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Antenna Matching and Smith Chart


Antenna Matching is very important work in client side, not only antenna supplier side.

To be precise, all antennas need impedance matching when applied to products. especially built-in antennas, such as spring type, chip type, pcb or fpc type antenna.


There are several ways to tune an antenna to achieve better performance. For resonant antennas the main factor is the length. Ideally the frequency which gives least reflection should be in the middle of the frequency band of interest. Thus if the resonance frequency is to low, the antenna should be made shorter. If the resonance frequency is too high, the antenna length should be increased.
Even if the antenna resonates at the correct frequency it might not be well matched to the correct impedance. Dependent of the antenna type there are several possibilities to obtain optimum impedance at the correct frequency. Size of ground plane, distance from antenna to ground plane, dimensions of antenna elements, feed point, and plastic casing are factors that can affect the impedance. Thus by varying these factors it might be possible to improve the impedance match of the antenna. If varying these factors is not possible or if the performance still needs to be improved, discreet components could be used to optimize the impedance. Capacitors and inductors in series or parallel can be used to match the antenna to the desired impedance. Figure 18 shows how inductors and capacitors can be used to change the impedance.



Figure 18. Series and Parallel Capacitors and Inductors in the Smith Chart





Freeware of Smith Chart tool

http://www.fritz.dellsperger.net/smith.html









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