Previous microstrip-to-waveguide transitions either required a hermetically sealed waveguide configuration, or a balun that needed to be tuned according to the frequency band of interest. In this design, the balun is realized using a double-Y junction to transition from microstrip to coplanar strip feeding a quasi-Yagi dipole array (see figure). The length of the feed (Lf) extending into the waveguide is 15.54 mm. The length of the ground plane below the ULTRALAM substrate is 7.75 mm. The lengths L1, L2, and L3 are 8.50 mm, 4.38 mm, and 2.14 mm, respectively. These lengths were computed via a preliminary optimization aimed at improving the return loss at the band edges.

The Microstrip-to-Waveguide (WR-90) Transition employing double-Y balun and modeled in HFSS.
The waveguide feed was designed to excite the TE10 mode in a WR-90 waveguide, and to operate over the recommended frequencies of 8.2 to 12.4 GHz. The feed employs a Rogers 6010 substrate (dielectric constant εr ≈ 10.2) bonded with a Rogers ULTRALAM substrate (εr ≈ 2.5). The ULTRALAM substrate serves to provide mechanical strength for 6010 substrate, and to mitigate loses due to parasitic modes (the ground plane is etched on the bottom of this layer due to the topology of the double-Y balun).

The double-Y balun transitioning from an unbalanced microstrip line to a balanced coplanar strip (CPS) line does not provide inherent impedance transformation; hence, Klopfenstein impedance tapers were synthesized to transition from 50 to 77 Ω in the microstrip section and from 77 to 110 Ω in the CPS section. At the balun junction, the CPS stub lengths were chosen such that the λ/8 resonance is pushed outside the bandwidth of operation. Also, the smallest allowable conductor width and gap spacing were chosen to meet acceptable manufacturing tolerances.

The microstrip-to-waveguide transition has been analyzed numerically using a commercial 3D finite-element electromagnetic solver. The WR-90 waveguide (10.16×22.86×25.56 mm) was modeled as an air box. The 6010 and ULTRALAM substrates were modeled to account for dielectric losses. The microstrip section of the waveguide feed was excited using a 50-Ω lumped port; the output face of the waveguide was modeled as a wave port. The waveguide achieves maximum insertion loss of 0.84 dB, and a minimum insertion loss of 0.32 dB from 8.0 to 12.4 GHz with the ULTRALAM substrate and additional ground. The resulting insertion loss at the band edges is significantly lower. Further improvement in the insertion loss of the waveguide feed can potentially be obtained with continued numerical optimization.

This work was done by Jaikrishna Venkatesan of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Electronics/Computers category. NPO-42667

This Brief includes a Technical Support Package (TSP).
Novel Microstrip-to-Waveguide Feed Employing a Double-Y Junction

(reference NPO-42667) is currently available for download from the TSP library.

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This article first appeared in the May, 2010 issue of NASA Tech Briefs Magazine.

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