# Problem with Pozar Derivation For Transmission Lines

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#### HPRF

##### Newbie level 4
Hi

I am reading through the proof for transmission line theory in Pozar and have a basic derivation step question that I am wondering if anyone can answer.

In the derivation Kirchoffs Voltage and Current laws are applied to give

V(z,t)-R*(delta)z*i(z,t)-L*(delta)z*(d/dt)i(z,t)-V(z+(delta)z,t)

for the voltage and something similar for the current.

The problem is that there is no explanation for where V(z+(delta)z,t) and I(z+(delta)z,t) go to when (delta)z-->0. They just seems to disappear from the derivation, without explanation, instead of cancelling with V(z,t) and I(z,t).

Any ideas would be much appreciated.

Not sure what you mean. Can you post the EQ ? (or Book & pg #)
If "(Delta)z" is the Calculus dz then it is most likely an integration, as dz goes to 0 .

look into the definition of differentiation in a calculus book. as dz goes to zero, the equation you mention turns into a differential equation. you'll be able to find this in a calculus book for sure.

Thanks, that solved it. A basic error.

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