3
\$\begingroup\$

There is a generator with ZG = 50 ohms connected to a transmission line of Z0 = 50 ohms, the length of the line can be any, the load is ZL = 200, this gives a reflection coefficient GammaL = 200-50 / 200 + 50 = 0.6, therefore there is a mismatch and a wave is reflected off the load and travels to the generator. When this wave reaches the generator, does it dissipate in ZG?

I have read this post (and many others here) and its references that helped me a lot:

Can we measure reflected power in a transmission line?

but I am not sure what happens when the reflected wave reaches the generator. I proposed the case ZG = Z0 specifically to make it simpler.

Thank you so much.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Yes, if the generator is 50 ohms and the line is 50 ohms, the reflection will be absorbed completely. \$\endgroup\$
    – mkeith
    May 13 at 3:50
2
\$\begingroup\$

Let us look at life from the perspective of the traveling wave front. It enters the transmission line, which has an impedance of 50 Ohms, and it is happy. There is no reflection because source and line are matched. V/I = 50. The wavefront travels in the transmission line and is happy in its V/I = 50 world. Then wavefront reaches the 200 Ohm load, and now it has a problem. V/I = 200. The energy that has traveled down the line cannot be delivered to the 200 Ohm load without changing V/I. SO, at that point, a reflection is generated that travels back toward the source. That reflection is a traveling wave in its own right, and behaves just the same way as if it was generated at the load.

The reflection travels back toward the source, and it is happy in its V/I = 50 world. Then it gets to the source, and the 50 Ohm resistor of the source allows it to deliver all of the wave energy to the resistor while still respecting V/I = 50.

\$\endgroup\$
1
\$\begingroup\$

This is true??

From: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/finite-length-transmission-lines/

A reflected wave returning back to the source will be dissipated entirely if the source impedance matches the line’s, but will be reflected back toward the line end like another incident wave, at least partially, if the source impedance does not match the line.

EDIT1: If the above is true, the reflected power in the load and the power dissipated in the Zg should also be provided by the source, but looking at this problem raised in "Microwave Engineering. Pozar.4th Ed" and solved in the manual of solutions, the reflected power is not included as provided by the source and neither as dissipated in Zg

enter image description here

EDIT2: I think it only makes sense to talk that the reflected power can be dissipated at the source only when the source is off at the moment of receiving the reflected wave. For example when the source sends only one pulse.

\$\endgroup\$
3
  • \$\begingroup\$ You seem surprised. Is there something about this that doesn't make sense to you? \$\endgroup\$
    – Hearth
    May 13 at 3:04
  • \$\begingroup\$ @Hearth I don't understand where the energy that is dissipated (lost) in the generator comes from? The generator only provides the energy that is dissipated in the load (I assume a line without losses) \$\endgroup\$ May 13 at 3:16
  • 2
    \$\begingroup\$ Your source provides the energy that's dissipated in the source too; it's not 100% efficient and can't be 100% efficient with a 50 Ω resistive output impedance. \$\endgroup\$
    – Hearth
    May 13 at 4:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.