高隔離度X-波段船舶雷達天線之研究

Abstract

本論文提出兩種由一維微帶天線陣列所組成之陣列天線。其中，一維子陣列中微帶天線單元的數目是可設計的，且其與欲設計之整體天線的規格相關，例如:天線增益、波束寬等。各天線單元係由串聯饋入的方式所連接。透過操控此饋入網路的各段傳輸線的特徵阻抗，易達成漸變的傳輸功率分布，該分布可有效抑制天線輻射旁波瓣的產生。 在第一個部分中，我們提出了一應用於船舶雷達的低旁波瓣平面印刷式天線。該天線共包含了32個相同規格且操作頻段包含在9.3 ~ 9.4 GHz (以1:1.5電壓駐波比[Voltage Standing Wave Ratio, VSWR]為基準)的方形微帶天線。微帶天線單元平均配置於四個支臂上，每一支臂擁有八個天線單元，並以具25 dB旁波瓣大小之柴比雪夫(Chebyshev)傳輸能量分布的饋入網路所串接。在海事雷達應用中，水平極化天線較垂直極化天線能降低海雜波(sea clutter)所產生的反射干擾。因此，我們在微帶天線單元上利用一狹縫去改變電流路徑，使其產生水平極化波。整體天線的增益、半功率波束寬、旁波瓣與輻射前後比分別為22 dBi、5.3度、−26.4 dB與38.5 dB。除此之外，我們將金屬天線擋板分別裝置於天線陣列之兩側，用於增加傳送與接收端天線的隔離度至60 dB。 天線擋板扮演轉態之角色，將電磁場由天線端轉換至自由空間(free space)中。傳統的天線擋板大都係由線性或分段線性金屬板構成。但在實際應用中，天線的物理尺寸有可能受到重量或成本考量而有所限制，使得擋板沒有足夠的長度去塑造適合的場強分布，造成輻射場型失真，產生較差的旁波瓣與前後比。因此，在次一階段中，我們提出了一波導天線，其由一一維線性平面式微帶陣列天線與有鰭式擋板所構成。一維線性平面式微帶陣列天線作為激發源;有鰭式擋板則是用於合成開口端的場強分布。藉由設計一維陣列天線的饋入網路，使其產生漸變的能量傳輸分布，進而抑制在陣列方向(水平方向)的旁波瓣大小。在另一方面，降低垂直方向的旁波瓣大小係利用由串接數個平行波導組成之有鰭擋板，透過這些有鰭結構可轉換電磁場，在開口端合成適當的場強分布。利用此兩結構可有效地同時降低垂直與水平方向的旁波瓣。量測結果除了證明此天線結構可有效降低旁波瓣，還可增加輻射前後比，並具有良好的交叉極化(cross-polarization)。

In this dissertation, we design and analyze two patch antenna arrays, both of which are composed of several 1-D series-fed patch subarrays. The number of patch in the subarray is designed to meet the system requirement (such as antenna gain and beamwidth along the array direction). The patch elements are connected to a feeding network in series. By manipulating the characteristic impedance of each feed line, the feeding structure is easy to achieve the taper distribution that can suppress the side lobe. First of all, a low-side-lobe printed antenna array for marine radar applications is presented. The antenna array is composed of 32 identical square microstrip patches operated at a center frequency of 9.35 GHz and has a 100-MHz bandwidth (subject to a 1.5:1 voltage standing wave ratio [VSWR]). The patch antennas are arranged in four arms, each of which contains eight elements and is series-fed using Chebyshev tapering (25-dB side-lobe level [SLL]). To apply the antenna in marine radar applications, an antenna with horizontal polarization was employed because, in comparison with vertical polarization, it can relatively reduce the sea clutter reflectivity. Therefore, a slit was carved on each patch element to change the current path, thereby enabling horizontal polarization. The antenna gain, 3-dB beamwidth, SLL, and front-to-back ratio (F/B) were 22 dBi, 5.3º, −26.4 dB, and 38.5 dB, respectively. Additionally, metallic baffles were introduced for increasing the isolation between the transmitting and receiving antennas to 60 dB. Baffles act as the transitions to transform the field from an antenna to the free space. Most of the conventional antenna baffles are composed of linearly tapered or two-step piecewise-linear plate. However, in practical applications, the physical size of the antenna is restricted by the factor of weight and cost concern. Due to short of transition length, the plates cannot shape a properly field distribution at the end of the baffle. The disordered field degrades the radiation field, causing poor SLL and F/B. Therefore, in the second part, we present a waveguide antenna consisting of a planar patch array served as an excitation source and a finned baffle for synthesizing the aperture fields to suppress the side lobe along both the vertical and horizontal planes. The side lobe reduction along the array direction (horizontal) is obtained by designing the Chebyshev tapered amplitude over the excitation source. On the other hand, the decrease in the side lobe along the vertical plane is achieved by electromagnetic field conversion via the cascaded parallel-plate waveguides. The measured results indicate that such a design can effectively reduce the antenna side lobe and cross polarization, and enhance the front-to-back ratio.