Electronics > Projects, Designs, and Technical Stuff

transmission line wave demonstration

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KTP:
Hi guys,

A section of my applied electromagnetics lab (due tommorow, better get busy!) had us answer some questions about a mechanical analogue to charges on a transmission line.  It described a continuous tube full of balls with springs between each ball and the tube elongated such that it had the appearance of a two wire transmission line.  I was bored yesterday and so whipped up a flash app. (first time ever doing anything in flash) to simulate this analogue.  It actually turned out fairly decent.  I added a damping factor to the end ball to represent a variable load...I guess the rest of the line would be considered lossless.  Some interesting values other than the default settings are increasing the amplitude, say to around 20 where you really start to see the traveling wave, and increasing the damping factor to a big value, say 2000 and reducing the spring constant, to say 10, and you start to see a standing wave pattern for awhile.  It probably has a lot of bugs, and certain values cause the balls to crash into each other or fly off the screen, but here it is nonetheless:

http://www.skyko.com/EE361/wave.html

Mechatrommer:
nice wave simulation, but i dont think its the real analogy to charges, as electrons move to +ve pole instead of static and "spring'ing" to each other.

saturation:
Very good job!  Safri, this is transmission line theory not just simple DC on a conductor.

http://www.skyko.com/EE361/wave.swf

ktp, I hope you don't mind me stripping off your html, looks better without it  ;)

alm:

--- Quote from: Mechatrommer on February 01, 2011, 04:37:53 am ---nice wave simulation, but i dont think its the real analogy to charges, as electrons move to +ve pole instead of static and "spring'ing" to each other.

--- End quote ---
Even for DC, this speed is insignificant compared to the signal velocity. Signal velocity is usually light speed times some factor (<1), electrons have mass, so they don't get anywhere near this velocity, especially without a strong force (eg. electric field). Depending on wire size and current, it's usually in the order of centimeters per hour, which is about 12-13 orders of magnitude slower than the signal velocity in copper.

And for AC (transmission lines are by definition AC), there's no net movement at all :).

KTP:

--- Quote from: alm on February 01, 2011, 12:12:18 pm ---
--- Quote from: Mechatrommer on February 01, 2011, 04:37:53 am ---nice wave simulation, but i dont think its the real analogy to charges, as electrons move to +ve pole instead of static and "spring'ing" to each other.

--- End quote ---
Even for DC, this speed is insignificant compared to the signal velocity. Signal velocity is usually light speed times some factor (<1), electrons have mass, so they don't get anywhere near this velocity, especially without a strong force (eg. electric field). Depending on wire size and current, it's usually in the order of centimeters per hour, which is about 12-13 orders of magnitude slower than the signal velocity in copper.

And for AC (transmission lines are by definition AC), there's no net movement at all :).

--- End quote ---

Right.  Well actually these could be representing positive charge (holes) if it helps to think of it that holes move in the direction of positive current flow.   You can see the bunching of *charges* on the simulation propagate down the line even though each charge is just sloshing back and forth with no net movement.   The electric field is contained between the wires in the dielectric and will be strongest in the area of these accumulated "charge pools" and thus will move down the line as a signal.  Since it is totally contained in the dielectric, the speed will depend on the permitivity of that material, but will be some fraction of light speed.