Compact Wideband Coupled-Waveguide Balun for Millimeter-Wave Applications

This paper presents a compact wideband balun for millimeter-wave (mmW) applications, which is constructed from tightly coupled half-mode substrate-integrated waveguides (HMSIWs). This balun takes full advantage of the frequency-independent out-of-phase property of the odd TE0.5,0mode of the coupled-HMSIW system. As a result, a phase difference of 180° and balanced amplitudes between two balun outputs are obtained inherently over a wide frequency range. Thanks to the similarity of partial transverse fields, this odd mode can be excited by a conventional slotline in a straightforward manner. This design concept is verified by both simulation and measurement. Measurement shows that this balun has a fractional bandwidth of 97%, from 17 to 49 GHz, with a return loss of 10 dB at its unbalanced port; the in-band amplitude and phase imbalances between two balanced ports are within ±0.6 dB and ±5° (±10° at around 48 GHz), respectively. In addition, the core-circuit size of the proposed balun is about 0.2 \lambda_{\mathrm{g}}\times 0.57\lambda_{\mathrm{g}}$, where $\lambda_{\mathrm{g}}$ is the guided wavelength at the center frequency of 33 GHz.