應用於2G/3G/4G/LTE之微小化平面式架構頻率可調手機天線

Abstract

隨著無線通訊技術發展，更多的人追求更快的傳輸速率，從以往GSM 2G/3G邁入現在熱門的LTE 4G，還有未來5G的發展，越來越多新頻段開放被使用，這些新頻段的到來增加了對天線設計的困難度。尤其在於低頻頻段需要符合現今4G與舊有2G/3G的規範，同時天線在手機裡只能占用緊湊的面積，這會是目前手機應用上最主要的需求。 多頻段手機天線的重點在於低頻頻段的設計，通常會以多分支架構作為天線主體，利用耦合饋入技術，耦合到低頻的操作路徑上，產生四分之一波長之共振，同時在饋入端加入一高頻四分之一波長的分支路徑，利用這樣的方法來產生多頻的操作頻段，但這樣勢必會在手機內部占用太大的面積（天線面積15 × 52.5 mm2），反而降低了應用在手機上的實用性。為了要解決占用太大面積的問題，因而產生頻率可調技術，同樣基於多分支架構，使用PIN-diode作為低頻頻段的切換機制，雖然面積相較於非頻率可調天線能縮小53.6%（天線面積10 × 36.5 mm2），但因為PIN-diode元件本身的使用特性，原本的高頻頻段在切換低頻頻段後會消失，使得低頻和高頻頻段不能同時使用，因而增加不預期的設計結果。 基於上述的問題，改進目前相關文獻論文之手機天線的缺失，故本論文所提出之手機天線是同時採用耦合饋入技術與頻率可調技術，設計出低頻和中頻採用完全共用的單一路徑之平面結構天線，以可變電容(varactor)簡單的作為頻率可調機制，且達到LTE600更低頻與體面積微小化的設計目標。 所提出之天線為應用於LTE600/LTE700/GSM850/GSM900/GSM1800/GSM1900/UMTS/LTE2300/LTE2500/LTE3700，即應用於2G/3G/4G/LTE頻段的頻率可調手機天線，其涵蓋頻率範圍為600 ~ 960 MHz/1710 ~ 2690 MHz/3600 ~ 3800 MHz，架構設計是採用平面倒F型天線為主要輻射體，並且設計出低頻(600 ~ 960 MHz)與中頻(1710 ~ 2690 MHz)共用電流路徑的耦合架構，其中低頻頻段具有頻率可調的特性。天線主體分成饋入端與共用電流路徑之耦合架構的短路端，利用耦合饋入的方式到輻射體，並在短路端產生二個低頻之共振頻率，其中一路短路端是流經一可變電容和一電感到地，另一路短路端是流經一電容和一電感到地。 由於天線主體，低頻和中頻是採用完全共用的單一路徑結構，所以和大部分多頻段手機天線相比，能夠縮小天線面積。再加上此單一路徑結構之低頻與中頻路徑是可以分區塊調整，因此更能有效地使用可變電容來做頻率可調，將低頻做成寬頻而不影響其中頻特性。同時使用可變電容，做成頻率可調天線的設計，更能節省手機空間縮小天線面積；此外蜿蜒架構的設計是利用帶線間的耦合在中頻產生多個共振點來做成中頻寬頻，以達到中頻所需要的頻寬。 LTE600的設計是基於天線本體的架構，在短路端額外加入一條電流共振路徑，使得低頻產生二個共振點，再藉由改變可變電容之電容值以達到低頻LTE600之寬頻；LTE3700的設計較為簡易，僅需要在天線輻射主體旁設計一小段分支就能達到此使用頻段。除此之外，藉由諧振電路之觀念，所引入之電感被動元件能降低天線操作的共振頻率，進而將天線面積大幅縮減。 最後本論文所設計之應用於2G/3G/4G/LTE之微小化平面式架構頻率可調手機天線，可以達到10.5 × 33 mm2的小面積。天線之模擬結果包含反射係數、輻射場型、輻射效率和天線增益，其設計細節及實驗結果也將在後面章節進行分析與討論，以及後續研究。

