In this communication, a high-gain planar antenna with a conical beam (CB) is presented for millimeter-wave drone applications. The proposed antenna comprises a vertical monopole and a circular side reflectors. The high-gain CB is achieved by in-phase reflections from the planar annular-distributed side reflectors which is realized by a series of via walls using the printed-circuit-board (PCB) fabrication. The proposed structure was numerically and experimentally demonstrated. Full-wave analysis shows a 10 dB impedance bandwidth (BW) of 26.50–29.96 GHz, corresponding to 12.26% of the fractional BW, whereas the measured data account for 11.6% in the frequency range of 26.63–29.91 GHz. Meanwhile, a maximum peak gain of 8.51 dBi is observed at a beam-pointing angle of 22°, with a high radiation efficiency of 88.3%. Owing to these valuable features of high gain, good CB radiation performance, and simple planar structure, the proposed antenna is a promising candidate for millimeter-wave drone applications.

In this study, we propose a broadband and wide-incidence angle optically transparent metamaterial absorber (MMA) using eight-resistive-arm cells. The MMA uses soda-lime glass as the substrate and indium tin oxide (ITO) as the conductive pattern. Leveraging these materials, the proposed MMA achieves optical transparency of over 78.06% at 550 nm. Simulation results indicate an absorption ratio exceeding 90% in the 7.49-10.01 GHz band for all polarizations under normal incidence. The proposed MMA is verified through fabrication and measurement. Under the oblique incidence of transverse electric and transverse magnetic modes, the absorption ratio is above 90% in the ranges of 8.0-9.01 GHz up to 50 ${}^{\circ }$ and 9.34-11.83 GHz up to 60 ${}^{\circ }$ .

In this paper, we propose a switchable band-pass/band-stop filter using liquid metal alloy as a fluidic switch. The filter is designed based on the Chebyshev response and implemented using a three-stage quarter-wavelength resonant structure. The fluidic switch is realized by injecting eutectic gallium–indium (EGaIn) in the microfluidic stubs, engraved in the polydimethylsiloxane (PDMS) material. When the fluidic switch selects the short stub using a micro-pump and microprocessor for switching, the filter acts as a bandpass filter (BPF) with the short stubs. When the fluidic switch selects the open stub, the filter acts as the bandstop filter (BSF) with the open stubs. At the BPF mode, the center frequency is 2.5 GHz and the 1-dB bandwidth is 1.75–3.07 GHz. The insertion loss is 0.5-dB ± 0.4-dB. At the BSF mode, the 15-dB bandstop bandwidth is 2.4–2.65 GHz with 2.5 GHz center frequency.