Progress in SIW distributed amplifiers
2nd Australian Microwave Symposium 2016, Adelaide, Australia
Substrate-integrated waveguide (SIW) leverages on the advantages of waveguide technology and planar circuit (PCB, LTCC or thin-film MIC) manufacture. The ultimate aim of this work is to develop a wide-band power amplifier that integrates the power transistors with SIW to leverage on the superior power handling capability of SIW compared to microstrip. Since rows of vias are used to realize the SIW side-walls, DC biasing of active devices directly connected to SIW is impossible. This typically means active circuits are realized in microstrip that is coupled into SIW components . To overcome this limitation, the author developed the half-mode corrugated SIW which has similar characteristics to half-mode SIW and yet permits direct connection of transistors and application of DC bias .
The first SIW distributed amplifier described comprised a pair of half-mode corrugated SIWs that were periodically coupled by transistors as shown in Fig. 1 . Despite demonstrating wide-band performance, the first SIW distributed amplifier suffered from a significant amount of passband gain ripple. The ripple has been attributed to spurious modes in half-mode corrugated SIW , and non-ideal performance of waveguide feeds . The former is resolved by a modified waveguide structure , and the later has been partially resolved by the development of an integrated DC decoupling for half-mode SIW . The remaining challenges include: (i) fundamental distributed waveguide amplifier theory, and (ii) improvement of the SIW feeds.
Fundamental distributed waveguide amplifier theory combines both circuit and electromagnetics to quantify coupling between the transistor and the fields within the waveguides. Paradoxically, FET parasitic capacitances equalize  the inherent gain roll-off . Theory of periodically loaded half-mode SIW  will be used to quantify gain equalization.
SIW feeds couple SIW to transmission lines such as coaxial cable or microstrip. Primarily, such feeds are used to connect SIW components to test equipment which uses coaxial leads. Whilst such feeds have been described in the literature, theoretical models based upon field theory are non-existent. Optimization of feeds typically requires time-consuming parametric analysis using field solvers. This approach lends little insight into fundamental operating principles and how they might be better designed. It will be the purpose of this work to model conventional SIW feeds so that optimization can proceed in an organized manner. The models will be used to develop new feeding strategies to best match fields in the SIW to fields within the feed, thereby optimizing coupling.
The presentation will describe the recent developments  –  and their impact on distributed amplifier performance as well as describe recent investigations addressing gain equalization and SIW feed improvement.