Transmission lines/electrical propogation

[Darren93]

We have just started to look at transmission lines/coaxial cables in one of my classes. This uses the concept of having 2 wires close together causing an impedance/capacitance that allows/controls propagation. However I have never really considered how electricity propagates in the first place. Surely electricity could propagate over 1 lead if the respective terminals where connected to ground? In these models that would be impossible. How does electricity propagate typically? Is it affected by the inductance/capacitance between two leads? Or is the transmission line theory just for high frequency signal propagation?

 

[Drakkith]

Two wires are used because it reduces electromagnetic interference from nearby electronics and reduces the power loss due to the conductors radiating at the signal frequency (among other reasons). Even short wires will radiate a large portion of their power outwards as an RF signal if they aren't paired with a second wire, since they are effectively acting as an antenna. Low frequency signals don't suffer this problem nearly as much since it is much, much harder to radiate a low frequency signal compared to a high frequency signal.

The inductance/capacitance of the conductors is very important. It determines the characteristic impedance of the line, which is necessary to know for impedance matching to avoid power and signal loss in the form of reflected and standing waves.

Or is the transmission line theory just for high frequency signal propagation?

It's for high frequency signal propagation. Low frequency signals have different requirements when it comes to effectively transmitting them down a conductor.

 

[Darren93]

Thanks very much! You answered 80% of what I was confused about in 1 post. I wasn't expecting that at all, thanks for taking the time to do so. I'm still unsure how electricity propagates at low frequency/dc along a lead other than at the drift velocity. That's a completely different question and requires some more research.

 

[Drakkith]

I'm still unsure how electricity propagates at low frequency/dc along a lead other than at the drift velocity.

Well, the first thing you'd need to understand is that "electricity" is a meaningless term. Electrical charges move to form electrical current, voltage can propagate or change, but "electricity" does nothing. It means nothing. It's simply a catch-all term for anything related to electrical circuits.

Electrical circuits (including transmission lines) is a complicated subject when you get into the details, so don't feel bad if you don't understand something. Many of the terms and concepts you've learned growing up are either utterly meaningless or downright wrong when you actually learn the subject.