This paper proposes a gain-enhanced metamaterial (MM) absorber-loaded monopole antenna that reduces both radar cross-section and back radiation. To demonstrate the proposed idea, we designed a wire monopole antenna and an MM absorber. The MM absorber comprised lumped elements of subwavelength unit cells and achieved 90% absorbance bandwidth from 2.42–2.65 GHz. For low-profile configurations, the MM absorber was loaded parallel to and 10 mm from the monopole antenna, corresponding to 0.09 λ₀ at 2.7 GHz. The monopole antenna resonated at 2.7 GHz with a 3.71 dBi peak gain and 2.65 GHz and 6.46 dBi peak gain, before and after loading the MM absorber, respectively. Therefore, including the MM absorber increased peak gain by 2.7 dB and reduced back radiation by 15 dB. The proposed antenna radar cross-section was reduced by 2 dB compared with a monopole antenna with an artificial magnetic conductor.

This paper proposes a deployable broadband millimeter-wave metasurface lens antenna. The deployability is realized by way of the 3D-printed accordion origami structure between the source and the lens. The metasurface lens is designed on the 3D-printed origami with two 3D-printed rectangular gratings and an inkjet-printed I-shaped pattern. The broad bandwidth is achieved with a high transmission efficiency while full 360º phase coverage is obtained from the orthogonal polarization conversion characteristic of the unit cell. The performances of the proposed metasurface lens antenna are numerically and experimentally demonstrated. Its measured 10-dB impedance bandwidth is 24.0–34.9 GHz (38%) and 3-dB gain bandwidth is 25.0–33.5 GHz (30%). Moreover, its overall volume is reduced to 1/3 of its original size when folded for idle mode. Our simple additive manufacturing process can produce the additional advantages of lightweight and low cost.

Despite the low cost and easy fabrication afforded by 3D printing technology, high dielectric loss of the 3D printing filament degrades the performance of 3D printed radio frequency electronics at high frequencies. In this paper, we propose a 3D-printed substrate-integrated waveguide (SIW) using a lossy polylactic acid or polylactide (PLA) filament. To minimize the insertion loss of the SIW, a non-uniform honeycomb is 3D-printed as the dielectric material of the SIW. The non-uniform honeycomb-shaped substrate is composed of large unit cells, which achieve low insertion loss (0.01 dB/mm) because of the low volume of the dielectric material. The performance characteristics of the proposed SIW was compared with those of SIWs made of solid and uniform honeycomb-shaped structures. The average insertion loss of the microstrip line-fed SIW with the proposed non-uniform honeycomb substrate is 0.92 dB in the frequency range of 1.97 to 3.35 GHz and those of the solid PLA and uniform honeycomb substrates are 2.49 dB and 1.38 dB, respectively. The proposed 3D-printed SIW additionally has the advantages of light weight and low cost.

This paper proposes a millimeter-wave lens antenna using 3-dimensional (3D) printing technology to reduce weight and provide stable gain performance. The antenna consists of a four-layer cylindrical gradient-index (GRIN) lens fed by a wideband Yagi antenna. We designed a fractal cell geometry to achieve the desired effective permittivity for a GRIN lens. Among different candidates, the honeycomb structure is chosen to provide high mechanical strength with light weight, low dielectric loss, and lens dispersion for a lens antenna. Therefore, the measured peak gain was relatively flat at 16.86 ± 0.5 dBi within 25−31.5 GHz, corresponding to 1 dB gain bandwidth = 23%. The proposed 3D-printed GRIN lens is cost-effective, with rapid and easy manufacturing.

In recent years, replacing the external control stimuli with internal control using characteristics of radio frequency (RF) signals (such as waveform or power level) has attracted considerable attention to the design of reconfigurable multifunctional RF devices. However, even with the most exciting techniques, the control process always needs a chip-based system to sense the power level or waveform of the incident RF signal, which is realised by additional supporting electronic components of the sensing and microcontroller circuits. Therefore, to achieve a batteryless structure, a majority of conventional works focus on using energy harvesters to convert energy from external environmental sources to DC energy for the electronic circuits. Herein, we propose a novel alternative approach to replace the traditional energy harvesters in batteryless RF devices with all-passive energy smart-sensing circuits. By exploiting the features of a nonlinear semiconductor device under different incident RF power values, the proposed network can passively self-sense the RF power level and dynamically self-control RF signal flow and power ratio. The operation of the proposed network can be considered as purely self-adaptation with control from the RF power level. Moreover, normal RF devices can be transformed to all-passive and batteryless purely self-reconfigurable devices via integration with the proposed structure. As proof of concept, the proposed network is integrated with a two-port antenna to experimentally demonstrate its purely self-reconfigurable polarisation. The proposed strategy is hereby expected to extend the field of batteryless self-reconfigurable multifunctional RF devices and pave promising new paths for the development of future intelligent, smart RF devices.