Question regarding the electrical signal traveling in circuits.

Studiot

You need to think in 3D for this.

yungman

I know this is a tedious question. Everyone know you can pick off a signal by solder on the top of the trace. I wonder anyone stop and think about how and why that happen!!!!

Studiot

I don't want to comment further until Sophiecentaur has confirmed my understanding of his picture, in case I am barking up the wrong tree.

sophiecentaur

I don't see the problem here. Where the wave is launched, there will be a delay (phase lag) in the fields on top. Without a dielectric to confuse things, the field at a point on top of the line will be the result of waves reaching it from under the line and further back down the line. At a large distance from the source, the path lengths will become equal to all points at a given distance from the source so the phases would be equal, I suppose. With a dielectric then you could get a phase tilt forward because the speed on top is nearer c than the speed below. But this tilt will settle down to a constant value as happens with an mf ground wave over a uniform ground plane.

@yungman
If you were using wide microstrip then I might suggest that the impedance presented by your soldered connection 'in the middle' of the line might be very different from the impedance if you terminated directly between trace and earth. But where there is a discontinuity, the situation would actually be different and the E fields and currents would be disturbed in any case. Your 'pick up point', soldered to the top of the line is only half of the connection; there needs also to be an earth connection if you want to take a signal from the line. Everything would be upset.

Studiot

My comments have nothing to do with launching a wave and everything to do with the geometry of the situation.

I am suggesting that the picture you posted is a cross section through an already fully developed (wave? signal?) (travelling?) at right angles to the plane of the section.

sophiecentaur

Yes. It shows the 'static' situation. Isn't that clear in the text? I'm afraid I recognised it immediately and didn't search for the detailed description. It shows that the field is stronger between the plates - not surprisingly.

Interestingly, when discussing what happens when you pick a signal off the line, it is the potential that makes the difference and not the fields. This is an old chestnut for ordinary circuits too.