With the development of wireless communication technology, people pursue a higher speed transfer rate. From previous 2G, 3G, present 4G and even 5G in the future…etc. Many new frequency bands will appear and new mobile communication technology standard will be defined, these are increasing the difficulty of handset antenna design. That lower frequency band need to include present 4G and previous 2G/3G standards, while the antenna must be only occupy a compact size and low profile in the handset, which is the most important requirements of the current mobile device applications. General multiband handset antenna design, which have focused on the low band to wideband, such as multi-branch architectures, by coupled-feed technique, the feeding strip can couple to the main branch strip, to generate the operation of the lower band with a quarter-wavelength resonance mode, while additional branch strips which also have quarter-wavelength resonance modes are used around the main branch strip for achieving required operating multiband, but this method is bound to take up too much internal space of mobile devices (the antenna size is 15 × 52.5 mm2), that decreasing the practicality for the handset applications. In order to reduce the antenna size, novel techniques are required, such as the frequency reconfigurable technique. There is use the PIN-diode as a mechanism for switching the operation of the lower band, also based on the design of multi-branch architectures. Although the frequency reconfigurable antenna reduces the general multiband antenna size by at least 53.6% (the antenna size is 10 × 36.5 mm2), the switching mechanism by using PIN-diode element will let the operation of the higher band disappear. However both the lower and higher bands can’t be used at the same time, the result is unexpected. Based on the above problems, in order to improve the current lack of relevant studies for mobile phone antenna has become a challenge. In this thesis, the antenna with the planar structure which is using coupled-feed technique and frequency reconfigurable technique is proposed, to design the low and middle band resonances entirely share the same current path, a tunable capacitor (varactor) as a simple mechanism for adjustable the lowest frequency band LTE600, and to achieve the miniaturized design goals of a compact size and low profile. The proposed frequency reconfigurable antenna can cover LTE600/LTE700/GSM850/GSM900/GSM1800/GSM1900/UMTS/LTE2300/LTE2500/LTE3700 for mobile handset applications. Covering the frequency range in the low band of 600 ~ 960 MHz, middle band of 1710 ~ 2690 MHz, and high band of 3600 ~ 3800 MHz. The architecture of the antenna is a PIFA (planar inverted-F antenna) as the main radiator, and the design of the coupled architecture which the low band (600 ~ 960 MHz) and middle band (1710 ~ 2690 MHz) are in common current path, wherein the low frequency band has a frequency tunable properties. The main radiator of the antenna is composed of the feeding strip and the shorting strips which with a coupled architecture of the common current path, by the coupled-feed method to drive the radiator, and generate two resonance frequencies for low band operation on the shorting strips, the shorting strip is the way in which flow through a tunable capacitor and an inductor, the other is flowing through a capacitor and an inductor to the ground. Since the antenna body, it is a completely common single path structure for low and middle frequency bands operations, and the antenna size compared with most of the multiband handset antenna is can be reduced. Coupled with this low frequency structure of a single path and the middle path can be adjusted in each area, and therefore it is more efficient by using a tunable capacitor to achieve frequency tunable, wherein the low band can be a broadband without affecting the characteristic of the middle band. By this way, it is make more compact size for frequency reconfigurable handset antenna design; moreover the design of meander line structure is the use of the coupling between each the strip line to produce a number of resonance frequencies for middle band, in order to achieve the desired middle bandwidth. The design architecture of LTE600 is based on the antenna body, added an extra current resonance path on the shorting strip, so that the low band generating two resonance frequencies, then by adjusting the value of the tunable capacitor in order to achieve the broadband low frequency band LTE600; the design of LTE3700 is more simple, requiring only an antenna radiating body next to a small branch of design can be achieved using this band. In addition, with the concept of the resonant circuit, the inductor of the passive element is embedded to reduce the resonant frequency of the antenna operation, which in turn will significantly reduce the antenna area. The thesis proposes a compact and tunable planar antenna for 2G/3G/4G/LTE handset applications. The size of the proposed antenna is only 10.5 × 33 mm2. The tested results include reflection coefficient, radiation patterns, efficiency, and gain which about the details of designing antenna and the results of experiments are discussed in this thesis, as well as follow-up study in later chapters.