A Miniaturized Low-Pass Filtering Balun for Cross-Band Isolation Enhancement in Triple-Band Shared-Aperture Antenna Arrays

Shared-aperture antenna arrays are promising for the integration of multiple antenna array apertures in a limited space, ensuring sufficient frequency bandwidth and angular coverage. However, these arrays often suffer from serious cross-band coupling, leading to deterioration in radiation performance. This letter proposes a novel balun integrated with miniaturized open-circuited stubs. By cascading multiple quarter-wavelength open stubs and strategically bending them to fit the maximum width, the proposed balun introduces transmission zeros that provide filtering capability for electromagnetic interference from other frequency bands. An equivalent circuit model is then developed to facilitate the integration of the balun with the antennas. With the novel miniaturized filtering balun and the equivalent circuit model, a triple-band antenna array for the fifth-generation base stations is developed. A prototype is fabricated and measured to validate the effectiveness of the proposed approach. Measured results illustrate that the isolation between the low-band (0.69 GHz to 0.96 GHz) and middle-band (1.90 GHz to 2.70 GHz) is improved from 18.0 dB to 32.0 dB in the middle-band, and the isolation between the low-band and high-band (3.30 GHz to 3.90 GHz) is improved from 25.0 dB to 36.0 dB in the high-band.