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.

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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 selfsense 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.

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