In-plane cavity-backed coplanar waveguide to rectangular ...

www.ietdl.orglow-cost microfabrication techniques such as lithography [17,18]. However, in these techniques, the height of thewaveguide is limited by the maximum thickness of the spunphotoresist, limiting the fabrication to the reduced-heightwaveguides which suffer from high attenuation. Takingadvantage of the ‘snap-together’ technique, a rectangularwaveguide was fabricated in two halves and then the halveswere put together to form a complete waveguide [19–21].An alternate technique to etch the waveguide is deepreactive ion etching (DRIE) of silicon. Unlike wet etchingwhich is dependent on the crystal planes of silicon, DRIE isisotropic and provides vertical sidewalls. Hence, DRIE is aviable approach for fabrication of a high-performancemicromachined waveguide structure. In [22, 23], a transitionusing microfabrication processes with separately fabricatedand assembled probe has been reported for both diamondand rectangular waveguide with 20% bandwidth. Anotherhigh-precision silicon micromachined transition with acapability to integrate filters has been proposed in [24, 25]and shows wideband characteristics at the same frequencyrange. However, limitations of microfabrication processesdo not allow fabricating many of the aforementionedtransitions because of the complexity of the geometriesand the number of steps needed in their assembly.In this study, we propose an in-plane cavity-backedcoplanar waveguide (CBCPW) line-to-rectangular waveguidetransition with prismatic features that does not requiremultiple parts and complex assembly. In this approach, theneed for fabricating suspended probe is eliminated andeffective transition is achieved using two resonant structures,namely, shorted CPW line over the waveguide followed by anE-plane step discontinuity. Since the design is very simplewith the features aligned with the Cartesian coordinate planes,it is highly compatible with microfabrication processes.However, it should be mentioned that since the design isusing short-circuited pin, the transition bandwidth issomewhat limited. The transition is modelled by an equivalentcircuit to help with the initial design which is then optimisedusing a full-wave analysis. To demonstrate the validity of thetransition and its model, a back-to-back structure is fabricatedby conventional machining methods at Ka-band and themeasurement results are compared to the simulations.1 Design considerations of waveguidetransitionsTraditional transitions based on E-plane probe excitation ofthe waveguide mode involve attaching a suspended probeto the centre conductor of CPW or coaxial linesperpendicular to the broad wall of waveguides as shown inFig. 1a. The suspended probes in waveguide can be viewedas an infinite array of dipoles and thus can be matched rathereasily to a coaxial line and provide octave bandwidth.The coaxial line-to-waveguide transitions are commonlyused at microwave frequencies since fabrication andassembly is rather straightforward. At high millimetre-waveand submillimetre-wave frequencies where dimensions arevery small, suspending small metalised probes and assemblywith the required tolerance is a challenging task. If the probeis extended, all the way to the lower plate of the waveguideto form a shorting pin then the pin can be formed on samesubstrate and can be bonded to a top substrate to make therequired electric connection. However, a short-circuitedprobe is not resonant, acts purely reactive and cannot bematched to the CPW line directly. In the next section, thearchitecture needed for making the transition from CPW torectangular waveguide using a shorting pin is described.2 Transition using a shorting pinTo properly excite a waveguide with a shorting pin, a resonantcondition must be achieved to eliminate the reactance of thepin. It is well known that a pin terminated by the broadwall of a rectangular waveguide acts as an inductiveelement whose inductance is inversely proportional to itsdiameter and the waveguide dimensions [26]. The geometryand the equivalent circuit model of a shorting pin areshown in Fig. 1b. For this case the transformer turn ratiocan be calculated from [27]√( )2a tan ka 2n =b ka(1)Fig. 1Design considerations of waveguide transitionsa Traditional resonant probe excitation for waveguideb Probe is terminated by the broad wall of the waveguide. In the equivalent circuit mode, L p is the equivalent inductance (X p ), C b is the series capacitance of theshort-circuited probe (X b ) [26] and Z 0 is the characteristic impedance of the waveguidec E-plane step discontinuity generates capacitances required for resonance needed for the mode conversion c the equivalent circuit model for the proposedresonanced The equivalent circuit model for the proposed resonance, X step is the equivalent capacitance of the step discontinuity444 IET Microw. Antennas Propag., 2012, Vol. 6, Iss. 4, pp. 443–449& The Institution of Engineering and Technology 2012doi: 10.1049/iet-map.2011.0194