Screen-Printable Flexible Textile-Based Ultra-Broadband Millimeter-Wave DC-Blocking Transmission Lines based on Microstrip-Embedded Printed Capacitors

In this paper, a novel multi-layered microstrip line with built-in parallel-plate capacitors is proposed for DC-blocking applications, with its transmission characteristics measured up to 50 GHz. The microstrip lines are fabricated via screen printing directly onto polyurethane films laminated on standard textile substrates which would otherwise be unsuitable for printing. Compared to a standard microstrip line on the same substrate, the proposed 10 cm-long line on felt (with an embedded 44 pF capacitance) suffers from less than 0.1 dB higher insertion loss up to 4 GHz. Furthermore, varying the overlapping length of the lines and hence the capacitance enables the realization of DC blocking and $-$ 3 dB high-pass filtering with pass-bands starting between 88 MHz and 1.2 GHz. This is achieved without altering the cut-off frequency of the microstrip line’s mode-free propagation, measured up to 50 GHz, exhibiting a low attenuation of 0.32 dB/mm at 50 GHz on a felt fabric substrate. Compared to a lumped capacitor, the proposed microstrip-embedded printed capacitor demonstrates a significant improvement in mechanical reliability, withstanding over 10,000 bending cycles, and RF power handling with under 6  $^{\circ }$ C temperature rise at 1 W. The lines are fabricated on two textile substrates and their transmission characteristics were measured up to 50 GHz, which represents the highest frequency characterization of textile-based lines to date, demonstrating a stable group delay and insertion losses. Based on the proposed multi-layered integration method, low-cost screen-printed microstrip-embedded capacitors on textiles can be used for microwave applications up to mmWave bands.