用以激發電路板輻射地緣電流之新微波元件：輻射電流驅動器

Abstract

在本論文中，我們提出一種名為電流驅動器之新型微波元件來激發地緣輻射電流。由於電流驅動器本身只作為一種小型的饋入裝置來將能量有效地饋入接地面產生輻射，因此對整體輻射的貢獻量很小，所以驅動器的尺寸可以設計得十分小而不影響整體天線特性。此種激發地緣電流作為輻射源的作法，相對於一般縮小化天線設計來說，在大幅地減小天線設計空間的同時，也不影響整體的輻射特性。此外本論文引用一種縮小化的巴倫等效電路模型來解釋電流驅動器的地緣電流產生機制並用來設計電流驅動器。在論文中，我們亦針對常用於隔離電磁干擾的金屬屏蔽盒對電流驅動器的特性影響做探討。結果指出即使在屏蔽盒緊臨電流驅動器的情況下，對驅動器的整體特性影響也不大。這顯示出電流驅動器在小型無線裝置的設計彈性。 基於上述的電流驅動器的優點，本論文提出二種基於電流驅動器的天線應用，此二種架構皆製作在常用的FR4 基板上。首先我們設計應用於WLAN 2.4 GHz的電流驅動器，此驅動器可利用集總電容或者是指插式電容來實現，整體尺寸僅 4 mm × 4 mm，並具備良好的天線輻射特性。在2.45 GHz時，量測的天線輻射效率大於60 %，並且在其接近全向性的平面上，量測的平均增益接近0 dBi。接下來，我們藉由合併二個單頻的電流驅動器來達成雙頻 WLAN 2.4/5.2 GHz操作，整體尺寸為 8.5 mm × 4 mm。我們將在論文中詳細呈現雙頻電流驅動器的設計流程。量測的天線特性在低頻 2.45 GHz 時與先前提出的單一2.4 GHz的電流驅動器特性大致相符，具有接近全向性的場型與大於60%的天線輻射效率。而在高頻5.2 GHz時，由於FR4基板在高頻的介質損耗，量測的天線效率降低至50%左右，但仍然保持接近全向性的場型。實際量測結果與模擬結果相當吻合。由於利用電流驅動器來達成天線應用具有小尺寸且具備良好的天線輻射特性等優點，因此很適合應用於無線通訊系統。

In this thesis, a new microwave component named current driver is proposed, which is capable of inducing ground edge current for radiation. Since the current driver serves as a small feeding structure with little contribution to radiation, it can be designed very small while good radiation properties are maintained. Compared to the miniaturized antennas, the approach that utilizes ground plane as the main radiation has smaller size and better antenna performance. Furthermore, a circuit model of the miniaturized balun (balanced to unbalanced transformer) is introduced to explain the current inducing mechanism and to provide some valuable physical insights into the properties of the current driver. The effect of a shielding metal box for the proximity circuitry near the current driver is also investigated. It is shown that the nearby shielding box has minor effect on the performance of the driver, which demonstrates the feasibility of the current driver for antenna applications in a compact wireless terminal. Given the benefits shown above, two antenna applications based on the current driver are presented. Both of them are fabricated on the low cost FR4 substrate of 0.4 mm. First, the current driver for WLAN 2.4 GHz designed with a lumped capacitor or printed capacitor has a small size of about 4 mm × 4 mm. The good radiation properties are obtained at 2.45 GHz with the measured radiation efficiency over 60 % and the nearly omni-directional radiation pattern with the average gain of about 0 dBi. Secondly, the dual-band current driver for WLAN 2.4/5/2 GHz applications is achieved by combining two single-band current drivers with the overall size of about 8.5 mm × 4 mm. The measured radiation performances at 2.4 GHz are similar to the ones for the previous single 2.4 GHz current driver, which has the measured antenna radiation efficiency higher than 60 % and nearly omin-directional patterns. At 5.2 GHz, the measured radiation efficiency is reduced to about 50 % due to the high dielectric loss caused by the FR4 substrate in the high frequency. The nearly omni-directional patterns are still maintained. The simulation and measurement results come to a great agreement. Based on the compactness and design flexibilities, the current driver is feasible for wireless applications