寬頻微帶洩漏波天線

Abstract

由於無線通訊蓬勃的發展，使得人們對頻寬的需求逐漸增加，因此設計寬頻且高增益的天線提供無線通訊應用所需成為一個很大的挑戰。本文提出寬頻洩漏波天線設計方法。為能完全展現這些寬頻洩漏波天線之寬頻特性，本文亦提出以微帶線設計寬頻平衡-不平衡電路來激發第一高階漏波模。 首先，在本文中提出反向平衡式微帶線設計方法。此反向平衡式微帶線乃利用平衡式微帶線結構而將基板上下之正負微帶位置互易而得。在本文中，將詳述如何將正負微帶位置互易的方法。由反射損失的量測結果可以看到反射損失小於-10dB的頻寬幾乎從 DC至17.4 GHz 或從 2.8 GHz至40 GHz。不同的頻寬是因為不同的設計參數而定。此反向平衡式微帶線跟平衡式微帶線結合使用則能形成兩對寬頻之平衡-不平衡電路，並用來當寬頻洩漏波天線之基本饋入結構。 其次，根據微帶線之第一高階漏波模特性，本文提出一新穎演算法則來設計寬頻緩變洩漏波微帶天線。此方法並可用於分析此類線性緩變之洩漏波微帶天線特性和作為近似模型。對此寬頻洩漏波天線之饋入結構亦一併提出。利用上述之兩對寬頻之平衡-不平衡電路，將其中一對饋入微帶天線而另一對接微帶線之地。此背對背饋入結構之反射損失的量測結果可以看到VSWR£2 的頻寬從 2.2 GHz至18.6 GHz。而此寬頻緩變洩漏波微帶天線反射損失的量測結果，可以看到VSWR£2 的頻寬從7 GHz至16.4 GHz，相對的頻寬約2.34:1。 另一例子是毫米波寬頻緩變結構之洩漏波微帶陣列天線，由四個單元所構成之一維陣列。此毫米波寬頻緩變洩漏波微帶天線反射損失之VSWR£2 的頻寬量測結果，從13.3 GHz至35.0 GHz。 由於直線型態的緩變寬頻洩漏波微帶天線相當消耗面積，為使得天線變得更緻密以節省所消耗面積，本文亦提出螺旋狀彎曲之緩變洩漏波微帶天線設計。此螺旋狀彎曲之緩變洩漏波微帶天線反射損失之VSWR£2 的頻寬量測結果，如同直線型態的緩變寬頻洩漏波微帶天線，相對的頻寬大於2.3:1。 最後，在本文中將提出單一導體帶狀天線結構。此單一導體帶狀天線結構在基板上只有單一帶狀導體而無任何其他實際接地面。對於此結構之第一高階漏波模特性，利用全波分析之積分方程法作一探討，發現擁有相當寬頻之輻射區段。為能激發此結構之第一高階漏波模，利用上述之兩對寬頻之平衡-不平衡電路，將其中一對饋入帶狀天線而另一對互接並拉回至原先接微帶線之地。所有數值計算和量測結果會在本文中做一說明。此單一導體帶狀洩漏波天線反射損失之VSWR£2 的頻寬量測結果，從6.55 GHz至13.75 GHz，相對的頻寬大於2.34:1。由於相位常數之特性，此單一導體帶狀天線結構之主波束位置幾乎不會隨頻率變動並幾乎維持在 endfire 方向。此單一導體帶狀洩漏波天線雖和緩變洩漏波微帶天線均同為寬頻洩漏波天線並幾乎操作在相同頻帶上，天線長度卻短少許多。

This thesis proposes the design methodology of broadband leaky-wave antennas. To exploit fully the broadband feature of these broadband leaky-wave antennas, also presented is the design method of the novel broadband planar balun used to excite the first higher order leaky mode. First, an inverted balanced microstrip line structure is proposed. This inverted balanced microstrip line structure is developed from the balanced microstrip line. The position of the positive strip on the upper substrate side is exchanged with that of the negative strip on the lower substrate side by the method proposed in this thesis. The measured return loss is observed to be better than -10dB over a frequency range extending from DC up to 18.6 GHz or from 2.8 GHz up to 40 GHz depending on the different design parameters. United with the balanced microstrip line, this inverted balanced microstrip line can be used to form into a pair of broadband baluns. Using this broadband balun can realize a broadband leaky-wave antenna. Secondly, based on the characteristics of the first higher order leaky-mode for the microstrip antennas, this thesis proposes a novel scheme for the empirical design of broadband tapered microstrip leaky-wave antennas. This scheme is demonstrated to be able to explain and approximately model the radiation characteristics of a linearly tapered leaky-wave microstrip antenna. Also presented is the broadband feeding structure that uses the balanced and the inverted balanced microstrip lines to form two pairs of broadband baluns. The balun on the upper substrate side feeds the microstrip antenna, while the one on the lower substrate side directly connects with the ground plane of the microstrip antenna. The measured return loss of the back-to-back feeding structures has a VSWR£ 2 from 2.2 GHz to 18.6 GHz. The measured bandwidth of the tapered microstrip leaky-wave antenna has a VSWR£2 from 7 GHz to 16.4 GHz, yielding a relative bandwidth of 2.34:1, more than an octave. Another example is the design of the millimeter-wave broadband tapered microstrip leaky-wave antenna array of 1´4 elements. The measured bandwidth of this millimeter–wave broadband tapered microstrip leaky-wave antenna array with 1´4 elements for a VSWR£ 2 is from 13.3 GHz to 35 GHz. With a straight profile, the broadband tapered microstrip leaky-wave antenna consumes area. To design a more compact broadband antenna, the tapered microstrip antenna is curled to save area. Presented is a compact broadband tapered microstrip leaky-wave antenna with a curled profile that has a bandwidth of more than 2.3:1 for a VSWR£ 2. Finally, proposed is a single-conductor strip antenna with only a single-conductor strip on a substrate without any practical ground plane. By using the full-wave integral equation method, the first higher order leaky mode of this single-conductor strip structure is investigated and found to possess a broadband radiation regime. To excite this first higher order leaky mode, a broadband planar balun is developed from the balanced microstrip lines and the inverted balanced microstrip lines. Like the feeding structure of broadband tapered microstrip leaky-wave antenna, the balun on the upper substrate side feeds the strip antenna with an offset from the edge, while the positive and negative balun strips on the lower substrate side are directly interconnected and returned back to the original ground plane of the microstrip line.Both the numerical simulation and the measurement data are presented. This single-conductor strip leaky-wave antenna has a bandwidth of 2.34:1 (from 6.55 GHz to 13.75 GHz) for a VSWR*2. Owing to the particular characteristics of its normalized phase constant, this single-conductor strip leaky-wave antenna has a fixed mainbeam radiation pattern in the endfire direction. As a broadband leaky-wave antenna, the length of this single-conductor strip leaky-wave antenna is much shorter than that of the tapered microstrip leaky-wave antenna.