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ResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering射频电路与天线(二)RFCircuitsandAntennas第第88讲讲天线的馈电天线的馈电褚庆昕华南理工大学电子与信息学院天线与射频技术研究所Tel:22236201-601Email:qxchu@scut.edu.cnSouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering第8讲内容引言考虑阻抗匹配的馈电方法考虑电流平衡的馈电方法-巴伦SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering8.1引言当天线通过传输线馈电，而与发射机或接收机连接时，有两个问题需要考虑：¾天线阻抗与传输线阻抗匹配¾天线上电流平衡问题SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering8.2考虑阻抗匹配的馈电方法天线阻抗与传输线阻抗匹配关系到：¾发射机的最大输出功率。当阻抗共轭匹配时，输出功率最大；¾接收机的灵敏度。当阻抗匹配时，接收机灵敏度最高；¾天线增益和效率。当阻抗匹配时，天线增益和效率最大；¾传输线功率容量。当阻抗匹配时，传输线功率容量最大；SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering实现天线与传输线阻抗匹配的方法：¾在天线与传输线之间采用阻抗匹配网络；¾改变馈电结构使天线与传输线匹配。采用阻抗匹配网络虽然可以实现良好匹配，但增加了结构尺寸和成本。网络阻抗设计属于射频电路问题。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering改变馈电结构调节天线输入阻抗的方法：¾改变馈电的位置，馈电点越靠近振子端点，阻抗越大。当振子长度小于半波长时，改变馈电位置不影响方向图但大于半波长时，对方向图也会有影响；¾采用并联馈电方式。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering几种并联馈电方法：Y匹配T匹配SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering8.3巴伦许多天线天然是对称的，因此电流也应该是对称的，即，是平衡的。但是，对称天线的电流平衡与馈电结构密切相关。对于对称振子，如果用双导线馈电，则电流平衡，但如果用同轴线馈电就可能出现电流不平衡的问题。电流的不平衡会影响天线的辐射特性：方向图、阻抗和极化方式等。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering同轴馈电由于有部分电流到同轴线的外表面，造成对称振子的方向图畸变。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering因此，需要采用平衡不平衡转换器，使得电流的不平衡变为平衡。平衡不平衡转换器的英文是balun，它是balance和unbalance的缩写。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering巴伦1：λ/4套筒-扼流环SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering巴伦2：折合巴伦SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineeringSouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineeringL是四分之一波长SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering巴伦3：U形环-1:4巴伦/4LRSouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering巴伦4：谐振式巴伦:SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering宽带微带切割式巴伦：SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering天然巴伦：SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering环天线的巴伦：SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering印刷巴伦SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering传输线变压器主要在低频应用(MHz)不仅有平衡-不平衡的变换作用，而且具有阻抗变换作用。优点是功率比较大、体积小、插损小、外磁场小，不需要屏蔽。可作为平衡转换器、阻抗变换器、混接网络，以及功率合成器中的分配器等。SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineeringSouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineeringSouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering巴伦和变压器的结合SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering第8讲要求了解并联馈电的原理了解各类巴伦的原理和基本分析SouthChinaUniversityofTechnologyResearchInstituteofAntennas&RFTechniquesSchoolofElectronic&InformationEngineering习题88-1分析右图所示巴伦的原理，给出电流分布和阻抗的分析。