-off frequency of dominant mode and obtain wider bandwidth than the conventional rectangular waveguide with same width and height,because of bigger frequency separation between the dominant mode and the first higher mode.Taking advantages of both AFSIW and ridged waveguide into account,the air-filled ridged substrate integrated waveguide(AFRSIW)is proposed in this paper.AFRSIW is a novel substrate integrated transmission lines with low-loss,compactness and wide mono-modal bandwidth.With the development trend of broadband,compact and low-loss of wireless communication systems,AFRSIW has a broad application prospect.Taking the air-filled substrate integrated ridge waveguide(AFRSIW)as the theme,the work of this paper is as follows.Firstly,the influence of ridge shape on transmission characteristics of AFRSIW is studied in Ku band and X band,such as cutoff frequency,characteristic impedance and mono-modal bandwidth.Secondly For connection from the AFRSIW to traditional planar circuit for planar circuit integration and easy measurement,a wideband transition between microstrip and AFRSIW,which is consisted of a tapered microstrip line and two suppressing stubs,is proposed.The tapered microstrip line is used to ensure good impedance matching,and two suppressing stubs preventing the undesired radiation are introduced to improve the impedance match broadening the operating bandwidth of the transition.For demonstration,a back-to-back transition prototype is fabricated and measured.A mono-modal bandwidth of 74.4% with 15 d B return loss is obtained from 8.65 to 18.9 GHz.Secondly,two directional coupler based on AFRSIW is design in this paper.They are all connected to the planar circuit through the proposed transition.The first directional coupler works in the X-band.Its compact structure and small size reflect the advantages of AFRSIW's small size.The width of the waveguide is the width of Ku-band standard waveguide,the measured operating bandwidth is 8.8-10.6GHz,and the coupling degree is 10 d B.The second directional coupler is a Ku-band 3d B broadband directional coupler,with the operating bandwidth of 11.24-18.64GHz(49.5%)and the variation range of the in-band coupler is 3.6±1d B.