# How transmission line circuit works

#### WBahn

Joined Mar 31, 2012
26,912
MOD NOTE: Looks like homework. Moved to Homework Help from General Electronics Chat.

#### MrAl

Joined Jun 17, 2014
9,350
Hi
can you sescribe to me what happens when the input volt Vs=230 V
what happens inside the circuit?
thanks

View attachment 94041

Hi,

What do you already know about a transmission line?

Without going into detail, the short answer for the simplified circuit is as follows...
When a voltage is first applied to the input there is a voltage differential across the inductor(s).
It takes time for the current to build in an inductor when there is a voltage across it.
The inductor pumps current into the second capacitor little by little.
It takes time for the voltage to build across the capacitor given a current (from the inductor) through it.
These time delays mean that the voltage takes some non zero time to reach the other end, but by that time the voltage near the center of the line (depending on wavelength) goes lower, and so now if all the energy is not absorbed in the load the remainder starts to travel backward down the line toward the start.

That is a very rough idea of what happens, but it would be better to use at least two inductors and three capacitors so you can think about the voltage at the center of the line too as being another node.

So besides that, what else do you know about the transmission line?
Also, is this really homework or you just want to know more about this topic?

#### Ali Alkhudri

Joined Nov 3, 2015
19
thank you .. this is not a home work but I started a course about amplifier design and our teacher mentioned the transmission lines so I just googled it and found that picture in internet ... So transmission line is totaly new for me #### Ali Alkhudri

Joined Nov 3, 2015
19
Hi,

What do you already know about a transmission line?

Without going into detail, the short answer for the simplified circuit is as follows...
When a voltage is first applied to the input there is a voltage differential across the inductor(s).
It takes time for the current to build in an inductor when there is a voltage across it.
The inductor pumps current into the second capacitor little by little.
It takes time for the voltage to build across the capacitor given a current (from the inductor) through it.
These time delays mean that the voltage takes some non zero time to reach the other end, but by that time the voltage near the center of the line (depending on wavelength) goes lower, and so now if all the energy is not absorbed in the load the remainder starts to travel backward down the line toward the start.

That is a very rough idea of what happens, but it would be better to use at least two inductors and three capacitors so you can think about the voltage at the center of the line too as being another node.

So besides that, what else do you know about the transmission line?
Also, is this really homework or you just want to know more about this topic?
thank you .. this is not a home work but I started a course about amplifier design and our teacher mentioned the transmission lines so I just googled it and found that picture in internet ... So transmission line is totaly new for me one more question : How is voltage depended on wavelength ?

Last edited:

#### MrAl

Joined Jun 17, 2014
9,350
Hi again,

Well let me clarify that first.

When i stated that i was assuming that the driver voltage was sinusoidal, because that is how a lot of transmission lines are driven and studied and because you stated "230v" which is a sort of common AC line voltage. In that case the voltage is partially dependent on wavelength because the voltage of a traveling sine wave varies with distance, even without considering any circuit reactions yet. So even if we had some sort of perfect transmission line we would measure different voltages at different points in the line, and if we could measure several points at the same instant we'd see voltages that varied as a sine wave at the same point in time (but at different distances along the line). The sine wave has a period that also relates to the wave length, and the wave length is after all a measure of distance.

If you are dealing with a DC voltage however (your 230v perhaps) then the response is mostly due to the elemental values. The response may still be oscillatory however, depending on the load and equivalent series resistances, so you may see a wave traveling backwards anyway.

It would be informative to model the simplest of approximated transmission lines by using two inductors and two or three capacitors (might not need the first cap if the drive voltage is a pure voltage source) and that way you can see what happens at both ends as well as in the middle of the line. You can then look at the wave at three different points along the line and start to gather conclusions about the basic operation. The more exact model requires the use of partial differential equations which i am not sure if you want to get into right now.

Maybe you can mention a few of the circuits you have studied up to now so we can get an idea where you are in your studies so far. That helps to understand what kind of information you would like to know and in what form.

#### Ali Alkhudri

Joined Nov 3, 2015
19
Hi again,

Well let me clarify that first.

When i stated that i was assuming that the driver voltage was sinusoidal, because that is how a lot of transmission lines are driven and studied and because you stated "230v" which is a sort of common AC line voltage. In that case the voltage is partially dependent on wavelength because the voltage of a traveling sine wave varies with distance, even without considering any circuit reactions yet. So even if we had some sort of perfect transmission line we would measure different voltages at different points in the line, and if we could measure several points at the same instant we'd see voltages that varied as a sine wave at the same point in time (but at different distances along the line). The sine wave has a period that also relates to the wave length, and the wave length is after all a measure of distance.

If you are dealing with a DC voltage however (your 230v perhaps) then the response is mostly due to the elemental values. The response may still be oscillatory however, depending on the load and equivalent series resistances, so you may see a wave traveling backwards anyway.

It would be informative to model the simplest of approximated transmission lines by using two inductors and two or three capacitors (might not need the first cap if the drive voltage is a pure voltage source) and that way you can see what happens at both ends as well as in the middle of the line. You can then look at the wave at three different points along the line and start to gather conclusions about the basic operation. The more exact model requires the use of partial differential equations which i am not sure if you want to get into right now.

Maybe you can mention a few of the circuits you have studied up to now so we can get an idea where you are in your studies so far. That helps to understand what kind of information you would like to know and in what form.
Hi again,

Well let me clarify that first.

When i stated that i was assuming that the driver voltage was sinusoidal, because that is how a lot of transmission lines are driven and studied and because you stated "230v" which is a sort of common AC line voltage. In that case the voltage is partially dependent on wavelength because the voltage of a traveling sine wave varies with distance, even without considering any circuit reactions yet. So even if we had some sort of perfect transmission line we would measure different voltages at different points in the line, and if we could measure several points at the same instant we'd see voltages that varied as a sine wave at the same point in time (but at different distances along the line). The sine wave has a period that also relates to the wave length, and the wave length is after all a measure of distance.

If you are dealing with a DC voltage however (your 230v perhaps) then the response is mostly due to the elemental values. The response may still be oscillatory however, depending on the load and equivalent series resistances, so you may see a wave traveling backwards anyway.

It would be informative to model the simplest of approximated transmission lines by using two inductors and two or three capacitors (might not need the first cap if the drive voltage is a pure voltage source) and that way you can see what happens at both ends as well as in the middle of the line. You can then look at the wave at three different points along the line and start to gather conclusions about the basic operation. The more exact model requires the use of partial differential equations which i am not sure if you want to get into right now.

Maybe you can mention a few of the circuits you have studied up to now so we can get an idea where you are in your studies so far. That helps to understand what kind of information you would like to know and in what form.
Thanks .. I just studied circuits of resistors, capacitors, inductors , diodes .. basics