Thiet ke anten vi dai UWB cho cac thiet bi di dong

Cong nghe bang thong sieu rong UWB (Ultra-WideBand) dang duoc xem nhu mot cong nghe khong day cua tuong lai voi uu diem noi troi cho phep truyen toc do du lieu cao cung nhu giam thieu duoc tac dong cua hien tuong da duong so voi cac cong nghe xuat hien truoc no. UWB hoat dong o dai tan khong can dang ky tu 3,1GHz - 10,6GHz. UWB duoc su dung trong cac he thong vo tuyen trong nha toc do cao, cac he thong khong day doi hoi tieu ton nang luong cuc it, mang vo tuyen ca nhan (WPAN - Wireless Personal Area Network), mang vo tuyen noi hat (WLAN) co nho, he thong Rada do tim, dinh vi trong quan su...

Ta Son Xuat, Tran Hai Nam, Vuong Hoang Nam

1. Nhu cau nghien cuu anten cho cong nghe UWB

Cong nghe bang thong sieu rong UWB (Ultra-WideBand) dang duoc xem nhu mot cong nghe khong day cua tuong lai voi uu diem noi troi cho phep truyen toc do du lieu cao cung nhu giam thieu duoc tac dong cua hien tuong da duong so voi cac cong nghe xuat hien truoc no. UWB hoat dong o dai tan khong can dang ky tu 3,1GHz - 10,6GHz. UWB duoc su dung trong cac he thong vo tuyen trong nha toc do cao, cac he thong khong day doi hoi tieu ton nang luong cuc it, mang vo tuyen ca nhan (WPAN - Wireless Personal Area Network), mang vo tuyen noi hat (WLAN) co nho, he thong Rada do tim, dinh vi trong quan su...

UWB cho phep truyen va thu nhan cac xung dang co ban duoc nen trong mien thoi gian khac voi viec truyen va thu nhan cac song hinh sin lien tuc duoc nen trong mien tan so nhu trong cac he thong thu phat truyen thong. Dieu do lam cho cong nghe UWB ve can ban khac so voi cac cong nghe khong day bang hep va trai pho tuong duong truoc do nhu cong nghe Bluetooth va 802.11a/b. UWB su dung mot bang cuc rong cua pho tan so de truyen du lieu. Do do, UWB co kha nang truyen tai nhieu du lieu hon so voi cac cong nghe vo tuyen truyen thong.

Cong nghe UWB ra doi co tam anh huong quan trong, mo ra nhung co hoi va thach thuc moi cho cac nha san xuat anten. Diem quan trong nhat trong thiet ke anten UWB la viec anten phai dat duoc mot bang thong rong, trong khi van phai duy tri duoc hieu suat buc xa cao tren toan bang [3]. Mot anten UWB phai co kha nang hoat dong tren ca dai tan 3,1-10,6GHz. Mot thong so quan trong khac cua UWB la do tre nhom (group delay). Do tre nhom duoc dinh nghia la dao ham cua pha goc tin hieu anten. Neu pha la tuyen tinh tren toan bo dai tan thi do tre nhom la hang so. Day la dac tinh quan trong the hien chat luong truyen di mot xung UWB va muc do bi anh huong meo hay tan sac cua xung do. Do thi buc xa va hieu suat buc xa cung la nhung dac tinh quan trong duoc xem xet khi thiet ke anten. Do thi buc xa dang huong la mot tieu chi thiet ke mong muon, co the cho phep dat vi tri may thu va phat o cac vi tri khong co dinh. Do do doi voi viec thiet ke anten cho cong nghe bang thong sieu rong phai co nhung yeu cau nhat dinh. Nhin chung, cong nghe bang thong sieu rong chu yeu huong toi cac thiet bi di dong, cam tay... Dieu do dong nghia voi viec anten duoc thiet ke phai co hinh dang, kich thuoc nho (anten vi dai) cho phep tich hop vao trong cac thiet bi.

