A novel wideband unidirectional antenna


Student thesis: Doctoral Thesis

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Awarding Institution
Award date15 Feb 2007


This thesis presents a novel wideband unidirectional antenna composed of a planar electric dipole and a shorted patch antenna for exciting an electric dipole and a magnetic dipole simultaneously. A new Γ-shaped feeding strip, comparing an air microstrip line and an L-shaped coupled strip, is embedded and proposed for exciting the dipole and the patch. The advocated architecture possesses advantages for the antenna having stable radiation pattern with low cross-polarization, low backlobe radiation, nearly identical E-and H-plane patterns and stable antenna gain across the entire operating bandwidth. In addition, two alternative feeding mechanisms: T-shaped and square-plate coupled lines are developed for demonstrating the flexibility in a change of feeding mechanism. Moreover, the antenna’s size and height reduction are performed by using folding technique. Furthermore, a bandwidth enhancement is examined by a U-bridge with a twin-coupled feed. The presented antennas find applications in many recent wireless communication systems like 2G, 3G, WiFi, ZigBee etc. Firstly, the antenna with combination of a planar dipole and a shorted patch antenna is studied. To demonstrate the performance of the proposed antenna, simulation results of input impedance, SWR and gain characteristics are presented. The ground plane effect on the radiation pattern is investigated. Radiation patterns of the conventional dipole and the proposed design are compared. Experimental data are obtained to verify the theoretical prediction. The antenna is simply excited by a Γ-shaped strip line. More than 50% impedance bandwidth for SWR≤2 and 8dBi maximum gain has been achieved. This new design antenna has many advantages such as simple structure, wide bandwidth, low cross polarization, symmetrical radiation pattern, and in particular, very low back radiation. Secondly, an extension analysis for the proposed antenna is actualized. Different shape of dipoles like trapezoid, triangle, and semi-circle are examined to determine their effect on the impedance bandwidth and the radiation pattern. Besides, alternative feeding structures are suggested. They are T-shaped strip and square-cap coupled lines. The performance of electrical characteristics among three feeds: L-strip, T-strip and square-cap are compared. Obtained results confirm that the use of T-strip line or square-cap coupled gives over 60% bandwidth achieved. Thirdly, in order to establish the proposed antenna becoming more competitive, a size and a height reduction are proposed by a folding technique. This approach can effectively abbreviate the size of the antenna and dedicates a small wideband unidirectional antenna element. There is a 59% reduction in the lateral length of the antenna and there is a 40% deduction in the antenna height. Finally, a bandwidth enhancement for the proposed antenna is investigated. One solution is to apply a twin-Γ feed to enhance the bandwidth for the proposed antenna. Another approach by using a stacked antenna element with a twin Γ-shaped strip fed is demonstrated. The antenna using the twin-Γ feed yields an impedance bandwidth of 69%. Then the antenna combined with a U-bridge element and the twin-Γ feed finds around 90% impedance bandwidth. Both proposed bandwidth enhancement techniques, can provide a wideband antenna with directional radiation patterns across the operating bandwidth.

    Research areas

  • Microstrip antennas, Broadband communication systems