A four-way filtering power divider (PD) based on a substrate-integrated waveguide (SIW) is presented. This structure is composed of two layers of the dielectric substrate. The eighth-mode SIW (EMSIW) technique is used to make four cavities in the upper substrate. In addition, an SIW cavity is provided in the lower substrate. To achieve an efficient transition between the two layers, four rectangular slots are etched on the middle metal planes. A circular slot is etched on the top metal layer of the EMSIW cavities to suppress the higher operating modes. To prove the validity, a four-way PD with a second-order bandpass filtering response is designed and fabricated. The measured results show that the PD achieves an insertion loss of 1.3 dB at 3.5 GHz, a return loss of higher than 17 dB, a 3-dB fractional bandwidth of 7.75%, and an out-of-band rejection of greater than 26 dB from 3.92 to 16.5 GHz. The results show that this PD has the advantages of compact size, high selectivity, and ultrawide out-of-band rejection.

This paper presents a fully combined sensor-antenna transmitter system for sensing relative permittivity of material at microwave frequencies. The transmit (Tx) antenna senses the relative permittivity of specimen and loads the radio frequency (RF) oscillator which translates the relative permittivity to the operational frequency of the carrier signal. At the same time, independent data, which can be the Radio Frequency Identification (RFID) information, are ON-OFF keying modulated over the carrier. At the receiver, a demodulator system extracts the data stream as well as recovers the carrier frequency using a zero-crossing detector. Therefore, the relative permittivity of the sample can be obtained from the extracted frequency shift as a digital number. As all the sensing elements in the proposed structure are passive, the sensor does not increase the power consumption of the system. The sensor-transceiver capability at low powers makes it ideal for sensor nodes in the Internet of Things applications. Since the sensing and demodulation are implemented at the same time, the complexity for detection reduces significantly compared with traditional RF/microwave sensing techniques which need complicated frequency spectrum monitoring equipment. The output of the system is a digital number correlated to the dielectric constant of the specimen and the independent data stream for communication. The proposed sensor node is fabricated at the 2.45-GHz ISM band as an evaluation and the measurement results with some known samples are presented.

The structure of a high-resolution microwave material sensor is presented based on the injection locking phenomenon. For a sample under test, the variation of center frequency and lock range of the oscillator with respect to a reference sample are measured to indicate the resonance frequency and quality factor of the resonator. A low power level and low phase noise injection locking can significantly reduce the lock range of the oscillator. Therefore, even small variations in the loop filter of the oscillator change the response and can be determined with a high sensitivity. The unknown sample is characterized based on the measured center frequency and quality factor of the resonator. An output power control (OPC) system increases the dynamic range of the sensor with respect to the dielectric constant of samples. Moreover, a method of material detection in the microwave frequency is proposed based on data from the OPC to eliminate the frequency analyzer from the system for Internet of Things (IoT) applications. As an example for validation, a 1-GHz sensor is fabricated and the measurement results are presented.

The Quadruple Loop (QL) antenna was designed and investigated to deliver a robust body-worn antenna for biomedical applications. The QL antenna is a very thin singlelayer groundless structure which allows for the installation of neighboring electronics and sensors which need to be extremely close to the epidermis layer. The performance of the proposed antennas was investigated for three main standards: GSM-900, GSM-1800, and Bluetooth Low Energy (BLE). Radiation pattern measurements were conducted to compare their results with their simulation counterparts. Moreover, data communication tests showed that the acquired BER readings could qualify the BLE and GSM-900 antennas to work as part of 4-QAM wireless links. Besides these hands-on endeavors, an equivalent and more efficient mathematical model of biological tissues which make up the human arm was computed. The equivalent model was tested in software and has the practical potential to be realized in the physical phantom making.

The emergence of the Internet of Things (IoT) necessitates the development of electronic components with various form factors and mechanical properties. 3D printing is an effective tool to realize objects with arbitrary form factors. Various 3D printable materials have recently been commercialized; among them, stretchable materials are particularly useful in the IoT because they enable adaptability in the dimensional change of the electronics. Most of these stretchable materials are, however, not electrically conductive; conductive coating can enable the functionality. Here, we propose a self-reinforcing conductive coating strategy, which reduced graphene oxide (RGO) self-assembles to wrap graphene nanoflakes (GNF) as a conductive binder that can also achieve mechanical integrity. The conductivity of the GNF-RGO coating reaches 4.47 × 10 ⁴ S m ⁻¹ . To demonstrate the potential applications of the GNF-RGO coating, applying the coating on 3D printed porous elastomers enabled flexible radio frequency (RF) antennas and strain sensors. The RF antenna shows high radiation efficiency and maintains excellent performance under bending conditions. The coating also produces a strain sensor with a gauge factor of ∼13 up to 40% of strain. We foresee that the electrically conductive GNF-RGO composite coating can provide versatile functionalization strategy in flexible electronics and in wearable biomedical devices.