Bang 1. Yeu cau cua anten cho thiet bi di dong bang thong sieu rong

Dai tan so hoat dong

3.1GHz - 10.6GHz

Hieu suat buc xa

Cao > 70%

Pha

Gan tuyen tinh, tre nhom khong doi

Do thi buc xa

Dang huong

Do rong bup song nua cong suat

Rong > 600

Hinh dang

Nho, gon, dong phang

He so dinh huong & He so tang ich

Thap

De tao anten vi dai bang rong can dua vao nhung anten bang rong co ban, sau do tim cach thay doi hinh dang va kich thuoc dua tren cac nguyen ly tao anten dai rong. Anten vi dai co the cong huong o nhieu tan so khac nhau, tuy thuoc vao hinh dang mieng buc xa, vi the co the co duoc mot anten bang rong bang cach thay doi sao cho cac tan so cong huong gan voi nhau.

2. Phuong phap thiet ke anten vi dai UWB

Cac buoc thiet ke anten duoc su dung o day bao gom:

Xem xet cac yeu cau ky thuat cua anten de lua chon anten thich hop

Mo hinh anten vi dai duoc dua ra co the duoc tich hop cho cac thiet bi di dong su dung cong nghe bang thong sieu rong. Mo hinh anten vi dai thiet ke la mot anten monopole dong phang buc xa dang huong voi kich thuoc nho, cau truc don gian va duoc san xuat bang cong nghe mach in nen gia thanh re. Voi cau truc phang, anten co the duoc dung trong cac thiet bi cua he thong may tinh ca nhan hoac cac thiet bi cam tay trong nha...

Chon nguyen ly tao anten bang rong thich hop de thiet ke

Ve mat ly thuyet, co 3 nguyen ly co ban de tao bang rong cho anten [5]: nguyen ly bien doi tu tu, nguyen ly tuong tu, nguyen ly tu bu. Tren thuc te, cac nha thiet ke co the su dung rat nhieu ky thuat khac nhau de tao bang rong cho anten. Trong mo hinh thiet ke anten vi dai UWB nay, nguyen ly bien doi tu tu da duoc chon su dung de tao bang rong bang cach tang dan dan kich thuoc cua duong tiep dien tu ong dan song dong phang ra mieng buc xa cua anten.

Chon phuong phap tiep dien thich hop

Trong qua trinh thiet ke anten, lua chon phuong phap tiep dien cho anten la mot buoc quan trong. Doi voi anten vi dai, co 3 cach tiep dien thong dung la: tiep dien bang cap dong truc, tiep dien bang duong truyen vi dai, tiep dien bang ong dan song dong phang. Lua chon phuong phap tiep dien phu hop de co the de dang phoi hop tro khang giua feeder va anten. Ngoai ra, cach tiep dien cho anten cung gop phan dang ke quyet dinh den kich thuoc va thuoc tinh cua anten. Trong mo hinh anten duoc thiet ke, cau truc tiep dien bang ong dan song dong phang duoc su dung. Ong dan song dong phang co cau truc mong, gop phan dang ke thu nho kich thuoc anten. Cac thiet ke anten gan day con cho thay viec su dung ong dan song dong phang co the cho phep mo rong bang thong.

Tinh toan thiet ke va su dung phan mem mo phong

Tu cac nguyen ly co ban va cac ky thuat tao bang thong sieu rong cho anten vi dai, giai phap dua ra la ket hop su dung phuong phap phan tu huu han FEM (Finite Element Method) de tinh toan thiet ke anten va su dung phan mem Ansoft HSFF de mo phong ket qua.

3. Ket qua thiet ke va mo phong

3.1 Anten co ban

Mo hinh anten co ban duoc chon la anten monopole hinh chu nhat, tiep dien bang ong dan song dong phang, duoc in tren mot lop de bang dien moi co chieu day h=0.5mm. Do UWB co dai tan hoat dong tu 3,1GHz - 10,6 GHz nen khi thiet ke anten se chon tan so cong huong trung tam vao khoang 7GHz. Khi do, kich thuoc cua mieng buc xa (monopole) se phu thuoc vao tan so trung tam, do rong cua bang tan va hang so dien moi hieu dung cua lop de [1]. Kich thuoc tinh toan cu the cua anten co ban nhu o Hinh 1.

