In this thesis the development of a high efficiency harmonic re-radiator, consisting of a diode doubler and conjugate-matched receive and transmit antennas, is described. Diode-based frequency multipliers and rectifiers, coupled with antennas, are of interest for quasi-optical applications, for energy-scavenging and for sensing applications. The device studied operates by receiving an interrogating signal at a frequency of 1.3 GHz and re-radiating a signal at 2.6 GHz. The primary goal of this research was to develop a passive, miniature and effective frequency doubler integrated with planar antennas. The system is referred to as a frequency doubling reflectenna, (FDR). Prediction of accurate performance was achieved by employing precise modeling and measurement methods. The FDR can be utilized in data collection applications. The footprint of the FDR is occupied primarily by the receive and transmit antennas. Therefore, a significant portion of the research focused on the development of compact and efficient planar antennas, which would provide for a miniature FDR. A first-generation FDR design was designed, which utilized quarter-wavelength shorted microstrip patch antennas. The choice of antennas provided a small prototype with dimensions equal to 44 mm by 17 mm. In order to further reduce the size of the harmonic re-radiator, meandered planar antennas were investigated and optimized for efficient operation. A second-generation FDR design, which utilized meandered microstrip patch antennas, was produced and a size reduction of 75% was achieved. Both first- and second-generation harmonic re-radiator designs were designed for low input power operation and provided maximum measured conversion efficiencies of approximately 4.5% and 1.8%, with the input to the diode doubler at -14.5 and -17.5 dBm, respectively. Re-configurable microwave devices, which dynamically operate at different frequencies, are often desirable for sensing applications. Therefore, to conclude this research, a tunable FDR was realized using a semiconductor varactor that provided the dynamic capacitance required for the tunability.

This thesis presented the research, design and fabrication associated with a unique application of rectenna technology combined with lock-in amplification. An extremely low-power harmonic transponder is conjoined with an interrogator base-station, and utilizing coherent demodulation the Remote Lock-In Amplifier (RLIA) concept is realized. Utilizing harmonic re-radiation with very low-power input, the 1st generation transponder detects a transmitted interrogation signal and responds by retransmitting the second harmonic of the signal. The 1st generation transponder performs this task while using no additional power besides that which accompanies the wireless signal. Demonstration of the first complete configuration provided proof of concept for the RLIA and feasibility of processing relevant information under "zero" power operating conditions with a remote transponder. Design and fabrication of a new transponder where the existing zero-bias transponder was modified to include a DC bias to the diode-based frequency doubler is presented. Applied bias voltage directly changed the impedance match between the receiving 1.3 GHz antenna and the diode causing a change in conversion loss. Testing demonstrated that a change in conversion loss induces an amplitude modulation on the retransmission of the signal from the transponder. A test of bias sweep at the optimal operating frequency was performed on the 2nd generation transponder and it was seen that a change of ~ 0.1 V in either a positive or negative bias configuration induced an approximate 15 dB change in transponder output power. A diode-integrated radar detector is designed to sense microwaves occurring at a certain frequency within its local environment and transform the microwave energy to a DC voltage proportional the strength of the signal impinging on its receiving antenna. The output of the radar detector could then be redirected to the bias input of the 2nd generation transponder, where this DC voltage input would cause a change in conversion loss and modulate the retransmitted interrogation signal from the transponder to the base station. When the base station receives the modulated interrogation signal the information sensed by the radar detector is extracted. Simulations and testing results of the fabricated radar detector demonstrate capability of sensing a signal of approximately -53.3 dBm, and accordingly producing a rectified DC voltage output of 0.05 mV. A comparison is made between these findings and the transponder measurements to demonstrate feasibility of pairing the radar detector and the 2nd generation transponder together at the remote sensor node to perform modulation of interrogation signals.