Transmission Line Question

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R. Fry SWL:
RE: ... excessive voltage or current at the antenna terminals resulting from a change in feedline impedance. ...

But a mismatch between the Zo of the termination at the far end of a transmission line to the Zo of that line does not really change the Zo of the transmission line.  That depends on the physical construction of the line, itself.

It causes an energy reflection back toward the input end of that line which in some situations can be enough to cause its failure, along with possible failure of the transmitter output stage.

KA2DZT:
It would seem that the phase angle between the transmitted and reflected waves at the transmitter would determine what excessive voltages or currents may or may not occur.  The phase angle is determined by the length of the transmission line between the transmitter and the mismatched load.

ashart:
To WA1STF:

You're not alone, OM!

Years ago, I assigned a tech to graph some data I had taken, and to do so on "log-log" graph paper.

He eventually reported back without the graph, but informing me that the log-log paper had itself given him a headache!

Big smile!

-al hart
al@w8vr.org

WD8BIL:
Quote

“The reflection at the mismatch gives rise to a second wave that travels back to the input of the line with the magnitude determined by multiplying the magnitude of the initial wave by the magnitude of [the reflection coefficient] ñ.  On arriving at the line input it sees total reflection ñ = 1 at the matching point in the network that achieved the conjugate match.”

OK, if I'm reading this right (phat chance) It states the reflection coefficient at the mismatch is some what less than 1. So the magnitude of the reflected wave is accordingly less than the incident. OK, I got that.

But you are correct. It still doesn't explain why, at the matched end, the reflection is perfect. I thought the whole idea of matched impedances was maximum power transfer? ??? If that is so, and a match occurs at source/line then why is the wave reflected and not transferred to the source?

We have to be missing something here! ???

KA2DZT:
Bud,

Unlikely that quoted statement is correct.  Hard to know exactly what Maxwell was thinking when he came up with this theory.

I think the reflected wave would add to the incident wave (depending on the phase relationship) at the transmitter and the sum of the two waves would travel back to the load.  Maybe this is what he is talking about.

My head is beginning to hurt.

Fred

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