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Radiating Systems

EE-402

 

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ELECTRICAL CHARACTERISTICS OF

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• These primarily depends upon the

construction of the transmission line.

• Important parameters: line capacitance,inductance and conductance (the leakage

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no dielectric is perfect).

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• A uniform transmission line’s section can be

like:

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• For many applications the values for lineresistance and conductance (leakage) can be

neglected so the model would look like:

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CHARACTERISTIC IMPEDANCE OF THE

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•

Lets thought of an infinite line.• What should be its impedance?

• Logically infinite.

• But that is not the correct answer.

• An infinite line can be represented by infinite

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numbers of inductors and capacitors, onesuch section is shown in the previous figure.

• If a voltage is applied across the line, there

would be a current that would flow thru the

infinite line as well.

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•

But that current would be having a finitevalue.

• The ratio of that a lied volta e to that

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current shall give the characteristicimpedance of the line.

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In other words Characteristic Impedance isthe finite impedance of the infinite line. It is

usually represented by Z0.

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Lets drive the expression for Z0 . Considerthe following circuit:

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• Hence we can say that the characteristic impedance

is independent of the length of the line.• Value of Z0 depends upon the distributed inductance

and the capacitance in the line.

• If the separation between the wires is increased,what shall happen to the inductance and capacitanceof the line?

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• Inductance shall increase, capacitance shalldecrease……why?

• Thus higher separation, greater characteristic

impedance of the line.• Conclusion: Characteristic impedance of the line thus

is a function of the line construction.

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• What would happen to the characteristic

impedance of the line if the diameter of the

conductors is reduced?

•Obviously this shall affect the capacitancemore than the inductance.

•  

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the capacitor plates.

• This would definitely reduce the capacitance.

• And if the capacitance is reduced, as per theformula for characteristic impedance (Z0 =

√L/C), it is bound to increase.

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•

For a coaxial or concentric line the formula is:

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• Coax is not to be stepped or crimped….

why?????

 

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THEREFORE ITS Z0 WOULD GET CHANGED.