Recon gurable Antennas Based on Varactor-Loaded Stubs

Abstract

The term “reconfigurable” is typically used for devices which exhibit some flexibility of functionalities and agility in their operation characteristics, with the aim of achieving high performance in various conditions. In antenna technology, the reconfiguration can be fulfilled through several techniques that provide an ability to modify the electrical current on the antenna’s structure, primarily to accomplish a physically realised new response. The main key to the reconfigurable antenna application is their potential to avoid the use of multiple antennas for multi-functionality, thus facilitating miniaturisation of the antenna system configuration. In this context, several novel reconfigurable antennas with a wide performance range are proposed in this thesis. Varactor-loaded stubs are used as tuning mechanism for these microwave antenna designs with improved performance throughout this thesis. Two types of electromagnetic structures are studied in this work, namely reconfigurable antennas and reconfigurable periodic structures, with these two main topics building the two main major parts of this thesis. In its first main part, the thesis proposes reconfigurable antenna designs with combined frequency and pattern reconfigurable characteristics. The main focus is first on the manipulation of near-resonant current distributions in a two-element array antenna as well as the optimisation of their feeding through T-junction power dividers. Each element has a controllable active component that allows the antenna to be tuned to different operating frequencies, while the concurrent adaption of the two elements is the basis of continuous beam scanning characteristics. Next, the thesis examines the exploitation of a single-element antenna structure based on the same operation principle. An optimisation procedure including a study of relevant design parameters is also presented. The core contribution for the two-element array and the single-element antenna is that they combine frequency-reconfigurability with effective beam scanning. The main difference between the two designs however is that they scan in the H-plane and the E-plane, respectively. In the second main part, the thesis focuses on a reconfigurable reflectarray antenna design. Potential applications of this advanced antenna design include the development of high gain antennas with various controllable reflection beam directions throughout a wide range of operation frequencies. The proposed reflectarray antenna unit cell is firstly proposed, together with an opimisation of the antenna characteristics in terms of reflection loss and phase range performance. It is further shown that the proposed antenna provides an excellent performance compared to the state-of-the-art. Performance measures include a near full phase tuning range of about 300 to 320 with a reflection loss of magnitude better than 3 dB within a fractional frequency range of operation of 18%. In contrast, most reflectarray antenna designs in the literature provide a limited phase range at a fixed operating frequency or within a narrower frequency tuning range. Experimental validation is provided with a 12-element linear reflectarray tested in twodimensional settings, for which the experimental challenges are also discussed in detail. The capability of reflected beam scanning is verified and successfully demonstrated.