Impedance matching Standing wave ratio

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swr used measure of impedance matching of load characteristic impedance of transmission line carrying radio frequency (rf) signals. applies transmission lines connecting radio transmitters , receivers antennas, similar uses of rf cables such cable television connections tv receivers , distribution amplifiers. impedance matching achieved when source impedance complex conjugate of load impedance. easiest way of achieving this, , way minimizes losses along transmission line, both source , load real, is, pure resistances, equal characteristic impedance of transmission line. when there mismatch between load impedance , transmission line, part of forward wave sent toward load reflected along transmission line towards source. source sees different impedance expects can lead lesser (or in cases, more) power being supplied it, result being sensitive electrical length of transmission line.


such mismatch undesired , results in standing waves along transmission line magnifies transmission line losses (significant @ higher frequencies , longer cables). swr measure of depth of standing waves , therefore measure of matching of load transmission line. matched load result in swr of 1:1 implying no reflected wave. infinite swr represents complete reflection load unable absorb electrical power, incident power reflected towards source.


it should understood match of load transmission line different match of source transmission line or match of source load seen through transmission line. instance, if there perfect match between load impedance zload , source impedance zsource=zload, perfect match remain if source , load connected through transmission line electrical length of 1 half wavelength (or multiple of 1 half wavelengths) using transmission line of characteristic impedance z0. swr not 1:1, depending on zload , z0. different length of transmission line, source see different impedance zload may or may not match source. deliberate, when quarter-wave matching section used improve match between otherwise mismatched source , load.


however typical rf sources such transmitters , signal generators designed purely resistive load impedance such 50Ω or 75Ω, corresponding common transmission lines characteristic impedances. in cases, matching load transmission line, zload=z0, ensures source see same load impedance if transmission line weren t there. identical 1:1 swr. condition ( zload=z0) means load seen source independent of transmission line s electrical length. since electrical length of physical segment of transmission line depends on signal frequency, violation of condition means impedance seen source through transmission line becomes function of frequency (especially if line long), if zload frequency-independent. in practice, swr (near 1:1) implies transmitter s output seeing exact impedance expects optimum , safe operation.







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