Abstract
In this paper, a filtering balun is proposed based on spoof surface plasmon polariton (SSPP) structure which composes of symmetric H-shaped periodic metallic strip, two types of microstrip-slotline cross-coupling structures and the transition structure between them. The SSPP structure is applied to support odd mode propagation with 180° phase difference and equal magnitude along both sides of the symmetric line, which could transmit the differential signal required for a microwave balun. And then, a new approach for effectively stimulating the odd mode of symmetric H-shaped SSPP structure based on waveguided slotline with a gradient transition structure is proposed and analyzed in detail. In addition, the dispersion characteristic of SSPP can intrinsically bring filtering feature with wide stopband in the balun design. Meanwhile, the two forms of microstrip-slotline cross-coupling structures can not only feed guided wave and realize a pair of balanced ports, but also improve the out-of-band rejection below the objective filtering band. For demonstration, a prototype filtering balun with operating frequency around at 2 GHz is designed, fabricated, and measured. The measured results agree well with the simulated ones which validate the proposed design method and reveal certain advantages of low insertion loss, good amplitude balance and out-of-phase performance as well as the wide stopband.
Highlights
Ultra-thin spoof surface plasmon polariton (SSPP) structure as a kind of novel electromagnetic (EM) wave propagation structure in microwave region has attracted much attention since it was first proposed [1]
It can be concluded that the parameter h is one of the dominant influence to the cut-off frequency of the SSPP structure, where an increase/decrease of the parameter h will lead to the decease/increase of the frequency
In order to utilize the outof-phase characteristics in the odd mode of SSPP as discussed above, here we propose to excite the odd mode SSPP based on the odd mode of coplanar waveguide (CPW) and apply it to the design of the balun
Summary
Ultra-thin spoof surface plasmon polariton (SSPP) structure as a kind of novel electromagnetic (EM) wave propagation structure in microwave region has attracted much attention since it was first proposed [1]. Various types of SSPP structures and functional devices in microwave components have been developed [3]–[14] including filters [4]–[7], antennas [9]–[11], splitters [12], [13] and so on Among these studies, the symmetricallymonometallic SSPP structures could support the odd-mode EM wave with equal-amplitude and reverse-phase signal [14]. The symmetricallymonometallic SSPP structures could support the odd-mode EM wave with equal-amplitude and reverse-phase signal [14] Such feature is very beneficial for the design of differential signal devices including balun with filtering characteristics. Filtering baluns that utilize the microstrip-slotline cross-coupling structure to construct balanced ports with intrinsic 180◦ phase difference are proposed in [18], [19] They are mainly based on the use of the traditional microwave transmission structures. The measured results clearly indicate that the proposed filtering balun could exhibit good performances which verifies the design feasibility
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