天线基本原理(PPT)

1、天線設計簡介 壹、傳輸線 貳、天線設計基本原理 參、天線規格 肆、實驗與驗證課程,听说过吗？躺在床上能和外教一对一练英语口语！适合职场中的你！免费体验史上最牛英语口语学习，太平洋英语,壹、傳輸線 根據傳輸線理論，一段無損耗傳輸線，輸入阻抗可表成：,ZL,Zin,Z0:特性阻抗 Zin:輸入阻抗 :相位常數,與訊號波長的關係為 = 2/ l:傳輸長度,波長=,光速,頻率 *, 介電常數,即= C/f*,如1KHz的信號在一介電係數為4的電路板上傳播時其波長()為,= C/f*=,3*108,103 4,= 1.5 105 公尺,低頻的訊號波長遠大於傳輸線的長度， l/=0 即l=0 ，tan(l

2、)趨近0，故Zin =ZL 故在分析低頻電路時，可以不考慮傳輸線的效應,涉及傳輸線效應，直角傳輸導線會有大的寄生電容,a=1.8W,同軸線阻抗,Dinner,Douter,Z0 =,138,log,Douter,Dinner,Outer編織線愈疏則造成insertion loss 愈大，相對的編織線愈密其insertion loss 愈小，地網與信號線間的介電常數愈趨近於空氣1，衰減愈小，故使用氮氣發泡是業界常用的方式。,RF Connectors Impedance,Z0 =,60lnD/d,D : 外導體內徑 d : 內導體內徑 : 絕緣介電常數,RF Connector的上限頻率 fu

3、= 2c/(D+d),/ 4 Transformers,Require Z0 / 4 = ZinZL,L= / 4,Zin =,Z0 / 4,ZL + jZ0 / 4 tan(l),Z0 / 4 + jZLtan(l),ZL,Zin,因l=(2/ )(/4)= /2,Z0 / 4 = ZinZL,Apply to real loads and sources Perfect match at spot frequency(10%-20% useable bandwidth),Impedance Match Rule,串聯L,並聯電容,並聯L,串聯C,天線設計基本原理 Sleeve Dipole

4、 Antenna,The current distribution of sleeve antenna is like the dipole antenna,這是目前業界最常見的SLEEVE偶極天線,銅管,/ 4約22mm,= C/f*約8.8cm,/ 4約22mm,實作驗證要不斷嘗試錯誤方有心得,此線長會有影響,Dipole Antenna with Center Driving,If the radius of wire, a, is very small, the input impedance of half wavelength dipole is 73 +j 42.5 ohm Th

5、e shorter the length of dipole, the real part of impedance will be smaller In order to match the 50 ohms coaxial cable, the length of dipole will be less than half wavelength The imaginary part of impedance will be changed form inductance to capacitance for shorter length of dipole The directivity o

6、f half wavelength dipole is 2.15 dBi,Whip Antenna for AMPS,This monopole antenna with higher directivity (5.162 dBi) for length 0.625 wavelength If the length is quarter wavelength, then the directivity is 2.15 dBi,這是常見5dBi的作法,Radiation Pattern with Center Driving,The definition of dBi is the antenn

7、a gain which compares to that of the isotropic antenna The definition of dBd is the antenna gain which compares to that of the dipole antenna The directivity of half wavelength dipole is 2.15 dBi The directivity of 1.25 (2x0.625) wavelength dipole 5.16 dBi,這是場型圖，請注意不同gain值的場型,VSWR: Return loss (RL)

8、Transmission Loss (TL),參、天線規格:,VSWR與Return Loss的關係是互換關係，若VSWR是2.0:1,則相對於Return Loss是約-10dB，其意是發射出去有90%的功率，反射回來有10%，反射愈多，幅射效率愈差,Examples VSWR=1.5, , RL=14 dB, TL=0.177 dB VSWR=2.0, , RL=9.5 dB, TL=0.512 dB VSWR=3.0, , RL=6.0 dB, TL=1.24 dB Lower power handling for higher VSWR The wider the bandwidth

9、the larger the VSWR,E-Plane Cut of antenna radiation pattern which contain the maximum field intensity and the direction of E field or electric current of antenna H-Plane Cut of antenna radiation pattern which contain the maximum field intensity and the direction of H field or magnetic current of an

10、tenna Principle planes Both E- and H-plane radiation patter,Beamwidth,Half power beamwidth E-plane beamwidth, H-plane beamwidth The wider beamwidth the wider angular coverage The wider beamwidth the more interferences input to the receiver,Polarization,Linear polarization Vertical polarization, Horizontal polarization, Slant linear polarization Circular polarization RHCP, LHCP Elliptical polarization,Transmit Receive Polarization Loss vertical horizontal infinity horizontal vertical infinity RHCP LHCP infinity LHCP RHCP infi