A substrate integrated waveguide (SIW) cavity backed active slot antenna oscillator is proposed, the operation frequency of which can be tuned by means of a properly connected varactor. The cavity backed structure permits overcoming potential problems such as heat dissipation and unwanted surface wave modes in large array implementations, and may provide better phase noise performance. The use of SIW technology permits a cost-effective fabrication process and simple implementation at millimetre-wave frequency, for radar and communication applications. Full-wave FEM-based analysis is used for the design of the radiating structure, while harmonic balance and transient simulations are employed in the design of the active antenna oscillator and in the study of the stability of its steady-state solutions. Tunable active antenna prototypes have been implemented and measured.Introduction: During recent years, the demand for low profile, compact and low-cost antennas has experienced unprecedented development, along with growing interest for radar and wireless communications. Substrate integrated waveguide (SIW) technology, the features of which have been widely studied in the literature, allows for the implementation of compact, cost-effective waveguide-like structures using standard printed circuit board (PCB) techniques. SIW structures feature low loss, high quality factor, good power handling capability, and can be easily integrated with planar circuitry. At present, several active and passive SIW circuits have been demonstrated [1]. Low profile cavity backed antennas use a cavity to suppress undesired surface-wave modes, and provide enough metal surface to prevent heat dissipation problems, resulting in great advantages for large array configurations [2 -4]. In addition, a proper design of the cavity can be used to improve the phase noise performance of the oscillator [5].In this Letter, a compact, tunable SIW cavity backed slot antenna oscillator is presented. The design has been carried out in two steps. First, the radiating structure has been designed using a finite element method (FEM)-based simulation software (Ansoft HFSS); then, harmonic balance (HB) and transient simulation have been used to design the active oscillator antenna and to verify the stability of the various steadystate solutions. The tuning of the frequency of operation has been obtained by properly connecting a varactor to the SIW cavity. The dependence of the oscillation frequency of the structure with respect to variations in the biasing voltage of the varactor has been studied, and the phase noise performance of the antenna oscillator has been measured. Compact, single substrate active antenna oscillator prototypes were built, thus demonstrating the validity of the design.

In this paper, a hybrid solar/electromagnetic (EM) energy harvester that operates at 2.45 GHz is presented. The proposed harvester integrates the solar cells in the same area as the rectenna element obtaining a compact implementation. The radiating element that forms part of the rectenna is a cavity-backed slot antenna based on substrate-integrated waveguide technology, which allows for a compact, single substrate implementation. The radiating element is connected to a circuit that provides both the rectification of the incoming EM signals and the collection of DC energy coming from solar cells. A single-substrate prototype has been implemented, demonstrating an overall power conversion efficiency up to 30%, depending on the incoming radio frequency signal level and the ambient light conditions.

This article presents a very efficient technique for the design of filters in substrateintegrated waveguide (SIW) technology. The proposed design approach is based on the combined use of equivalent circuit models of SIW discontinuities and a ''calibrated'' space-mapping optimization technique. The effectiveness of this design technique is demonstrated through some examples.

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