Compact Wideband Balun and Balun Bandpass Filter Based on Broadside Coupled Three-Line Structures

An ultra-wideband balun prototye and a wideband balun bandpass filter (BPF) with compact sizes are proposed based on the coupled three-line structure. Firstly, the ultra-wideband balun is constructed by a partially coupled three-line where the terminal of central half-wavelength
$(\lambda_{\mathrm{g}}/2)$
input line is grouded and two
$\lambda_{\mathrm{g}}/2$
sidelines are open-circuited with two output feeding ports oppositely placed. By improving the coupling strength with broadside coupling implemented by two-layer stacked subatrates, ultra-wideband performance can be achieved. Then, by loading two
$\lambda_{\mathrm{g}}/4$
short-ended stub resonators at both open terminals of the
$\lambda_{\mathrm{g}}/2$
sidelines, high selective filtering response with two transmission zeros around the passband is obtained. Then, according to the design equations derived from equivalent circuit, two prototypes of the proposed balun and balun BPF are designed in theory and simulated. Final results show that the wideband balun BPF is centered at 1.7 GHz with a 3-dB fractional bandwidth of 85.8% and a compact size of
$0.35\lambda_{\mathrm{g}}\times 0.23\lambda_{\mathrm{g}}$
. Within the passband, the phase difference is within 180°±2° and the amplitude imbalance is less than 0.2 dB.