開放式平面傳輸線之傳播特性與能量洩漏之研究及其應用

Abstract

在本論文中，開放式平面傳輸線波漏現象及其相關的傳播特性與應用被加 以研究。論文第一部分包含了表面波漏及其相關傳播特性的分析，三種在 開放式平面傳輸線常遭遇之模態問題被發現皆存在於共平面帶狀線( coplanar strips)中，首先在較低頻率範圍存在著類表面波模(surface- wave-like mode)，再則在較高頻率範圍存在著表面波洩漏(sur-face wave leakage)的問題，這個衰減常數產生一個尖銳的極小值，且發生在 洩漏開始的頻率附近，就像在共平面波導所發現的現象。然後則是當共平 面帶狀線之金屬帶寬超過某一數值時在整個高頻率範圍會同時存在另外一 個會洩漏的主模(additional leaky domin-ant mode)。另外，共平面導 與共平面帶狀線的背面金屬板對傳播特性的影響在本論文中亦被加以研究 以便設計適當的電路支撐架構。最後，寄生在大部分支撐結構下的類表面 波模的本性，在本論中也以槽線(slot line)的類表面波模為例子加以描 述，並且這些類表面波模存在的條件也被加以解釋。在論文的第二部分包 含了空間波漏相關特性的研究及其應用。首先，本論文提出了一個解析的 方法去證明微帶線第一階高次模(first higher order mode)的本性及分 析它的放射效益(radiation efficiency)。根據複數的功率流向(complex power flows)，這個微帶線高階模的本性便可獲取，而藉由在頻域分析 法(spectral domain approach)中積分圍線(integration contours)的選 定，表面波漏與空間波漏的多寡可分別取得，使得微帶線高階模的放射效 益得以加以分析。接著在微帶線高階模的特性被掌握之後，我們對如何有 效激發這個高階模以便應用其為一漏波天線加以研究。三種有效饋送結構 在本論文中被提出，並且以頻域積分方程法加以研究並歸納出設計準則， 對於數值分析所得的結果，在論文中有一個實驗加以驗證，另外一個改良 的數值方法亦被提出以增進數值分析的效率，最後一個新的共平面式的漏 波線源(leaky wave line source)亦在本論文中被加以研究，這個新的漏 波線源是來自槽線的高階模(slotline higher order mode)。幾何相關性 以及有效金屬板對這一個新的高階模的影響亦被深入探討，然後三種有效 饋送結構也在本論文中被提出，這個新的高階模的存在性，以及一個共平 面式漏波天線的可行性亦經由實驗得以證實。 The leaky-wave-related propagation properties and their applications in openplanar transmission lines are investigated in this thesis. In part I of thethesis, the analyses of the surface-wave-leakage-related propagationcharacteristics in open planar transimissiom lines are included. Threecommonly encountered moding problems in open planar transmission lines are allpresent in the coplanar strips (CPS) cases. First, in the lower frequencyregime, there is an extra surface-wave-like mode exsiting. Respective fieldand current distributions of the dominant CPS mode and the surface-wave-likemode are presented since this surface-wave-like mode may be inadvertentlyexcited. This surface-wave-like mode is also essential in explaining the modeevolution around the spectral gap frequency regime when the bound CPS modeevolved into a leaky complex mode. Second, after the onset of leakage intothe surface wave of the supporting structure, the normalized leakage constantof the leaky CPS mode exhibits a sharp an deep minimum as witnessed in thecoplanar waveguide (CPW) case. Third, as the strip width of the CPS isincreased beyond certain values, both the leaky mode and the bound mode canexist simultaneosly in the full higher frequency regime. Also, the effect ofthe nearby back conductor plane on the surface wave leakage of the CPS and CPWcircuits is investigated for the design of appropriate supporting structures.The variation of mode evolution patterns between the CPS and the conductorbacked coplanar strips (CBCPS) are presented. How the back conductor planesinfluence the frequency point associated to the surface wave leakage of CPWis discussed. Moreover, the common features of the surface-wave-like modeparasitizing in most supporting structures are described by an example of thesurface-wave-like mode in slotline. The conditions of the existence of thesesurface-wave-like modes are explained. In part II of the thesis, the space--wave-leakage propagation characteristics and their applications are included. Fisrt, an analytical method for theoretical verification and analysis of themode nature of the microstripline first higher order mode is proposed. Theradiation efficiency of the microstrip line first higher order mode as aplanar leaky-wave line source is also investigated. The spectral domainapproach is used to calculate complex power flows propagating toward differentdirections. By observing the complex power flows, the mode nature in differentfrequency regimes can be clearly distiguished. Also, the amount of powercarried by the surface wave leakage and that by the space wave radiation canbe separated by choosing the integration paths appropriately in the spectraldomain. By comparing the amounts of these two types of power excited by themicrostrip line first higher order mode, it is found that the microstrip lineis more efficient as a leaky- wave line source when its substrate is thin, thedielectric constant of the substrate is low and the antenna main beam is awayfrom the endfire direction. Second, after the analyses of the mode nature ofthe microstrip line first higher order mode, the efficient excitation methodsof this planar leaky-wave line source are investigated. Three arrangements ofthe feeding structure for the excitation of a microstrip leaky wave antennaare proposed. And a full-wave spectral domain integral equation methodcombined with the fundamental mode sampling technique is applied to determinethe reflection coefficient of the excitation source. Tabulation technique isused to reduce the computational effort. Dependence on structural parameterssuch as line width, line spacing and overlap length is fully analyzed toobtain the design criteria for a microstrip leaky wave antenna. Also, anexperimental setup is used to check the validity of our numerical resultsand verify the radiation nature of the microstrip line first higher ordermode. Additionally, a modified dynamic model in integral equation methods forcharaterizing the discontinuities problems is proposed to improve thecalculation efficiency. Third, a new uniplanar leaky wave line source forrealization of a uniplanar leaky wave antenna is investigated with thespectral domain approach. This line source comes from the first higher ordermode of slotline. The leakage constant and the radiation band of this firsthigher order mode is larger than that of the microstrip line first higherorder mode. The geometric dependences and the effect of finite conductorplanes are analyzed. The undesired dominant modes (surface-wave-like modes)in cases of slotline with finite and infinite conductor planes are alsodiscussed. Moreover, feeding structures are proposed for efficient excitationof the leaky wave line source Finally, experimental results show agreementwith our numerical results and further identify the existence of the slotlinefirst higher order mode and the possibility to implement an uniplanar leakywave antenna.