The Killer Picture.

Written and posted yesterday,   this item requires more thought. The waveforms given to us by the voltage probe do not give the whole story. A probe loses information as to whether it is seeing a pulse travelling from left to right or from right to left.

On my websites . 1 , 2  is a discussion on what an oscilloscope probe sees when attached to the middle of a coaxial cable, when pulses travel through each other in opposite directions. When a probe captures a little part of a single 10v pulse, it turns it through 90 degrees and sends it off to the oscilloscope. If it captures part of a pulse travelling in the opposite direction, it turns this through 90 degrees and sends it off to the oscilloscope. During the capture, information as to the direction of travel of the original pulse is lost. Both present the same 10v signal on the oscilloscope.

When two pulses travel through each other, these two signals are superposed by the oscilloscope probe and the result is a single 20v pulse or a single 0v pulse on the oscilloscope, depending on the relative sign of the two pulses. If of the same sign, they add to 20v. If of different sign, they cancel.

When we see a 20v pulse on the oscilloscope, this is false. At that point in time, there was not a 20v voltage difference. There were two completely independent 10v pulses travelling through each other. The “signal” was two signals, and no signal was 20v or 0v. The oscilloscope falsified the picture by presenting 20v or 0v.

Not look at “The Killer Picture”. In discussion with Forrest Bishop yesterday, we agreed that it is difficult to make deductions from the picture, because of the above problem. If we look at a coaxial cable in the middle, it is reasonably easy to deduce that two pulses are travelling through each other. However, in the bottom trace of “The Killer Picture” we are looking at a point at the end of the 75 ohm coaxial cable when it is wrongly terminated at 40 ohms. The probe is looking at a signal and at the same signal partially reflecting at the same point. Terminated in an open circuit, we know we would see voltage doubling. However, terminated at half the characteristic impedance, it requires more thought. “The Killer Picture” results from the realisation that during discharge, electric current goes back into the top capacitor plate and charges it. This is difficult to think through, because the very idea that electric current is involved collides with "The Catt Question" .

"The 109 Experiment" is much clearer. I stumbled on the idea three years ago and at last Tony Wakefield has done the experiment. It clearly shows that a “steady charged capacitor” is not steady at all. All the time, energy is reciprocating from end to end at the speed of light. It was after Tony competed this experiment that I suggested that he try other things, for instance to terminate the cable with 40 ohms instead of with 75 ohms. These further experiments require more thought.

Ivor Catt 12 May 2012

Probing pulses in a TEM Wave down a coaxial cable. 1 , 2