Vertical patch antennas
Student thesis: Master's Thesis
Related Research Unit(s)
|Award date||3 Oct 2006|
This thesis presents a detail study of a recently developed antenna, designated as the vertical patch antenna (VPA). The geometry of the VPA is different from any existing antennas, which is found to have a non-distorted radiation pattern, wider bandwidth and occupying less space than the conventional (horizontal) patch antenna. The primary objective of the study is to provide a small but wide-band, high-gain antenna with non-distorted radiation patterns. VPAs with different characteristics have been designed, fabricated and tested. In addition to linear polarization designs, other designs with circular polarization, dual-band operation, wide-band operation and backlobe suppression have been studied and developed. Simulation studies of the antennas are performed using an EM simulator IE3D, and most of them are confirmed by measurements. Firstly, a circular vertical patch antenna (circular-VPA) using proximity-coupled feeding mechanism is investigated. Its occupied area is even smaller than that of a corresponding annular-ring patch since the perimeter of the VPA is smaller than the outer circumference of the annular-ring patch. Results show that the VPA not only can provide wide bandwidth of over 7% and high gain of 8 dBi, but also can preserve the broadside radiation patterns. For some applications, other shapes may be more preferable and convenient. It is found that rectangular- and triangular- shaped VPAs, can achieve similar performance. It is expected that the VPA design is suitable for other shapes as long as the perimeter of the vertical patch is about a wavelength. Slightly difference in bandwidth, gain, projection area, etc. can be expected. As the hollow structure of the VPA allows another antenna (another VPA or patch) to be located inside itself, a compact multi-band antenna can be built. Secondly, based on the single-band circular-VPA with proximity-coupled feed, another circular-VPA with smaller size is placed inside to form a dual-band VPA. The measured bandwidth (SWR < 2) in the lower band centered at f1 = 4.25 GHz is 7% and at the upper band centered at f2 = 7 GHz is 26 %, with gain of around 7dBi at both operating frequency bands. The VPA radiates towards broadside with symmetric patterns in the H-plane. Also, the patterns maintain broadside across the passbands. An antenna with dual frequency operation can be attained by the double resonance technique such as the stacked patches geometry in microstrip antenna design, which can also be applied to the VPA. Under a series of research, a single structure dual-band VPA can be performed, which is constructed by simply folding the rim of a square-VPA to form a fractal-like antenna. It not only has advantages of small size and single-element structure, but also has impedance bandwidth of 5.2% and 2.3% across the lower and upper bands respectively, with broadside patterns at two operating frequencies. Although the bandwidth is not wider than that of the original single-band circular-VPA, it shows the special features of VPA and performs dual-band radiation by a single structure with small projection area. Thirdly, for bandwidth enhancement, the fractal concept is applied to design the VPA. Likewise, making use of the characteristic of its hollow structure, a wideband VPA consisting of two connected Koch loops is created. Using a dual Koch loop structure, a wideband VPA with 42% bandwidth and 8dBi gain at the center frequency is designed and tested. Symmetrical broadside patterns are obtained at the passband. Comparisons between Koch fractal VPAs of different iterations and a circular-VPA with same size and same height are also carried out. Fourthly, to suppress the cross-polar level and backlobe radiation, a circular vertical patch antenna using corrugated ground plane is presented. The corrugated ground plane consists of three circular layers with different diameters and heights. The proposed VPA has significant reductions in cross-polar (< -20 dB) and backlobe radiations (< -22.5 dB) across the entire operating band (SWR < 2 and Gain > 9 dBi). A cross-polarization of around -25 dB and a backlobe of about -30 dB were recorded at the center frequency. The antenna radiates towards the broadside direction within the operating frequency band with symmetrical patterns. It is clearly seen that the improvement in cross-polarization and backlobe is substantial when the corrugated ground plane is used. A 15 dB reduction in both cross-polarization and backlobe are obtained compared with the case using the usual planar ground plane structure. Finally, apart from linear polarization, a circularly-polarized vertical patch antenna (CP-VPA) is examined systematically. A new technique is proposed to excite the vertical patch antenna (VPA) for circular polarization radiation. The CP-VPA maintains small size and yields reasonably wide bandwidth. It is simply constructed by adding two stubs at two opposite sides of the circumference of the antenna to excite two orthogonal modes with 90o phase difference for circular-polarization. The SWR is less than 2.0 over a frequency range of about 18% and a gain of 8dBi is observed. An axial ratio bandwidth of 4% is achieved, which is wider than that of an annular-ring microstrip antenna with broadside radiation pattern at the operating frequency. The VPA is novel in design, together with its unique hollow structure, the VPA can occupy less space. The VPA is also simple in structure and easy to manufacture, using standard industrial techniques such as the molded intrusion design (MID). With its excellent performance, the VPA should make an impact on antenna technologies.
- Microstrip antennas, Design and construction