| 摘要: | 本論文的目的在研究多種平面槽孔天線的特性，最後並將之和平面單極天線的特性進行比較。本論文探討的平面槽孔天線結構包括簡單細槽孔天線、長方形寬槽孔天線以及正方形環狀槽孔天線，並針對後兩者進一步將其接地面裁小(縮小化)以提高其實用性，並探討接地面裁小後的特性變化。
對於接地面未裁小的天線，吾人首先探討簡單細槽孔天線，發現簡單細槽孔天線的輻射場型與極化方向，和由微帶線饋入的單極天線 (monopole antenna)幾乎完全一樣；另外，增大接地面尺寸可使簡單細槽孔天線的天線增益略為增加。接著，探討一個由”一字型”饋入長方形寬槽天線，於不同的頻率，吾人可以觀察到其電流及電場的分佈清楚展現出基頻模態與倍頻模態，接著針對此寬槽天線，改利用U字型端的饋入線以作為寬頻匹配之用。並進一步探討各結構參數對頻寬的影響。
吾人發現，U型饋入端和槽孔下緣的距離Lg對天線的頻寬影響相當敏感，值得注意是Lg=0.8mm是一個達到寬頻特性的重要設計參數，這和超寬頻(UWB)單極天線的設計參數要求類似。另一個影響天線阻抗頻寬的重要設計參數為槽孔長度Ws，Ws對天線的頻寬影響相當嚴重，且主要影響最低通過頻率fL，適度的調整Ws可使頻率比fH/fL達到1.9左右。
接著，吾人將天線槽孔的上方及左右的接地面尺寸裁小(縮小至2.5mm的寬度)，方便將其應用於個人電腦存取的PCMCIA卡，以提高其實用性。接地面尺寸裁小後的天線，其頻率比fH/fL可達到2.15左右，和接地面未裁小的天線相比（Ws=40.88mm），其頻寬並不因接地面的裁小而變小，甚至還稍微增加。
有關正方形環狀槽孔天線的探討方面，眾所周知槽孔邊長為四分之一波長時會產生基頻共振：針對基頻進行確認，在槽孔週邊吾人可觀察到電流呈現兩半波長駐波的基頻共振現象，針對倍頻，在槽孔週邊吾人可觀察到電流呈現四個半波長駐波的倍頻共振現象。
若進一步將正方形環狀槽孔天線的接地面直接裁小，並和接地面未裁小的正方形環狀槽孔天線特性比較，吾人發現接地面裁小可使正方形環狀槽孔天線的基頻由2.45GHz降至2GHz左右，但原先倍頻模態的匹配性會變差。另外，接地面裁小後，正方形環狀槽孔上側的電場有比下側的稍微大一些，這將使得主輻射的角度由 theta=0° 偏移至 theta=10°左右。
接著，吾人在把正方環形迴圈內部金屬縮小，觀察其天線的影響，此時倍頻模態的匹配性會回復，S11響應有兩個匹配共振的頻率，分別是2.13GHz與4.61GHz。遠場輻射主要來自上下兩側槽孔電場的貢獻，且倍頻模態的槽孔上側電場的貢獻比下側的大了許多，這將使得主輻射的角度由 theta=0° 偏移至 theta=20°左右。
最後，本論文探討將環形迴圈的內部金屬被提到饋入線平面並和饋入線重疊，使結構由方形環狀槽孔天線變成方形寬槽孔天線，然後再進一步將槽孔去掉，使變成一個簡單的平面單極天線，後者是一相對窄頻的天線；要注意的是，此一平面單極天線的單極模態在方形寬槽孔天線中未必能被有效匹配/激發。
In this thesis, the characteristics several planar slot antennas are investigated, which are then compared with those of the planar monopole antenna. The antenna structures concerned include a simple thin slot, rectangular wide slots, and square ring slots. For the latter two structures part of the ground plane around the slot is then cut to reduce the antenna size in order to easily utilize them in practices. The characteristics of those after cutting the ground are compared with those without cutting.
For the structures with normal ground plane, we first investigate the simple slot antenna, it is found that the radiation pattern and polarization are well compared to those of a simple planar monopole antenna fed by a microstrip line. In additions, increase of the size of the ground plane can slightly increases the gain. Next, a rectangular wide slot antenna fed by a simple microstrip line is studied. For different resonant frequencies, it can be observed that the surface current on the ground plane and the corresponding E fields on the aperture exhibit clearly the corresponding modes of base frequency and the harmonic frequency. Then, a microstrip line with U-shape end is employed to feed the rectangular wide slot antenna in order to increase the bandwidth of the return loss. The influences of the structure parameters of this rectangular wide slot antenna upon the bandwidth and the characteristics of the return loss are also studied.
It is found that the bandwidth is very sensitive to the gap distance Lg between the U-shape end and the slot. It should be mentioned that setting Lg around 0.8mm is a very convincing design, which is quite similar to the case of ultra wideband (UWB) planar monopole antenna designs. Another important factor that can largely influence the bandwidth is the slot width Ws , which can shift the lower end frequency fL of the passband easily. Therefore, by suitable adjusting the width Ws the frequency ratio fH/fL of the passband can achieve ~1.9.
Next, the top, left and right parts of the ground plane around the slot is cut (reducing to width=2.5mm only) in order for easy applications, such as the wireless link of computers through the PCMCIA cards. It is found that, after the cutting, the the frequency ratio fH/fL of the passband can achieve ~2.15. As compared the original ones without cutting (Ws=40.88mm), the bandwidth is not sacrificed due to the size-reduction at all. On the contrary, it increases a little bit.
Next structure concerned is related to the square ring slot antennas. It is well known that each side of the slot should be about a quarter wavelength in order to resonate at the base frequency. To confirm the characteristics at the base frequency, the surface current on the ground plane exhibits two half-wavelengths standing wave patterns. While, for the second harmonic frequency, the surface current on the ground plane with four half-wavelengths standing wave pattern is observed.
Afterward, the top, left and right parts of the ground plane around the square ring slot is cut (reducing to width=1mm only) for easy applications, The characteristics of those after cutting the ground are compared with those without cutting. It is found that cutting the ground plane can effectively reduce the first resonant frequency, from 2.45GHz to 2GHz in our cases, but the matching for the second resonant frequency would become worse. In addition, the cutting would render the E fields on the ring aperture unbalanced, of which the top-side become bigger than the bottom one a little bit. This would then make the main beam shifted from theta=0° to theta=10°.
We then increase the ring slot by reducing the inner metal part of the aperture. By doing so, the matching for the second resonant frequency would be restored, which render the ring slot a dual–band antenna again. The resonant frequencies are 2.13GHz and 4.61GHz, respectively, in this case. Similarly, reducing the inner metal part would render the E fields on the ring aperture more unbalanced, of which the top-side become even bigger than the bottom one. This then make the main beam shifted from theta=0° to theta=20°.
Finally, the inner metal part is raised to the feed line plane to coincide with the feed line. This would make the ring slot antenna become a square wide slot antenna. Furthermore, the slot is completely removed to make a planar monopole. The latter is relatively narrow band antenna. It is noted that the monopole mode embedded in the square wide slot antenna is not effectively excited. |
