An inkjet-printed, fully passive sensor capable of either humidity or gas sensing is presented herein. The sensor is composed of an interdigitated electrode, a customized printable gas sensitive ink and a specialized dipole antenna for wireless sensing. The interdigitated electrode printed on a paper substrate provides the base conductivity that varies during the sensing process. Aided by the porous nature of the substrate, a change in relative humidity from 18% to 88% decreases the electrode resistance from a few Mega-ohms to the kilo-ohm range. For gas sensing, an additional copper acetate-based customized ink is printed on top of the electrode, which, upon reaction with hydrogen sulphide gas (H₂S) changes, both the optical and the electrical properties of the electrode. A fast response time of 3 min is achieved at room temperature for a H₂S concentration of 10 ppm at a relative humidity (RH) of 45%. The passive wireless sensing is enabled through an antenna in which the inner loop takes care of conductivity changes in the 4–5 GHz band, whereas the outer-dipole arm is used for chipless identification in the 2–3 GHz band.

A dual-band UHF/ISM compact rectenna with wideband UHF matching (855-935 MHz) covering all, EU, US and Chinese UHF RFID standards is presented. The proposed design consists of a compact single-port, dual-band antenna, a 3-port dual-branch matching network, and two voltage doubler rectifiers implemented with Schottky diodes, and terminated with a common 10 kΩ load. For each band the measured RF-to-DC efficiency is around 60% for a -5 dBm unmodulated signal. The wideband behavior of the rectenna supports harvesting of multi-tone signals. The use of a six-tone signal increases the maximum RF-to-DC efficiency of the dual-band rectifier, from approximately 60% for a single-tone UHF signal (890 MHz), to 80% for a six-tone signal, for a received RF power of -5 dBm in both cases. A signal generator is used to generate a six-tone signal of equally weighted and equally spaced (10 MHz separation) monochromatic tones, with a total bandwidth of 50 MHz. The measurement results demonstrate that the DC voltages rectified from the two separate RF signals are effectively superimposed on the load, providing a clear advantage compared to single-band rectennas. The single-layer, compact design with a size of 80 × 45 mm2, with a high RF-to-DC efficiency, is an excellent candidate for wirelessly powered RFID tags.

A dynamically reconfigurable dual-layer UWB antenna integrated with an energy harvesting system for powering a GaAs FET switch is presented. The UWB antenna dynamically creates a notch-band in the presence of an interfering signal at 5.6 GHz and it goes back to normal UWB operation when the interferer is removed. For the switching operation, the FET switch is powered using only harvested energy carried by the interfering signals. The UWB antenna on the front layer is a microstrip-fed monopole with an embedded elliptical slot. Inside the slot a quarter wavelength linear stub acts as a resonator and it is connected and disconnected using the low-power FET switch. The UWB antenna shares the RF ground with a very compact energy harvesting system, that consists of a planar inverted-F antenna, a very compact voltage doubler rectifier and a passive DC-to-DC boost converter. The boost converter elevates the rectified voltage to above the 3.3 V threshold, which is the minimum voltage needed for the actuation of the FET switch. The dynamic notch-band reconfiguration of the UWB antenna without the need for any external DC power source is made possible when the collected power at the input of the rectifier is higher than -12 dBm.

A reconfigurable bandpass filter (BPF) with constant centre frequency and wide tuning range of bandwidth is presented. The proposed filter is based on the novel configuration of two modified asymmetrical π-sections with folded shorted stubs loaded on a ring resonator. The novel topology allows the implementation of three reconfigurable passband states. A detailed theoretical analysis for calculating the transmission zeros using ABCD parameters is shown, and the design approach of implementing the random offset -x-, asymmetrical π-sections with the folded stubs to get the bandwidth reconfigurability is thoroughly explained. PIN diodes are utilized as switching elements to achieve narrowband/mediumband/wideband response. The measured results indicate an absolute bandwidth tuning range of 1.3 GHz, whereas the -3 dB fractional bandwidth (FBW) can be varied from 37% to 92% while the centre frequency remains constant at 2.4 GHz. The fabricated prototype reconfigurable BPF demonstrates one of the widest 3-dB FBW tuning ranges and one of the highest absolute tuning ranges with a sharp-skirt characteristic. It has also more than 20 dB attenuation in the upper stopband which is extended to 2.7 times the centre frequency.

A novel dynamically reconfigurable bandpass filter (BPF) employing stepped-impedance resonators (SIRs) that can operate as either a single-band or a dual-band filter is demonstrated. The reconfigurable BPF uses four positive-intrinsic-negative (p-i-n) diodes as switching elements. With the four p-i-n diodes in the ``OFF'' state, the filter behaves as a low-loss (0.85 dB) single-band BPF with a passband around 2.45 GHz. The reconfigurable SIR filter can operate as a dual-band BPF with two center frequencies at 1.6 and 2.45 GHz. The diodes are dynamically set to the ``ON'' state in the presence of a 1.6-GHz RF signal that is received by an RF triggered power management unit (PMU), integrated on the back side of the microstrip filter in a dual-layer architecture. The RF triggered PMU consists of a planar inverted-F antenna, a high-efficiency voltage doubler rectifier (47% at -13 dBm) and an active dc-to-dc power booster. The rectified output voltage is used as the enabling voltage for the dc-to-dc power booster. This, in turn, provides the required dc power for the diodes biasing. The filter switches from single-band to dual-band when a wireless input RF signal (>-13 dBm) is received by the RF triggered PMU antenna.