reflections on transmission line

reflections in transmission lines

Hi! I have some basic problems need help ...
plz see the picture, I dont know why the "V total" equ E inc + E refl ,"I total" equ I inc -I refl ?

Any good paper or something can help me understand this problems?

Thanks in Adnance!

-cqcq

reflections in transmission lines

Alright, Einc and Erefl are called "wave amplitudes". They are similar to voltages, but they "move" along the transmission line. What do I mean by that? Well, imagine you are on vacation at the beach, and you decide to go stand in the water. Well, the height of the surface of the water is always changing due to the waves. If you look far out, you can see a wave peak, and it is travelling towards you. When it hits you, the water height is a maximum.

So that would be like the Einc on a transmission line, some measureable thing that has an amplitude and is moving "towards" the load.

Now, if you were back on the beach, and decided to go stand in the water, but in front of a big concrete sea wall. You might notice that the waves come in from the sea, go past you, hit the sea wall, and reflect backward out to sea. If you watched the water from far out to sea, you could see the waves coming in. If you watched the sea walls, you could see the waves reflecting back.

But if you stared staight down at the water surface where you were standing, you would not see any waves at all, you would just see a surface height of the water. And although you could see that surface height changing, you would not really know what was making up the changing water height. In other words, you could measure the amplitude of the water height at that one point, and it would be a superposition of the incident waves AND the reflected waves at that one physical location/time instant.

same with microwaves. At the load you can measure the "height" of the waves (in volts if it is a wire transmission structure, or in some Efield intensity if in a waveguide). This "voltage" is the superposition of incident and reflected wave voltages, so it is Einc + Erefl.

It is important to note that the wave phases are very important. For instance, if the load is an open circuit, and the reflected wave is in phase with the incident wave, then the voltage measured at the load is 2 x the incident wave amplitude.

reflection in transmission lines

Hi biff44,Thank you very much!
As you said , I need to move the Reflected Wave(Erefl) to LINE1?



I have do some simulations in ADS I found If I consider the Reflected Wave on the LINE2 is not satisfy "V total" equ E inc + E refl.

square wave transmission line

I wonder, what you are simulating, cause there's no transmission line in your schematic. Also, if you assume 50 ohms transmission line impedance, there's zero reflected wave, cause you have a matched termination.

Although ADS is a powerful tool, it can't be expected to give meaningful answers to unreasonable questions.

advantage of reflections in transmission line

Yes, I would also like to see a transmision line in your simulation. For instance, use a transmission line of 50 ohms characteristic impedance, dielectric constant of 2.2 (good for common BNC type connection cable), 1 meter length, and ue a 30 ohm load. Look at the pulses at the generator, and at the load. If you simulator is capable of accurately showing things, the pulses at the generator should show some ringing. As you vary the length of the line you will see the location of the "ringing" move around. As you vary the load resistance from 10 to 50 to 200 ohms, you willl see the amplitude of the rings change, and something odd when you go from <50 ohm load to >50 ohm load. Play around.

But, this being a microwave board, I assumed you were sending an analog signal, down the transmission line. My comments were valid for a single frequency.

If you are sending a square wave down the line, things are a little different. A swaure wave will have a lot of frequencies in it (the fundamental frequency f, 3f, 5f, 7f....),

There are time domain ways of thinking of transmission lines. That is more like what you want to understand. They would better explain how small amounts of pulses get reflected.

Rich
www.maguffinmicrowave.com