Research On Passive Intermodulation In The Base Station Antennas

With the development of the third generation mobile communication technology, themobile terminal communication rate is increased. Not only to transmit the voice signal, andcan be send images, real-time video and high-speed Internet connection. Therefore, To build ahigh quality base station equipment is very important. The antenna base station system isterminal equipment, and transform the baseband signal into electromagnetic wave, itsperformance directly affects the quality of the communication system. In addition to the basicelectrical performance parameters, passive intermodulation indicators as a important factorthat is affect the quality of the base station system, gradually be get more attention by themobile operators.The phenomenon of intermodulation generation due to transmission of multiple carriersthrough passive components is generally called passive intermodulation(PIM).PIM productsin high power multiple channel communication systems have become one of the spuriousinterference.The generation mechanism is discussed.Passive intermodulation interferencesare mainly originated two types of passive nonlinearities,namely,the contact nonlinearity andmaterial nonlinearity. The general expression of the amplitude of intermodulation products asa function of intermodulation orders in two carrier case are derived by Fourier series method.To build the blocks of the base station antenna, were analyzed according to the theoreticalmechanism of passive intermodulation generated. The constituent key materials of the basestation are used the basic metal die-casting or stamping and PCB etching, combined with thecoaxial cable composition. Electro-facing is the common style to the metal surface. Withdifferent plating thickness and plating method, the passive intermodulation indicators ofperformance has different levels. Compared to traditional FR4substrate,the PCB designingand manufacturing processes, have more requirements, especially for to protection of theoriginal PCB copper foil layer is very important, PCB copper foil surface roughness can beaffect the passive intermodulation.Firstly, the most important parts of the base station antenna are analyzed,then to analyzehow to decrease the assembling influence of all parts of base station antenna,the design and insulate principle must be carry out. Soldering factors for passive intermodulation influencecan not be ignored. The soldering wire and soldering temperatures are very important. Inaddition, the stability of the passive intermodulation is also very important content. About thebase station antenna passive intermodulation testing, the basic principles and testing methodsare discussed. As the passive intermodulation signal is very sensitive, to calibration andstability the test system is necessary for avoiding systematic errors at the testing process.At the practical using occasion,the interferences of passive intermodulation of the basestation, seriously have affect the communication quality of the base station system. Thispaper proposed some advisement to deal with it.