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1、Antenna BasicsIntroductionAntennas are a very important component of communication systems.By definition,an antenna is a device used to transform an RF signal,traveling on a conductor,into an electromagnetic wave in free space.Antennas demonstrate a property known as reciprocity,which means that an
2、antenna will maintain the same characteristics regardless if it is transmitting or receiving.Most antennas are resonant devices,which operate efficiently over a relatively narrow frequency band.An antenna must be tuned to the same frequency band of the radio system to which it is connected,otherwise
3、 the reception and the transmission will be impaired.When a signal is fed into an antenna,the antenna will emit radiation distributed in space in a certain way.A graphical representation of the relative distribution of the radiated power in space is called a radiation pattern.Antenna GlossaryBefore
4、we talk about specific antennas,there are a few common terms that must be defined and explained:-Input ImpedanceFor an efficient transfer of energy,the impedance of the radio,of the antenna and of the transmission cable connecting them must be the same.Transceivers and their transmission lines are t
5、ypically designed for 50impedance.If the antenna has an impedance different from 50,then there is a mismatch and an impedance matching circuit is required.-Return loss The return loss is another way of expressing mismatch.It is a logarithmic ratio measured in dB that compares the power reflected by
6、the antenna to the power that is fed into the antenna from the transmission line.The relationship between SWR and return loss is the following:963名师资料总结-精品资料欢迎下载-名师精心整理-第 1 页，共 15 页 -Return Loss(in dB)=?20log10SWRSWR-1-Bandwidth The bandwidth of an antenna refers to the range of frequencies over whi
7、ch the antenna can operate correctly.The antennas bandwidth is the number of Hz for which the antenna will exhibit an SWR less than 2:1.The bandwidth can also be described in terms of percentage of the center frequency of the band.?BW=100FH-FLFCwhere FHis the highest frequency in the band,FLis the l
8、owest frequency in the band,and FC is the center frequency in the band.In this way,bandwidth is constant relative to frequency.If bandwidth was expressed in absolute units of frequency,it would be different depending upon the center frequency.Different types of antennas have different bandwidth limi
9、tations.-Directivity and Gain Directivity is the ability of an antenna to focus energy in a particular direction when transmitting,or to receive energy better from a particular direction when receiving.In a static situation,it is possible to use the antenna directivity to concentrate the radiation b
10、eam in the wanted direction.However in a dynamic system where the transceiver is not fixed,the antenna should radiate equally in all directions,and this is known as an omni-directional antenna.Gain is not a quantity which can be defined in terms of a physical quantity such as the Watt or the Ohm,but
11、 it is a dimensionless ratio.Gain is given in reference to a standard antenna.The two most common reference antennas are the isotropic antenna and the resonant half-wave 964名师资料总结-精品资料欢迎下载-名师精心整理-第 2 页，共 15 页 -dipole antenna.The isotropic antenna radiates equally well in all directions.Real isotropi
12、c antennas do not exist,but they provide useful and simple theoretical antenna patterns with which to compare real antennas.Any real antenna will radiate more energy in some directions than in others.Since it cannot create energy,the total power radiated is the same as an isotropic antenna,so in oth
13、er directions it must radiate less energy.The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power.Usually we are only interested in the maximum
14、 gain,which is the gain in the direction in which the antenna is radiating most of the power.An antenna gain of 3 dB compared to an isotropic antenna would be written as 3 dBi.The resonant half-wave dipole can be a useful standard for comparing to other antennas at one frequency or over a very narro
15、w band of frequencies.To compare the dipole to an antenna over a range of frequencies requires a number of dipoles of different lengths.An antenna gain of 3 dB compared to a dipole antenna would be written as 3 dBd.The method of measuring gain by comparing the antenna under test against a known stan
16、dard antenna,which has a calibrated gain,is technically known as a gain transfer technique.Another method for measuring gain is the 3 antennas method.,where the transmitted and received power at the antenna terminals is measured between three arbitrary antennas at a known fixed distance.-Radiation Pattern The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna,at a constant distance.The radiation pattern is a reception pattern
