天线基本原理精品文稿.ppt

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1、天线基本原理第1页，本讲稿共19页壹、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：ZLZinZin=ZoZL+jZ0tan(l)Z0+jZLtan(l)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l第2页，本讲稿共19页波長=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*108103 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)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第3页，本讲稿共19页涉及傳輸線效應，直角傳輸導線會有

2、大的寄生電容WR 3Wa=1.8W第4页，本讲稿共19页同軸線阻抗DinnerDouterZ0=138logDouterDinnerOuter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。第5页，本讲稿共19页RF Connectors ImpedanceZ0=60lnD/dD: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=2c/(D+d)第6页，本讲稿共19页/4 TransformersRequire Z0/4=Zin

3、ZLL=/4 Zin=Z0/4 ZL+jZ0/4 tan(l)Z0/4+jZLtan(l)ZLZin因l=(2/)(/4)=/2 Z0/4=ZinZL Apply to real loads and sourcesPerfect match at spot frequency(10%-20%useable bandwidth)第7页，本讲稿共19页Impedance Match Rule串聯L並聯電容並聯L串聯C第8页，本讲稿共19页天線設計基本原理Sleeve Dipole AntennaThe current distribution of sleeve antenna is like t

4、he 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第9页，本讲稿共19页Dipole Antenna with Center DrivingIf the radius of wire,a,is very small,the input impedance of half wavelength dipole is 73+j 42.5 ohmThe shorter the length of dipole,the real part of impedance will be s

5、mallerIn 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 dipoleThe directivity of half wavelength dipole is 2.15 dBi第10页，本讲稿共19页Whip Antenna for AMPSTh

6、is monopole antenna with higher directivity(5.162 dBi)for length 0.625 wavelengthIf the length is quarter wavelength,then the directivity is 2.15 dBi這是常見5dBi的作法第11页，本讲稿共19页Radiation Pattern with Center DrivingThe definition of dBi is the antenna gain which compares to that of the isotropic antennaTh

7、e definition of dBd is the antenna gain which compares to that of the dipole antennaThe directivity of half wavelength dipole is 2.15 dBiThe directivity of 1.25(2x0.625)wavelength dipole 5.16 dBi這是場型圖，請注意不同gain值的場型第12页，本讲稿共19页VSWR:Return loss(RL)Transmission Loss(TL)參、天線規格:VSWR與Return Loss的關係是互換關係，若

8、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，幅射效率愈差第13页，本讲稿共19页ExamplesVSWR=1.5,RL=14 dB,TL=0.177 dBVSWR=2.0,RL=9.5 dB,TL=0.512 dBVSWR=3.0,RL=6.0 dB,TL=1.24 dBLower power handling for higher VSWRThe wider the bandwidth the larger the VSWR第14页，本讲稿共19页E-Plane Cut of antenna radiation

9、 pattern which contain the maximum field intensity and the direction of E field or electric current of antennaH-Plane Cut of antenna radiation pattern which contain the maximum field intensity and the direction of H field or magnetic current of antennaPrinciple planesBoth E-and H-plane radiation pat

10、ter第15页，本讲稿共19页BeamwidthHalf power beamwidthE-plane beamwidth,H-plane beamwidthThe wider beamwidth the wider angular coverageThe wider beamwidth the more interferences input to the receiver第16页，本讲稿共19页PolarizationLinear polarizationVertical polarization,Horizontal polarization,Slant linear polarizat

11、ionCircular polarizationRHCP,LHCPElliptical polarization第17页，本讲稿共19页 Transmit Receive Polarization Loss vertical horizontal infinity horizontal vertical infinity RHCP LHCP infinity LHCP RHCP infinity RHCP or LHCP linear 3 dB linear RHCP or LHCP 3 dB vertical vertical 0 dB RHCP LHCP infinity horizontal horizontal 0 dB 45度slant Linear V or H 3 dB Polarization loss第18页，本讲稿共19页第19页，本讲稿共19页