Hinh 1. Anten co ban

Kich thuoc cua duong dan dien o giua, kich thuoc cua khe hep va hang so dien moi tuong doi cua lop de se quyet dinh tro khang dac tinh cua ong dan song dong phang. Voi kich thuoc ghi tren hinh 1, khi chon lop de lam bang FR4 co ε= 4.4, thi tro khang dac tinh thu duoc la 50 Ohm.

Tu cac yeu cau cua anten cho cong nghe bang thong sieu rong co the chon cac thong so ty so dien ap song dung VSWR (Voltage Standing Wave Ratio), do thi buc xa va he so tang ich de danh gia anten. Doi voi cac anten thiet ke, yeu cau phai dat VSWR < 2 trong dai tan so hoat dong [1]. (Dieu nay tuong duong voi viec trong nhieu thiet ke anten su dung tieu chi danh gia he so ton hao nguoc RL-Return Loss nho hon -10dB trong dai tan so hoat dong).

Hinh 2: Ket qua mo phong VSWR cua anten co ban

Tu do thi mo phong VSWR co the thay gia tri VSWR< 2 chi dat duoc trong khoang tan so tu 3,2GHz - 9,5GHz, do do anten nay chua dam bao hoat dong tot trong dai tan cua UWB (3,1GHz - 10,6GHz).

3.2 Mo hinh anten vi dai UWB

Mo hinh anten vi dai UWB duoc thiet ke dua tren mo hinh anten co ban da duoc trinh bay trong 3.1. Anten UWB duoc tao ra bang cach su dung nguyen ly bien doi tu tu de tao bang rong nho viec tang dan dan kich thuoc cua duong tiep dien tu ong dan song dong phang ra mieng buc xa cua anten.

Anten vi dai UWB co kich thuoc 30 x 40 mm, duoc in tren lop de FR4 co ε = 4.4 va chieu day 0.5 mm, voi bo bien doi tu tu nhu tren Hinh 3.

Hinh 3: Anten vi dai UWB

Duoi day la mot so ket qua mo phong cho mo hinh anten UWB duoc thiet ke o tren. O Hinh 4, co the thay gia tri VSWR < 2 trong dai tan cua UWB (3,1GHz - 10,6GHz). Hinh 5 bieu dien do thi buc xa cua anten o 3.5GHz va 9GHz trong 2 mat phang 0º va 90º. Qua do thi co the thay do thi buc xa anten duoc thiet ke gan dat dang huong trong toan dai tan so hoat dong. Hinh 6 la do thi tang ich cua anten .

Hinh 4: Ket qua mo phong VSWR cua anten vi dai UWB

Hinh 5: Do thi buc xa cua anten UWB o tan so 3,5GHz va 9 GHz.

Hinh 6- Do thi tang ich cua anten vi dai UWB

Hinh 7. Mot mau anten UMB duoc thiet ke

4. Ket luan

Mau anten vi dai UWB duoc thiet ke co bang thong bao trum dai tan tu 3,1GHz den 10,6GHz, dam bao buc xa dang huong tren toan bo bang tan, kich thuoc nho, cau hinh don gian, de che tao. O tan so cao, do thi buc xa cua cac anten thong thuong co xu huong bi meo. O mo hinh anten vi dai UWB thiet ke, khi xet den tan so 11GHz, anten van dat buc xa dang huong. Anten nay da dap ung duoc yeu cau cua anten cho cac thiet bi di dong su dung cong nghe bang thong sieu rong.

Tai lieu tham khao

[1] I.J.Bahl, P.Bhartia, Microstrip Antennas, Artech House, 1980.

[2] David M. Pozar, Microwave Engineering, 2nd edition, John Wiley & Sons, Inc, 1997.

[3] Johnna Powell, Antenna Design for Ultra Wideband Radio, Master thesis, Massachusetts Institute of Technology, 2004.

[4] Phan Anh, Truong dien tu va truyen song, Nha xuat ban Dai hoc Quoc Gia Ha Noi, 2002.

[5] Phan Anh, Ly thuyet va ky thuat anten, Nha xuat ban Dai hoc Quoc Gia Ha Noi, 2002.

(Bài do Cong ty Viẽn thong Quóc té VTI tài trọ)