Higher-order mode antennas with improved functionalities

Abstract

This thesis focuses on the designs of a wideband dual-polarized antenna, a wideband circularly polarized (CP) slot antenna, two gain-enhanced metasurfaced CP antennas, and a high gain CP slot antenna array. A unique feature is that high-er-order mode or multi-mode is excited to design these antennas. The first antenna employs a cross slot with inductor loadings for dual polariza-tions. To achieve a compact size, a very small ground having the same hypotenuse length as the slot, is adopted. The inductor loadings are used at both ends of the slot to further reduce the size. Simulated results show that the loaded inductors contribute to a more constant impedance to the frequency response at 1-λ mode, and the impedance moves towards the lower frequency with inductors. Based on the constant impedance of the inductor-end slot at 1-λ mode, a wideband dual-polarized antenna using two orthogonal slot radiators is designed with stable beamwidths. The second antenna uses the 1- resonant mode of a slot radiator with single feeding for bandwidth enhancement. The impedance of a simple slot radiator is studied using computer simulations. A large square slot is used at both ends of the slot to reduce the required slot length. Based on it, a wideband CP cross slot an-tenna (CSA) with a single feeding is then designed. By loading reactive elements with appropriate values on each slot near the feed line, the two slots generate two orthogonal linearly polarized electric fields with 900-phase difference for CP oper-ation. In the third antenna, an metasurface (MS) is applied to a slot antenna for CP op-eration. The antenna consists of a slot antenna and an MS placed at a short distance above the slot antenna. The slot antenna has two slots placed in orthogonal posi-tions, with distributed elements for CP radiation. Measured results show that, with the use of the MS, the operating bandwidth can be greatly increased by exciting both fundamental and higher-order modes. Within the axial ratio bandwidth (ARBW), the realized co-polarized gain can be increased by about 3 dB on aver-age. By using two cross slots and an MS, a dual wideband CP antenna can be ob-tained, where the two bands can be tuned independently. In the last antenna, the higher-order mode and ground-edge diffraction of a slot antenna are used to achieve a high gain. To inhibit the multi-lobes at higher-order mode, ground-edge diffraction is utilized to form a single main lobe by changing the size of the ground plane of the slot radiator. Then, this slot antenna is employed to construct a windmill-shaped array antenna for CP radiation. A 4-way sequential-phase feeding network is designed to feed the four slot elements which are placed in orthogonal positions to form a windmill shape. The slot elements are excited in the higher-order resonant mode, instead of the fundamental resonant mode, and ground-edge diffraction of the slot elements is used to make the radia-tion pattern form a single strong lobe with increased gain at the boresight direction.