結合類蕈狀結構與矩形環狀天線之 三頻雙極化小型天線設計

Abstract

本論文利用類蕈狀結構天線與矩形環狀天線之結合，提出了三頻雙極化小型天線，此三頻為一線雙圓極化的電磁波特性。類蕈狀結構是由蕈狀結構改良而成的 ，改良方式主要為了配合圓極化特性而在蕈狀結構上板金屬的正交方向挖槽。這種類蕈狀結構可以用複合式左右手傳輸線模型來等效，且可以實現提供零階線極化及正一階圓極化的雙頻極化天線。而此正一階模態是利用對向截角的方式來使正一階達到圓極化特性。頻率的可調性則可改變左手電感值或右手電容值來調整零階共振頻率，然後使用不同的挖槽長度來改變正一階電流共振路徑達到正一階頻率可調且零階共振頻率不受影響。 為了實現第三個頻率特性，本論文引用了矩形環狀天線來和類蕈狀結構天線作結合，把類蕈狀結構放在矩形環狀天線中。一般的微帶天線操作在主模時為二分之一等效波長共振，而矩形環狀天線透過在微帶天線中間挖空而成的，其目的為增加本身電流共振路徑使共振頻率下降而達到天線縮小化，然後在天線對向作截角形成圓極化特性。利用同軸饋入加耦合金屬片的方式直接對兩支天線作訊號饋入，可以很輕易的達到匹配。此三頻雙極化天線優點為所有天線皆實現在同一層基板金屬上與只需要單一訊號饋入方式就能達成三頻雙極化特性，三個共振頻分別操作在1.9GHz、2.45GHz、3.5GHz。

In this thesis, a compact tri-band and dual-polarized antenna, using a combination of mushroom-like structure and square-ring patch antenna is presented. This antenna can excite one linear polarization and two circular polarization. Mushroom-like structure is evolved from conventional mushroom structure for circular polarization by slotting slits along the diagonal direction of patch. This mushroom structure can be equivalent to composite right/left-handed transmission line model, and can realize dual-band and dual-polarized antenna that provides linear polarization at zeroth-order resonance (ZOR) and circular polarization at first-positive-order resonance (FPOR). Circular polarization is excited by using truncated corner patch on mushroom structure at FPOR. In order to achieve frequency tunability, the ZOR frequency can be adjusted by changing the left-handed inductance value or the right-handed capacitance value. Furthermore, the FPOR frequency can be adjusted by slotting different length of slits to change different current path, and the ZOR frequency is almost fixed. For realizing the third resonant frequency, the mushroom-like structure antenna combines with square-ring patch antenna by placing mushroom-like structure inside square-ring patch antenna in this thesis. Conventional microstrip patch antenna operating at fundamental mode is equal to half-wavelength resonance, but square-ring patch antenna that can decrease resonant frequency by extending current path is able to achieve antenna miniaturization, also uses truncated corner ring to excite circular polarization. Taking advantage of coaxial feed and coupling-strip feeds directly signal to two antennas, so impedance matching is easy to reach. The advantage of this tri-band and dual-polarized antenna is that all of the antennas are realized on upper metal of the same substrate, and only need single signal feeding to reach tri-band and dual-polarization. Finally, this antenna operates at 1.9GHz for ZOR, 2.45GHz for fundamental mode of square-ring patch antenna and 3.5GHz for FPOR.