impedance matching

Importance of impedance matching and coupling mechanism(1)

And older story in microwave and THz  or higher frequency range is finding a way that we have high effieceincy coupling. So i will discuss about mechanism coupling and phase and impedance matching.

In electronic circuits,we learn first law impedance matching,when i had  a source with impedance XS and the load has impedance  Xfor high efficeicny we should had condition matching.

RL=RS

XL=-XS

In microwave ad comunication systems, we may face with some loads can not satisfy this condition,so we should use some extra circuit that make coupling effieciecny with out changing paramter loads.

Single Stub Matching:

There are two design parameters for single stub matching:

  • ‰ The location of the stub with reference to the load dstub
  • ‰ The length of the stub line Lstub

Any load impedance can be matched to the line by using single stub technique. The drawback of this approach is that if the load is changed, the location of insertion may have to be moved.for more info please see this file handbook.

single stub matching

Fig1:single stub matching

The transmission line realizing the stub is normally terminated by a
short or by an open circuit. In many cases it is also convenient to
select the same characteristic impedance used for the main line,
although this is not necessary. The choice of open or shorted stub
may depend in practice on a number of factors. A
short circuited
stub is
less prone to leakage of electromagnetic radiation and is
somewhat easier to realize. On the other hand, an
open circuited
stub may be
more practical for certain types of transmission lines,
for example microstrips where one would have to drill the insulating
substrate to short circuit the two conductors of the line.

 

Since the circuit is based on insertion of a parallel stub, it is more convenient to work with admittances, rather than impedances.stub matching admittance

Fig2:stub matching admittance

For proper impedance match:

YA=Ystub+Y(dstub)=Y0=1/Z0

In order to complete the design, we have to find an appropriate location for the stub. Note that the input admittance of a stub is always imaginary (inductance if negative, or capacitance if positive):
Ystub = jBstub

A stub should be placed at a location where the line admittance has real part equal toY0:

Y(dstub)=Y0+JB(dstub)

For matching, we need to have

Bstub = – B(dstub )

Depending on the length of the transmission line, there may be a
number of possible locations where a stub can be inserted for
impedance matching. It is very convenient to analyze the possible
solutions on a Smith chart.

 

stub matching

Fig3:YA in stub matching

So in the next post we with same title we will discuss about other mechanism mathcing.

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