# Voltages along on a transmission line...

• posted

I know this should be obvious, but I just can't visual the power or voltage distribution on a short, matched transmission line that is, lets say, just one wavelength long: For such a short line, wouldn't the voltage or power vary in amplitude along the matched 360 degree long line, considering that this signal will sinusoidally start out at zero volts at the beginning of the transmission line (0 degrees), then

90 degrees later reach a peak, then 90 degrees later drop to 0V, etc? Yet I never here mention of any amplitude variations on a lossless transmission line except when I read of standing waves on a mismatched line -- but why don't we get measurable amplitude variations due to the rise and fall of the RF signal itself (which would affect RF circuits vs. distance at high microwave frequencies)??

Confused,

-Bill

• posted
** Groper in the dark alert

** You are indeed.

Try to figure out the difference between "amplitude" and instantaneous value.

All continuous waves have both.

...... Phil

• posted

Yes, at any one instant it might look like that. A little later, it will look different. The generator is extruding a sine wave that travels along the line, towards the load, at a goodly fraction of the speed of light. At any intermediate point along the line, there's the same sine wave voltage, just shifted in phase.

If you measure any point along the line with an oscilloscope, you'll see the sine wave rise and fall. If you measure it with a voltmeter, it will look the same everywhere. It's a matter of the time scale over which you take the measurements.

If the line is unmatched, there will be waves traveling in both directions, in which case the voltmeter will see different voltages in different places, as the waves add in some locations and subtract in others.

John

• posted

Fantastic info John -- that really cleared up my confusion on this subject!

Thanks again for the great help!

-Bill

• posted

Although you seem to have cleared this up, here's a thought exercise:

You have a driver output that has an effective rise time of 300 picoseconds (not uncommon nowadays). It is driving a 6 inch track.

On FR4, the transmission velocity is approximately 160 picoseconds per inch. So at the initiation of the transmission, the line is 0V everywhere.

At 50% amplitude output (at the driver, and assuming 3.3V systems and a linear output) the voltage 1 inch away **is still zero**; i.e. the energy has not yet reached this point.

Now continue this until the entire system has stabilised.

Cheers

PeteS

• posted

A matched line will have uniform RF voltage along its length, since there are no standing waves. A (theoretical) time domain voltage probe would show the same signal, delayed in phase by (vf * distance from source)/wavelength in free space.

Frank Raffaeli

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